Fish in the coastal seascape exploring ecological processes and connectivity for conservation oftemperate fish communities

 Thomas Staveley

Thomas Staveley    Fish

in th

e coastal seascape

Doctoral Thesis in Marine Ecology at Stockholm University, Sweden 2019

Department of Ecology, Environment andPlant Sciences

ISBN 978-91-7797-590-8

Thomas Staveley

This thesis includes the following papers:       Seascape structure and complexity influence temperate seagrass fishassemblage composition. Staveley TAB, Perry D, Lindborg R,Gullström M. (2017) Ecography, 40, 936-946.      Habitat connectivity of fish in temperate shallow-waterseascapes. Perry D, Staveley TAB, Gullström M. (2018) Frontiers inMarine Science, 4, 400.       Sea surface temperature dictates movement and connectivity ofAtlantic cod in a coastal fjord system. Staveley TAB, Jacoby DMP,Perry D, van der Meijs F, Lagenfelt I, Cremle M, Gullström M.Accepted in Ecology and Evolution.      Exploring seagrass fish assemblages in relation to the habitat patchmosaic in the brackish Baltic Sea. Staveley TAB, Hernvall P, StjärnkvistN, van der Meijs F, Wikström SA, Gullström M. In review.

Fish in the coastal seascapeexploring ecological processes and connectivity for conservation oftemperate fish communitiesThomas Staveley

Academic dissertation for the Degree of Doctor of Philosophy in Marine Ecology at StockholmUniversity to be publicly defended on Friday 3 May 2019 at 10.00 in Vivi Täckholmsalen (Q-salen), NPQ-Huset, Svante Arrhenius väg 20.

AbstractThe need to understand patterns and processes in the marine environment has never been so profound as today, particularlyas anthropogenic pressures upon coastal regions are drastically affecting habitats and species across a vast range. Oneapproach to further understand these patterns and processes is through the use of seascape ecology methods. Pertinently,fish are ideal candidates to use in many seascape ecological studies due to their mobility and potential to connect a multitudeof patches and habitats throughout their life cycle. They also serve as fundamental components in coastal food webs andare of economic benefit. This thesis strives to answer how fish assemblages are affected by ecological and environmentalpatterns and changes in temperate seascapes throughout the Swedish Skagerrak and the Baltic Sea.

Initially, the spatial arrangement of benthic habitat patches in coastal Skagerrak was investigated in relation to the fishcommunity inhabiting seagrass meadows. Seascape structure and complexity was shown to create optimal or sub-optimalareas for certain parts of the fish community. For instance, simpler seascapes (e.g. less habitat patches and edges) werefound to have a higher density of juvenile fish, while wrasse densities were related to more complex seascapes. Thisoffers insights into the consequences of spatial patterning in the marine environment and possible effects of habitat lossin the ecosystem (paper I). Through surveying fish assemblages in common, shallow-water habitats, the more structurallycomplex habitats, i.e. seagrass and macroalgae, were found to harbour a greater fish abundance compared to the lesscomplex unvegetated soft bottoms. However, all three habitats were deemed important for their role in supporting juvenilefish species, thus suggesting that embayments in this environment might function as seascape nurseries (paper II). Theimportance of connectivity of a marine predator was discovered using acoustic telemetry and network analysis. This studydemonstrated that sea surface temperature was of major importance for Atlantic cod movement dynamics within a fjordsystem as well as revealing the significance of localised connectivity at varying spatial and temporal scales (paper III).Finally, spatial pattern relationships and fish assemblages were explored in Baltic seagrass meadows. Fish assemblageswere dominated by meso-predators (i.e. three-spined stickleback) both during summer and autumn, with a noticeable lackof larger piscivorous species throughout both seasons. Correlative analysis showed that fish densities were influenced byseagrass habitat structure (negatively), area of bare sediment (negatively) and habitat patch diversity (positively) (paperIV).

This thesis has lifted a central role in addressing important seascape ecology questions and tools in the temperate marineenvironment. Specifically, it highlights the importance of analysing patterns and processes at multiple scales to gain a morecomprehensive understanding of the relationships between fish and their environments, which is relevant for marine spatialplanning and conservation.

Keywords: seascape ecology, landscape ecology, seagrass, marine habitats, fish, spatial analysis, connectivity,conservation, Sweden, temperate.

Stockholm 2019http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-166242

ISBN 978-91-7797-590-8ISBN 978-91-7797-591-5

Department of Ecology, Environment and PlantSciences

Stockholm University, 106 91 Stockholm

FISH IN THE COASTAL SEASCAPE 

Thomas Staveley

Fish in the coastal seascape 

exploring ecological processes and connectivity for conservation oftemperate fish communities 

Thomas Staveley

©Thomas Staveley, Stockholm University 2019 ISBN print 978-91-7797-590-8ISBN PDF 978-91-7797-591-5 Cover: Seascape and landscape around Gullmarsfjorden, Bohuslän, Sweden. Photo by Felix van der MeijsPapers I-IV cover: Fish illustrations by Karl Jilg / Linda Nyman, published with permission from the SwedishSpecies Information Centre (ArtDatabanken), SLU Back: Photo by Lieselotte van der Meijs www.vdmfoto.se Printed in Sweden by Universitetsservice US-AB, Stockholm 2019

To my daughters,Aurelia & Isabella. May you continued to beamazed by the naturalworld & its beauty.

Svensk sammanfattning

Behovet av att förstå mönster och processer i havsmiljön har aldrig varit så påtaglig som idag, särskilt som det antropogena trycket på våra kustnära ekosystem drastiskt påverkar arter och deras livsmiljöer. Havslandskapsekologi erbjuder nya tillvägagångssätt för att förstå ekologiska konsekvenser av rumsliga mönster i havsmiljön. Fisk är en idealisk modell-organism när man studerar landskapsekologi i havet på grund av att fiskar rör sig över stora områden och därför fungerar som länkar mellan olika livsmiljöer under hela deras livscykel. Fiskar är dessutom grundläggande komponenter i kustnära födovävar och av ekonomiskt betydelse som havsresurs. Denna avhandling syftar till att svara på hur fisksamhällen påverkas av ekologiska samband och miljö-relaterade mönster och förändringar i tempererade havslandskap längs den svenska Skagerrak-kusten och i Östersjön.

I den första studien så undersöktes hur den rumsliga fördelningen av kustnära bentiska habitat påverkar fisksamhällsstrukturen i sjögräsängar i Skagerrak. Havslandskapets struktur och komplexitet visade sig skapa optimala eller suboptimala miljöer för vissa fiskarter. Exempelvis visade sig enklare havslandskap (baserat på t ex färre habitat-delar inom ett område och mindre påverkan från kanteffekter) ha en högre densitet av ungfisk, medan densiteten av läppfiskar var relaterade till mer komplexa havslandskap. Studien visar hur rumsliga variationsmönster i utbredningen av habitat påverkar organismer i havsmiljön och även potentiella effekter av habitatförlust i ekosystemet (manuskript I). Genom att kartlägga fisksamhällen i den grunda kustnära havsmiljön så visade resultaten från den andra studien att de strukturellt mer komplexa livsmiljöerna, dvs sjögräsängar och tångbälten (makroalger), hade en högre fiskdensitet jämfört med den mindre komplexa mjukbotten utan vegetation. Alla tre livsmiljöerna visade sig emellertid vara viktiga för ungfiskar. Generellt visar den här studien att fiskar fungerar som potentiella funktionella länkar mellan närliggande grunda habitat i vår havsmiljö (manuskript II). I den tredje studien undersöktes betydelsen av konnektivitet hos en marin rovdjursfisk med hjälp av akustisk telemetri och nätverksanalys. Studien visar att havstemperaturen vid ytan har stor inverkan på torskfiskars rörelsesdynamik inom ett fjordsystem och att lokal konnektivitet har stor betydelse på olika tids- och rumsskalor (manuskript III). Slutligen undersöktes rumsliga mönster och fisksamhällen i sjögräsängar i Östersjön. Fisksamhällen kvantifierades under såväl sommar som höst och resultaten visade att storspigg var betydligt mer abundanta än andra fiskarter och att det var en märkbar brist på större rovdjurfiskar. Korrelationsanalyser visade att sammansättningen av fisk i sjögräsängar drivs av faktorer såsom strukturen hos sjögräshabitat (som var positivt korrelad till fiskdensitet), area av mjukbottensediment utan vegetation (negativt korrelerad) och diversitet av habitat-”patcher” (positivt korrelerad) (manuskript IV).

Denna avhandling visar betydelsen av landskapsekologisk förståelse och dess verktyg när man studerar den tempererade havsmiljön. Specifikt betonas vikten av att analysera mönster och processer på olika skalor för att få en mer ingående förståelse av förhållandet mellan fisk och deras livsmiljöer. Studierna är högst relevanta som underlag för marin rumslig planering och i samband med kustzonsförvaltning och bevarandefrågor.

List of papers This thesis is based on the following papers, referred to in the text by roman numerals I-IV:

I Staveley TAB, Perry D, Lindborg R, Gullström M. (2017) Seascape structure and complexity influence temperate seagrass fish assemblage composition. Ecography, 40, 936-946. doi:10.1111/ecog.02745

II Perry D, Staveley TAB, Gullström M. (2018) Habitat

connectivity of fish in temperate shallow-water seascapes. Frontiers in Marine Science, 4, 400. doi:10.3389/fmars.2017.00440

III Staveley TAB, Jacoby DMP, Perry D, van der Meijs F,

Lagenfelt I, Cremle M, Gullström M. Sea surface temperature dictates movement and connectivity of Atlantic cod in a coastal fjord system. Accepted in Ecology and Evolution.

IV Staveley TAB, Hernvall P, Stjärnkvist N, van der Meijs F,

Wikström S, Gullström M. Exploring seagrass fish assemblages in relation to the habitat patch mosaic in the brackish Baltic Sea. In review.

My contributions to the above papers: Planning and design of study (papers I-IV), fieldwork / data acquisition (papers I-IV) analysis of data (papers I, III and IV), writing the papers (main responsibility in papers I, III and IV, active participation in paper II). Additional published papers relevant to the thesis: Perry D, Staveley TAB, Hammar L, Meyers A, Lindborg R, Gullström M. (2018) Temperate fish community variation over seasons in relation to large-scale geographic seascape variables. Canadian Journal of Fisheries and Aquatic Sciences, 75, 1723–1732. All published papers are open access.

Table of contents

Scope of the thesis ...................................................................................... 1

Background ............................................................................................... 3Seascape ecology ...................................................................................................... 3Connectivity .............................................................................................................. 4Temperate coasts and dominating habitats ......................................................... 5

Seagrass ................................................................................................................. 6Macroalgae ........................................................................................................... 6Unvegetated soft sediment ................................................................................ 7

Study sites .................................................................................................. 8

Study species ............................................................................................. 8Atlantic cod ............................................................................................................... 9Sea trout .................................................................................................................. 10Pipefish .................................................................................................................... 10Sticklebacks ............................................................................................................. 12

Methods .................................................................................................... 13Collecting fish data ................................................................................................ 13

Beach seine ........................................................................................................ 13Underwater stereo video cameras .................................................................. 14Acoustic telemetry ............................................................................................ 15

Seascape data analysis ............................................................................................ 16Spatial pattern metrics ...................................................................................... 16Network analysis ............................................................................................... 17

Main findings ........................................................................................... 18Fish assemblages related to seascape patterns ................................................... 18Nursery habitat or nursery seascape? .................................................................. 21Diel changes in the fish assemblage .................................................................... 23Spatiotemporal movement patterns of marine predators ................................ 24

Conservation issues and future perspectives ........................................... 25

Acknowledgements .................................................................................. 28

Financial support ...................................................................................... 29

References ................................................................................................ 30

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Scope of the thesis Coastal regions around the world encompass important and productive ecosystems, which are imperative for a multitude of physical, chemical and biological processes, as well as of significance for humans (Barbier et al. 2011; Hyndes et al. 2014). However, pressure from human activities and climate change upon the world’s oceans and seas has likely never been so profound as today (Brierley and Kingsford 2009; Toropova et al. 2010). Major pressures include habitat degradation, overexploitation, eutrophication and litter, often causing severe and irreversible consequences on marine life (Kappel 2005; Carleton and McCormick-Ray 2014; Halpern et al. 2015).

Many marine organisms are intrinsically linked to their environment through the distribution of resources, physical conditions and other abiotic and biotic factors. In particular, fish have an integral role in marine ecosystems, such as in coastal food webs where predator-prey relationships can be vital to maintain a healthy and resilient system (Hughes et al. 2005; Klemens et al. 2014). As a consequence of increasing pressures upon the marine environment, anthropogenic and environmental changes have the potential to dramatically affect and re-structure fish populations and communities (Jackson et al. 2001b; Worm et al. 2005). It is therefore imperative that fish dynamics (e.g. movement patterns) as well as fish-habitat relationships are investigated in a seascape ecological setting, paramount for the survival of species, healthy ecosystem functioning and effective marine stewardship. Thesis aim

Knowledge relating to the ecological significance of spatial patterning and connectivity at multiple spatiotemporal scales in temperate seas is often lacking though well requested in management frameworks, such as marine spatial planning. In order to address some of these knowledge gaps, the focus of this thesis was to investigate fish assemblages and habitat distributions using seascape ecology concepts and applications throughout shallow-water coastal environments in Sweden. Thus aiming to answer the overreaching question: How are fish assemblages affected by ecological and environmental changes in the temperate seascape?

Initially, I wanted to understand how fish assemblages responded to seascape patterning. Here we related how the seascape, composed of a mosaic of different benthic habitat patches, influenced fish communities within a central seagrass meadow. From this, important ecological relationships were revealed based on the structure and complexity of the seascape (I & IV).

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To investigate similarities and differences in the fish community found in common shallow-water habitats; fish species richness, diversity and habitat preference was used to understand the importance of connectivity. Further important notions regarding habitat and seascape nursery functions were also addressed (II).

To assess the importance of connectivity of a mobile species and further understand more about behavioural ecology across the shallow-water seascape, predatory fish were tracked throughout coastal areas where their movement activity patterns were related to environmental conditions (III). In this complex coastal system horizontal spatial movement patterns were assessed during different seasons, which demonstrated areas of particular importance (or hotspots) for small-scale movement.

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Background Seascape ecology In recent years, seascape ecology (or sometimes referred to as marine landscape ecology) has evolved considerably posing novel questions, quantifying new relationships and pushing boundaries surrounding spatial patterning and ecological processes in the marine environment (Pittman 2018). This opens up a field where spatially relevant questions can be posed and addressed which are not only ecologically beneficial but also increasingly needed for ecosystem-scale management, such as marine spatial planning (Pittman et al. 2011; Caldow et al. 2015).

The emergence of seascape ecology stems from concepts and applications developed from its terrestrial counterpart; landscape ecology. On land, over the last few decades, this discipline has grown incredibly and gained a strong momentum demonstrating the relevance of exploring ecology at multiple scales (Turner 2005; Gergel and Turner 2017). Due to the rise of seascape ecology, new terms and concepts have developed and evolved, many derived from landscape ecology (Box 1).

In parallel with landscape ecology, seascape ecology extends from the more traditional ecological view of focusing on single habitats or species, while instead broadens the scope and investigates patterns and processes at relevant spatial and temporal scales, which are often deemed more applicable at a management level. Scale is an important consideration in seascape ecology (Boström et al. 2011; Schneider 2018) thus, spatially, often incorporating multiple patches and habitats within the extent of the focus study (e.g. seascape structure and complexity, Box 1). Temporal scale is also fundamental in many seascape ecology studies, as over time (e.g. days, seasons, years) changes in mechanisms and processes may become more apparent or weakened. For instance, on a relatively small temporal scale of day and night some fish species, linked to ecological processes (e.g. grazing, predation), can exhibit shifts between prey and habitats (thus also accounting for the importance of spatial scale) (Grant and Brown 1998; Beets et al. 2003). Studies over seasons, for example monitoring fish movement, can reveal important information about species behaviour, residency and movement pathways (Biggs and Nemeth 2016; Kessel et al. 2016) which can be used to reveal connectivity patterns in the seascape (Box 1) and information regarding ontogenetic movement (Huijbers et al. 2015).

Geographically, most applications of seascape ecology have been conducted in tropical and sub-tropical coastal environments (e.g. Pittman et al. 2004; Gullström et al. 2008; Grober-Dunsmore et al. 2008; Gilby et al. 2018).

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In more temperate regions, such as Europe, consequences of habitat patterning particularly relating to fish assemblages are becoming better known. For instance in the North Atlantic (Jackson et al. 2006a; Staveley et al. 2017) and more recently in the Mediterranean (Ricart et al. 2018; Scapin et al. 2018b).

Adapted from Grober-Dunsmore et al. (2009) and Staveley et al. (2017)

Connectivity As with seascape ecological questions in general, connectivity research in the marine environment has been predominantly focused on tropical coastal systems (e.g. Berkström et al. 2012; Gullström et al. 2013; Pittman et al. 2014; Olds et al. 2016; van Lier et al. 2018; Baker et al. 2019), where important discoveries have been made linking habitats through, for instance, ontogenetic migration (e.g. Berkström et al. 2013a) and diel movement patterns (e.g. Hitt et al. 2011a). Connectivity can also differ considerably within a life-stage of a species, for example, Hogan et al. (2012) reported that fish larvae from coral reefs in central America could be found hundreds of kilometres from their home reef despite other conspecifics which showed high reef self-recruitment. This shows the importance of exploring different scales as well as connectivity (Box 1) which is not only imperative to further ecological understanding but necessary in management applications (e.g. spatial marine

Box 1: Terms used in seascape ecology

Patch – a defined area of habitat

Mosaic – an area consisting of multiple patches of differing habitat types

Seascape – a heterogeneous area varying in size and structure

Seascape structure – the composition (e.g. habitat type) and configuration (e.g. edges) of patches

Seascape complexity – a gradient-based assessment based on the spatial structure of habitat patches

Structural connectivity – how the physical structure links aspects of the seascape

Functional connectivity – linkages of functional importance across seascapes

Actual connectivity – a direct measurement of how organisms move through the seascape

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planning, marine protected areas) to ensure that a more holistic approach is considered (Olds et al. 2018a; Ortodossi et al. 2018). Connectivity can also identify important species pathways or corridors, which is crucial when implementing protection and conservation measures in the marine environment (Pendoley et al. 2014).

Network analysis applications (often coupled with biotelemetry data) have become more frequent in the marine environment where large fish species are often the study organisms (e.g. Jacoby et al. 2012; Wilson et al. 2015; Lédée et al. 2016; Williams et al. 2018). This not only increases our understanding of connectivity at multiple scales, but furthermore this information can be used in evidence-based conservation efforts (Lea et al. 2016; Engelhard et al. 2017). Of particular prominence, Lea et al. (2016) found using network analysis a positive link between the sizes of marine protected areas and shark movement dynamics, which resulted in increased national protection for these marine predators in the Seychelles.

Several connectivity studies have been conducted in temperate coastal systems, such as climate change impacts on estuarine population connectivity (Chust et al. 2013), genetic connectivity and seed dispersal of marine angiosperms (Ruiz-Montoya et al. 2015; Jahnke et al. 2018) and adaptive conservation management for the protection of marine organisms (Coleman et al. 2011). However, in general there is a profound knowledge gap in connectivity research regarding marine species, habitats and ecosystems in the temperate seas. In order to gain more knowledge about connectivity in the temperate coastal environment, and address some of the data gaps, information relating to fish and their environment was gathered in papers I-III. Temperate coasts and dominating habitats Temperate coastal environments are productive systems which can support many ecosystem services (Rönnbäck et al. 2007). Such as, climate regulation (e.g. sequestration of carbon in seagrass meadows; Röhr et al. 2018), food security (e.g. nursery grounds for commercial fish species; Seitz et al. 2014) as well as many cultural services (e.g. recreation, education, aesthetic value; Beaumont et al. 2007). Due to the close connection and reliance humans have with coastal systems (plus subsequent pressures imposed), in addition to the relative ease to conduct field research (compared to often more challenging environments e.g. open ocean, deep sea) the majority of seascape ecology studies have focused on these near-shore environments (Boström et al. 2006, 2011; Gullström et al. 2008; Pittman et al. 2011; Olds et al. 2018a).

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Seagrass Seagrass habitats in the marine landscape are a classic habitat that has been used for seascape ecology applications (Robbins and Bell 1994; Irlandi et al. 2006; Boström et al. 2006). Seagrasses are photosynthesising marine flowering plants that grow in soft sediment coastal areas throughout most of the world. Their distribution in depth is limited by light availability, which can be impaired through increased eutrophication and sedimentation in the coastal zone. Around the Swedish coast, eelgrass (Zostera marina) is the species most commonly found (Fig. 1a) which can form large, continuous underwater meadows. Other seagrass species occur such as the dwarf eelgrass (Zostera noltii) and beaked tasselweed (Ruppia maritima) but with a much more limited distribution (at least in the surveyed study sites). Eelgrass forms meadows or beds of varying size and structure and are found in sheltered sites on the northern Swedish west coast to fairly exposed areas in the northern Baltic Proper (I and IV), where it begins to reach its lower salinity tolerance (5-6) (Boström et al. 2014). Eelgrass habitats are important for many different functions and roles within the environment such as sediment stabilisation, carbon sequestration, as well as a habitat for marine organisms (Nordlund et al. 2016). However, like many other habitats around the world, seagrass is declining globally (Waycott et al. 2009), with an average worldwide loss of eelgrass of 1.4% annually (Short et al. 2010). The loss of eelgrass on the Swedish west coast has been reported as severe, with up to 80% disappearing in some areas over two decades (Baden et al. 2003). Macroalgae Hard rocky substrate comprises the majority of the shoreline in the Swedish Skagerrak and the northern Baltic Sea proper, making this ideal for the colonisation and growth of macroalgae (Fig. 1b). Compared to seagrass, these habitats are much less studied in terms of their fish assemblage around this region, but they may serve similar functional roles due to their three-dimensional structure (II; Jenkins and Wheatley 1998; Tano et al. 2017). In particular, species in the wrasse family (Labridae) are reliant on these habitats and the underlying hard-substrate for vital ecological processes, such as reproduction (Kullander et al. 2012), and have been found to be positively associated with a complex seascape consisting of multiple habitats (I). Common macroalgal species around the shallow-water west coast are, for example, Ascophyllum nodosum, Chorda filum, Fucus spp., Laminaria digitata and Ulva spp. In comparison, much fewer larger macroalgal species (e.g. mainly Fucus vesiculosus) grow on the hard substrate in the northern Baltic Proper.

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Figure 1 Common marine habitats found in coastal Sweden. Eelgrass or Zostera marina (a), macroalgae (west coast) e.g. Fucus spp., Chorda filum (b) and unvegetated soft sediment (c). Photos: T. Staveley Unvegetated soft sediment A large proportion of shallow-water habitat (<10 m) around the northern Swedish west coast is composed of unvegetated soft sediment (e.g. mud, sand) (Fig. 1c; Stål and Pihl 2007). However, despite the abundance of this habitat, it is perhaps the least understood compared to seagrass and macroalgae, particularly in terms of faunal biodiversity in shallow-water areas. The importance of the extent of unvegetated soft sediment bottoms in a spatial patch mosaic perspective in coastal Sweden has shown to have both positive- (I) and negative effects (IV) in relation to the seagrass fish community. The significance of this habitat is further emphasised by the strong site attachment and movement patterns of fish (III; Thrush et al. 2002; Fetterplace et al. 2016). Due to lack of high-resolution spatial movement data of many marine mobile species, and more research focused on vegetated coastal habitats, unvegetated soft sediment bottoms are often underrepresented which is of consequence when management decisions are made for marine and fisheries protection.

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Study sites Research in this thesis was conducted in shallow-water environments around the coast of Sweden (Fig. 2). On the northern west coast of Sweden (I-III), the Skagerrak Sea is joined to the North Sea to the west and Kattegat linking to the Baltic Sea in the south. This area has low-tidal amplitude, with a mean salinity around 15-34 (Kristiansen and Aas 2015). The coastline is dynamic and complex, consisting of archipelagos, fjords, and fjord-like systems creating a myriad of bays, coves and inlets. This has given opportunity for a multitude of organisms to create habitats, live and thrive in these sheltered environments. Throughout this region there has been various marine protection initiatives implemented, such as the Koster Havet National Park, the marine reserve of the Gullmar Fjord, numerous Natura 2000 sites and localised bans on fishing certain species (e.g. Atlantic cod Gadus morhua).

Moving around the southern part of Sweden into the semi-enclosed Baltic Sea, salinity decreases rapidly creating a physiological stress to marine organisms that results in naturally lower species richness. The further north in the Baltic Sea the more brackish and fresh-water species dominate (coupled to lower salinity). Upon reaching the northern Baltic Sea proper on the Swedish east coast (Fig. 2; IV), this archipelago region has soft sediment bottoms where a mix of submerged aquatic vegetation is dominated by freshwater species, particularly in shallow bays, and macroalgae (seaweeds) around the rocky shores. The Baltic Sea has no influence of tides, and in this region surface salinity is around 6.

Study species In Sweden approximately 140 species of fish are found regularly throughout fresh and saltwater environments. Of these, roughly 60 species are frequently observed in shallow-water coasts (<10 m) in the Skagerrak region (Pihl and Wennhage 2002). Half of these were found in the studies on the Swedish west coast (I-III), while in comparison species richness in the northern Baltic proper was lower with more influence of species from freshwater origin (IV). Some common /focus taxa from the studies within this thesis (I-IV) are described in more detail below.

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Figure 2 Location of the study sites and associated papers in the Skagerrak and the Baltic Sea, Sweden Atlantic cod The Atlantic cod is one of the most important predatory fish found in the region. In coastal areas, juveniles’ benefit where they can seek shelter in vegetation, prey on a wide range of available food (e.g. worms, crustaceans, fish) and use these relatively shallow seas as a nursery area (Figs. 3, 4). The economic importance of this species across its distribution (North Atlantic) is vast, however, in recent times intense pressure through fishing has led to the substantial depletion of many populations (Jackson et al. 2001b). Atlantic cod is currently listed as ‘vulnerable’ on the IUCN Red List (however the current assessment needs updating; Sobel 1996, IUCN 2018) most likely due to drastic decreases in populations coupled with the collapse of some stocks in the beginning of the 1990s (Frank et al. 2005; Fox et al. 2008). Throughout the Skagerrak region of the North Sea, the situation is the same as in the rest of the

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North Atlantic, where populations have historically been abundant (Jonsson et al. 2016), though nevertheless still currently subjected to commercial fishery pressures (European Union 2018). Although since 2012 in the Gullmar Fjord fishing pressure have been significantly reduced due to an all year-round fishing ban on Atlantic cod as well as Haddock (Melanogrammus aeglefinus) and Pollack (Pollachius pollachius). Regarding Baltic Sea cod populations, a reduction in available habitat (e.g. due to hypoxia; Casini et al. 2016) and decreases in growth have also become apparent (Svedäng and Hornborg 2014). Together with fishing pressures it makes the situation critical with little signs of improvement, particularly in the Eastern population (ICES 2018). Sea trout Another important predatory fish found commonly in the study sites on the Swedish west coast (I) is the anadromous sea trout (Salmo trutta). This can be a highly migratory species, but in the Skagerrak region they are thought to be more stationary and can stay around coastal waters year round during some life stages (Olsen et al. 2006), perhaps due to a relative lack of riverine systems in the region. Sea trout is one of the largest predatory fish species found in eelgrass meadows on the Swedish west coast (I), where they prey on fish (e.g. gobies) and crustaceans (e.g. shrimps) (Wennhage and Pihl 2002). Even though there are sea trout in the Baltic Sea, none were found during investigations in paper IV. Sea trout is classified as ‘least concern’ by the IUCN Red List (Freyhof 2011). Pipefish In Swedish waters, six species of pipefish (family: Syngnathidae) are found throughout coastal habitats. In the study areas investigated here, four of the species were recorded. The broadnosed pipefish (Syngnathus typhle) was most common in seagrass habitats on the Swedish west coast (I and II) and also found in the Baltic Sea. While in the Baltic Sea, the more slender, straight-nosed pipefish (Nerophis ophidion) was more prevalent (IV). Both these species show strong links with seagrass habitat (Scapin et al. 2018a). The snake pipefish (Entelurus aequoreus) and the greater pipefish (Syngnathus acus; Fig 5) were found only on the west coast and in lower densities than the former species. These cryptic species are often associated with structurally complex vegetation such as seagrass- or seaweed beds, where they prey upon small crustaceans and occasionally small fish (Kullander et al. 2012). All four species are listed as ‘least

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concern’ by the IUCN Redlist (Pollom 2014, 2015; Schultz 2014; Smith-Vaniz 2015).

Figure 3 Atlantic cod (~33 cm) swimming through an eelgrass meadow in Bohuslän, Western Sweden. Also, smaller, two-spotted gobies Gobiusculus flavescens can be seen in the middle of the picture just above the eelgrass canopy. Photo: D. Perry

Figure 4 The life cycle of Atlantic cod. Source: Molly Thomson/ Piscoweb.com

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Figure 5 A Greater Pipefish (Syngnathus acus) among the seagrass Zostera marina and filamentous algae in the Gullmar fjord, western Sweden. Photo: T. Staveley Sticklebacks Several species of stickleback (family: Gasterosteidae) were found within the brackish and marine waters around Sweden’s coast, however, in seagrass meadows, the three-spined stickleback (Gasterosteus aculeatus) vastly dominated in both the summer and autumn seasons not only in this family but often in the total fish assemblage (I and IV). This opportunistic species has flourished in recent years (Baden et al. 2012; Bergström et al. 2015) due to more favourable conditions, such as lower predation pressure from diminished piscivorous fish populations and increased eutrophic conditions, resulting in more filamentous algae among seagrass meadows (Pihl et al. 2006). This is also coupled with naturally low predation rates upon this species from piscivores as other prey is often preferred (e.g. gobies; Wennhage and Pihl 2002).

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Methods Collecting fish data In order to collect data on fish assemblages and movement patterns in the coastal seascape, three methods were employed (Fig. 6). These were beach seine (I and IV), remote underwater stereo video cameras (II) and acoustic telemetry (III).

Figure 6 The seascape and landscape around the Gullmar Fjord, western Sweden. Papers and associated methodology regarding fish survey and investigation are shown in different parts of the coast. The Sven Lovén Centre for Marine Infrastructure – Kristineberg (University of Gothenburg) (top-centre) was used as the research base for most of these studies. Note: paper IV not shown as conducted in the Baltic Sea. Photo: F. van der Meijs

Beach seine The beach seine (Fig. 7a) is a net that encompasses a given area of habitat and by closing the ends together traps fish within. Fish are directed to the cod end and then hauled onto land for species data collection. The beach seine sampling is used as a favoured method for quantifying the fish assemblage in seagrass meadows (e.g. Pihl et al. 2006; Unsworth et al. 2008; Ford et al. 2010; Smith et al. 2011), although it could underestimate some species’ densities (Nagelkerken et al. 2001). This method has been shown to offer more exact catch estimates

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compared to other fishing methods, such as beam trawling (Guest et al. 2003). However, other studies have shown preference to the beam trawl, particularly when sampling during day and night (Jackson et al. 2006b). Nonetheless, this also depends on the size of respective nets and depth limitations for each method. The beach seine used in papers I and IV was 30 m in length and 3.5 m in height (at the cod end), enabling collection at relatively deep depths for this method. Due to the size of the net this was deployed using boat and 2-3 people, which enabled smooth deployment over the seagrass habitat.

Figure 7 Pictures showing the different methods of fish data collection; beach seine haul (a), underwater stereo video cameras (b) and acoustic telemetry receiver setup (c). Photos: T. Staveley / M. Gullström Underwater stereo video cameras The use of remote underwater video (RUV) cameras to collect ecologically relevant fish data is a growing method around the world (e.g. Hammar et al. 2013; Unsworth et al. 2014; Perry et al. 2018; Henderson et al. 2019), especially where water clarity allows good observations. Different methods include the

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use of a single camera or a calibrated stereo system (as used in paper II; Fig. 7b) with or without the use of bait (often crushed up fish / crustaceans placed in front of the camera to attract fish or other target species). This method is a useful way to quantitatively assess mobile marine life as it is non-destructive and non-invasive and the system can be left in situ to record for long intervals (depending on battery life). The use of a stereo camera system can be advantageous over the single camera, as after calibration it allows post-recording analysis to extract additional information, such as fish size (II). This information can then be used to better understand the fish assemblage in terms of life stage and community dynamics. A drawback of this method is re-counting fish that swim in and out of the field of view, thus leading to overestimates of some species. Though, recent methods have been introduced using rotating cameras to enable 360° viewing (Denney et al. 2017), which may help eliminate this issue.

Acoustic telemetry Acoustic telemetry is one of the most widespread forms of biotelemetry that is used to track the movements and gain important ecological knowledge of animals in the aquatic environment (Hussey et al. 2015). As mobile marine organisms, many fish species are ideal candidates to employ this method in

Box 2: Method comparison - beach seine and RUV

Many methods can be used to survey and quantify fish assemblages in seagrass habitats, such as different types of nets (passive or active), traps, underwater visual census and cameras. Due to discernible differences in survey methods, comparisons have been undertaken to assess the similarities and differences found between methods (e.g. Harmelin-Vivien and Francour 1992, Petrik and Levin 2000, Baker et al. 2016).

Comparing paper I (beach seine) and Perry et al. (2018) (RUV), where both methods were used in the same seagrass meadows simultaneously, some trends reveal that although most species (e.g. three-spined stickleback, broad-nosed pipefish) were observed using both methods, some demersal fish (i.e. shorthorn sculpin Myoxocephalus scorpius and viviparous eelpout Zoarces viviparus) were only recorded with the beach seine. This can suggest that species that are strongly associated with the sea floor and extremely stationary may be underrepresented by certain survey methods. Further investigation and potential standardisation between these two methods needs to be conducted to ensure their appropriateness as single- or combined fish survey methods.

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order to collect continuous, high resolution movement data (Furey et al. 2013; Fetterplace et al. 2016; Abecasis et al. 2018). Through the use of acoustic receivers (Fig. 7c) and transmitters (fitted internally or externally to the organism), this method not only offers continuous monitoring data, but also explicit information is gained about an animals’ whereabouts at varying spatial and temporal scales. This kind of data can be overlaid upon environmental features in the seascape (e.g. habitat type, bathymetry) which gives valuable information on how individuals / species are connected with their environment (Hitt et al. 2011). Information from acoustic telemetry has also paved a way forward into the efficacy of marine protection for marine species (Afonso et al. 2008; Alós et al. 2011; Marshell et al. 2011; Lee et al. 2015), the conservation of marine environments and in marine spatial planning (Pittman et al. 2014; Espinoza et al. 2015; Lea et al. 2016; Olds et al. 2016). Furthermore, discoveries in the abrupt absence of detections have been recently reported, revealing potential presence of illegal fishing activities (Tickler et al. 2019).

However, even though technology is rapidly advancing the development of acoustic transmitters and improving the collection and quality of data (Cooke et al. 2004; Hussey et al. 2015), there remains a trade-off between battery life (i.e. size of transmitter) and the size of the study organism. Also, in order to gain detections, a tagged individual needs to be within the range of an acoustic receiver, which highlights the prerequisite of a well researched and designed study with perhaps some preconception of movement patterns. Seascape data analysis Spatial pattern metrics Spatial pattern metrics can be used as appropriate tools to assess spatial patterning in the coastal and marine landscape (e.g. I, IV; Wedding et al. 2011; Young et al. 2018). Metrics often represent the composition (e.g. number of patch types, area of habitat) and configuration (e.g. distance measures, edge characteristics) of elements in the seascape, which can be used to determine seascape structure. In paper I, fifteen metrics were used (see Table 1 in paper I for more details) to quantify the area, density, diversity, edge, shape and connectivity of the two-dimensional habitat patch structure of thirty seascapes (600 m in diameter). Metrics were then modelled against fish assemblage densities found in the central seagrass meadows. Following this, seascape complexity (based on influential metrics) was determined to form a broader understanding of how spatial patterning could be optimal or sub-optimal for the fish assemblage (I). Seascape structure was also determined in paper IV,

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though using fewer numbers of seascapes and spatial pattern metrics. However, the overall aim was similar to above, in understanding the consequence of seascape structure in relation to fish densities in seagrass meadows. Network analysis Collecting continuous spatial movement data on species (e.g. through acoustic telemetry) can often create very large datasets (hundreds of thousands to millions of detections), which can be challenging to extract relevant information at appropriate scales (Heupel et al. 2006). The application of network analysis (Jacoby and Freeman 2016) can be one way to present these data and show it in a spatially meaningful way (through nodes and edges), particularly across larger temporal scales. Investigating movement patterns of predatory fish in shallow-water areas of a coastal fjord (III) enabled the use of network analysis to better understand spatial linkages of individual fish at relevant scales. Connectivity metrics (i.e. node strength, degree) from networks were calculated and modelled against environmental data to understand significant abiotic effects upon fish movement and connectivity in this region.

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Main findings The main findings from the research in this thesis is outlined (Box 3) and presented below.

Fish assemblages related to seascape patterns Surveying fish assemblages in the shallow-water coastal zone in Sweden has revealed that there was a general dominance in the three-spined stickleback and the two-spotted goby (Gobiusculus flavescens) (I, II and IV). There was a prominent lack of larger predatory piscivorous fish (especially in the Baltic Sea; IV), which is certainly a sign of pressures from overfishing, habitat destruction and effects from increased pollution in the coastal waters (Österblom et al. 2007; Klemens et al. 2014). Some fish species are of economic importance in this region (e.g. Atlantic cod, saithe Pollachius virens), which use these coastal areas for parts or all of their life cycles. Others have a strong social importance, such as the trout, caught by the recreational fishery.

Fish assemblages in seagrass meadows were the focus for understanding influences of habitat spatial patterning in the coastal seascape (I and IV) At the smaller habitat scale, seagrass shoot height and density was found to negatively affect, more so in the summer, different fish trophic levels (i.e. low-, mid- and high-level carnivores) as well as juvenile densities in the Baltic Sea (IV). While in contrast, these habitat characteristics had very little

Box 3: Main findings

• Spatial habitat pattering affected temperate fish assemblages in different seasons.

• Seascape complexity was found to be optimal and sub-optimal for parts of the coastal fish assemblage.

• Structurally complex coastal habitats harboured greater fish abundances than less-complex unvegetated environments; but all habitats deemed functionally important as a nursery.

• Sea surface temperature hindered spatiotemporal connectivity of a marine predator.

• Low-level carnivorous fish, with little evidence of larger predatory species, dominated Baltic Sea seagrass meadows.

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significance on the fish assemblage in the Skagerrak (I). At a larger scale, seascape complexity was investigated (I) and found that for some parts of the fish assemblage (e.g. juveniles, pipefish) a less complex seascape (e.g. less patch diversity, larger patch size and reduced edge) was more beneficial to fish densities found in the seagrass meadows (Fig. 8). While interestingly, in the Baltic Sea, the response of fish densities to some elements of the seascape structure showed opposite results. For example, juvenile fish densities were higher when the patch mosaic was most varied (or ‘complex’; i.e. a higher Shannon’s diversity index, IV). These dissimilarities may be in part due to differences in fish biodiversity between the Baltic Sea and Skagerrak, however, this also underlines how the consequences of spatial habitat patterning may differ so greatly, even when focusing on the same focal habitat (i.e. eelgrass).

Figure 8 A conceptual seascape showing how patch dynamics (i.e. seascape complexity) change from less complex, on the left, to more complex towards the right. Examples of fish species and life stage are shown that relate to either less or more complex seascapes. Adapted from Staveley et al. (2017). Fish illustrations by Karl Jilg and Linda Nyman, published with permission from the Swedish Species Information Centre (ArtDatabanken), SLU

Structural connectivity, that is the physical structure of the seascape (e.g. patch size, number of patches), was assessed in respect to the spatial

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patterning of benthic habitat patches that could benefit or impede fish densities in seagrass habitats (I), and also how common shallow-water benthic habitats were connected via fish species assemblages (II). In paper I, connectivity was not shown to be a major factor influencing fish assemblages; perhaps limited by scale issues in this instance. It may be worth investigating larger seascapes sizes than this study (i.e. >600 m), or other modelling approaches (e.g. Caldwell and Gergel 2013), around temperate coasts as spatial scale is often central in understanding seascape connectivity (Pittman and Brown 2011; Gullström et al. 2011; Berkström et al. 2013b; Scapin et al. 2018b). Paper II explored similarities and differences in fish assemblages in adjacent habitats within the same seascape where predictions on habitat connectivity were discussed. Results from this study predominantly showed that structurally complex habitats (i.e. seagrass and macroalgae) had a significantly higher abundance of fish compared to unvegetated habitats, but interestingly no difference in species, which highlights the importance of how fish may be using and indeed connecting multiple habitats within the shallow-water coastal environment.

The Broadnosed pipefish was found in higher densities where the seascape was less complex (I; Fig 8) and with lower amounts of bare sediment (IV). As this species is highly associated with seagrass (Scapin et al. 2018a), fragmentation and reduction of this habitat, as what has been observed particularly on the Swedish west coast (Baden et al. 2003; Eriander et al. 2017), may have significant impacts for the survival of this species in this region. Effects of loss of habitat due to various reasons including physical disturbance, eutrophication, and climate change make this a cause for concern regarding these Syngnathid species that are reliant on seagrass habitat (Vincent et al. 2011).

Paper IV not only uses a seascape approach to understand how the surrounding habitat structure influences fish found in seagrass meadows, but it offers quantitative information on the fish assemblage that can be found in these habitats at different times of the year in the Baltic Sea. Although not so surprising, the three-spined stickleback was extremely prevalent throughout (as it has been reported in large abundances in other habitats in the Baltic Sea; Bergström et al. 2015), but perhaps more surprising was the overall lack of larger piscivorous fish, possibly indicating a break down in food-web structure in the coastal northern Baltic Sea.

Other recent investigations, at a large seascape-scale (km) in the temperate Skagerrak, assessed how changes in seagrass fish assemblages were linked to open ocean, deep water, wave exposure and latitudinal position (Perry et al. 2018). One important result showed a higher abundance of Atlantic cod when seagrass meadows were closer to deep water (> 20 m) and towards more northern latitudes (Fig. 9). This encourages the question of addressing broader-

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scale approaches that may be necessary for understanding patterns of organisms which can offer further insight into marine ecosystem dynamics.

Figure 9 Illustration showing that larger seascape / geographic variables, such as the proximity to deep water and northern higher latitudes, can play a positive role in the abundance of Atlantic cod in seagrass meadows in western Sweden. Images: Courtesy of the Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/symbols/) Nursery habitat or nursery seascape? The notion of the nursery function for specific marine habitats has been well-discussed in the literature (e.g. Beck et al. 2001; Jackson et al. 2001a; Nagelkerken et al. 2001; Heck 2003; McDevitt-Irwin et al. 2016), particularly in seagrass meadows where these habitats can often harbour more juveniles of fish and invertebrate species than adjacent habitat types in the surrounding seascape (Bertelli and Unsworth 2014; Lilley and Unsworth 2014; Unsworth et al. 2018). A concept recently approached (Nagelkerken et al. 2015) is that of assessing parts of or the whole seascape (varying scales) as one when it comes to assessing the nursery function as mobile species, like fish, can be dependent on multiple habitats depending on life stage, foraging, shelter and other survival strategies (Litvin et al. 2018).

In paper II, results showed that while structurally complex habitats, such as seagrass meadows and seaweed belts, harboured a higher density of fish compared to unvegetated soft bottoms, juvenile composition was relatively similar and high (ca. 75%) across these habitats, thus indicating the notion of the seascape approach to assessing the nursery concept. Albeit, this survey was conducted in summer (June) and other times of the year may exhibit

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differences in juvenile composition depending on species’ reproduction and survival strategies. For example, fish density data from seagrass meadows during summer and autumn (I) revealed a distinct shift in not only species composition but also the life stage of the goldsinny (Ctenolabrus rupestris) and corkwing wrasse (Symphodus melops) (Fig. 10). Thus highlighting the temporal variation in species composition and the possible function of a nursery at certain times of the year. Additionally, in the surveyed eelgrass meadows in the Baltic Sea juvenile fish densities were 17 times higher in the autumn compared to the summer (IV), mainly attributed to the summer spawning of the common three-spined stickleback (Bergström et al. 2015). This highlights the significance of considering the functional needs of individual species and their habitat preference. For example, some groups (e.g. pipefish) are adapted to and strongly depend on certain habitats, while others, such as larger predatory fish (e.g. pollack, sea trout) utilise multiple habitats within the marine environment (Jackson et al. 2001a).

Figure 10 Density of two wrasse species found in eelgrass meadows on the Swedish west coast (I). There was a high abundance of adult goldsinny (left picture) in the summer and juvenile corkwing in the autumn (right photo). Fish illustration by Karl Jilg, published with permission from the Swedish Species Information Centre (ArtDatabanken), SLU. Photo: Paul Naylor

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Diel changes in the fish assemblage An important part of assessing any fish assemblage is not only to take into account seasonal differences, but investigating the fish structure throughout the daily cycle, as fish communities can differ depending on the time of day (Jackson et al. 2006b; Bertelli and Unsworth 2014; Shoji et al. 2017). Six seagrass meadows (same as surveyed in Paper I) were also surveyed at night during 2013 and 2014 to quantify the fish assemblage (unpublished data). Although more replicates are needed to gain a more comprehensive understanding of the structure of the fish community between day and night in this region, some trends were apparent. For instance, during the night, the European eel (Anguilla anguilla) and some species of the codfish family were more abundant. However, no new species were observed at night compared to those already present during the day.

In addition, from research connected to Paper III, while tracking juvenile cod using acoustic telemetry, distinct changes in spatiotemporal movements were detected in individuals exhibiting strong diel migrations (unpublished data; Fig. 11). These patterns highlight the importance of examining the whole diel cycle of fish species, which can lead to new information on behaviour, habitat requirements, as well as predator-prey relationships. In the example in Figure 11, this shows crepuscular movement of a juvenile cod from one local to another where physical structural differences were apparent (% seagrass habitat), thus potentially contributing to changes in ecological processes and survival strategies (e.g. foraging and shelter).

Figure 11 Detections from a juvenile Atlantic cod over 10 days in September 2015 showing changes in location based upon time of day. Each box represents a detection. Grey and white columns represent night and daytime, respectively

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Spatiotemporal movement patterns of marine predators Using biotelemetry methods, 48 Atlantic cod were tagged and tracked throughout parts of a temperate fjord system, which gave spatially explicit movement data over time (III). This, together with network analysis applications, investigated actual connectivity of this species. Foremost was the significant effect of sea surface temperature on the movement of individuals. Here we found as tracking progressed from summer through to winter there was a substantial decrease in connectivity across the seascape at varying spatial scales (100s m to km) (III). In general, more wide-ranging movement pathways, such as linking the two study areas, disappeared after summer and even more local (smaller distance) movements were reduced during the colder winter months (Fig. 4 in Paper III). Even though some fish left the study area (receiver array) throughout the monitoring period, some fish were detected for the majority of the time (hence a high residency index). Thus emphasising how vital these shallow-water ecosystems, composed of habitats such as, seagrass, macroalgae and unvegetated soft bottoms, are for the sustainability of Atlantic cod populations which are a pivotal part of the coastal food web in this region.

Additionally, recent acoustic telemetry data from tagged sea trout (n=20) have given further insights regarding the spatial movement patterns of large marine predators in this region (Gullmar fjord). Preliminary results suggest that the majority of tagged individuals stayed around shallow-vegetated areas during summer and autumn exhibiting high site fidelity, however, most left the study area during the colder winter months (unpublished data). As they were seldom detected in or around local river systems, this suggests that when they left the relatively shallow study area, they may have moved to deeper waters in the fjord or further into the fjord or even offshore, perhaps seeking warmer waters. These results together with paper III strongly suggest that localised areas (i.e. receivers D & E, Fig. 1 in paper III) are very important for both sea trout and Atlantic cod during certain times of the year.

Identifying differences in and improving the understanding of spatial and temporal movement patterns of marine organisms, while incorporating fish-ecosystem linkages, is vital for continued conservation efforts in shallow-water seascapes (Olds et al. 2018b; Treml and Kool 2018).

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Conservation issues and future perspectives With an increasing human population and subsequent pressure on coastal regions the demand on natural resources is as never before. We need to address existing problems pragmatically while establishing and governing effective management decisions to achieve a sustainable future for the marine environment. There is a growing science base in marine connectivity research and understanding the importance at multiple spatial and temporal scales, however, this is often overlooked in the application of management (Magris et al. 2014; Olds et al. 2018a).

The general state of fish populations fished by humans is poor or overfished (Costello et al. 2016) with European waters far from exempt as many countries are fishing above scientific advice (Carpenter and Heisse 2019). Challenges of correct management practices even in relatively rich parts of the world, such as Europe, can harbour illegal, unreported and unregulated (IUU) fishing practices (Fig. 12). As a consequence, this is damaging managed wild fisheries, biodiversity and conservation efforts. Even in protected areas such as the Gullmar fjord, western Sweden, the permanent ban on Atlantic cod fishing maybe thwarted by illegal fishing activities (pers. comm. H. Svedäng), thus severely affecting local recovery of this species.

Future hope may be on the way as public awareness is increasing, particularly with the rapid increase in online media forums and if political will is on the side of long-term marine conservation (Fig. 12). Promising signs were the reformed EU common fisheries policy in 2014, which introduced, for example, a ban on discards and long-term management plans (European Commission 2011). However, signs of illegal dumping of fish by commercial fisheries are still occurring today in Swedish waters (WWF 2019). Nonetheless, stringent quotas and enforced management practices are needed to be set in place (Fig. 12), especially as the EU adhered to make all commercial fishing practices sustainable by 2020 (European Commission 2018).

With regard to fishing and marine protected areas (MPAs), recent evidence shows that within European waters trawling is startlingly higher in MPAs compared to non-protected areas (Dureuil et al. 2018). This raises the pertinent question of MPA effectiveness, but taking into account specific goals of individual MPAs (e.g. Sala et al. 2018). In Sweden (as with most EU member states) marine protection is less than its terrestrial counterparts and with generally poor management and governance (Grip and Blomqvist 2018). This is because, as suggested by Grip and Blomqvist (2018), that the establishment of marine protected areas in Sweden, particularly those with higher levels of protection, have been hindered due to strong pressure from the commercial fishery and other stakeholders, such as land owners.

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In addition to the Atlantic cod, the European eel which utilises temperate coastal environments (I and II), is also under threat (critically endangered - IUCN Red List; Jacoby and Gollock 2014). Though some conservation measures have been adopted (i.e. seasonal fishing bans) fishing and consumption still occurs in many countries, including Sweden. Unfortunately, species survival and fisheries management are severely undermined as, in Europe, it is estimated that illegal eel fishing activities extract twice that of the regulated fishery (Sandström et al. 2019).

Figure 12 Conceptual model showing various inputs and outputs (not exhaustive), which can contribute and affect the future of fish sustainability. Atlantic cod is shown as an example of a species subjected to a high fishing pressure, whilst ecologically important. Society: there needs to be sufficient motivation from the government and public (e.g. marine strategy framework directive). Fisheries: realistic quotas should be set by society with these and other practices enforced, as well as tackling illegal, unreported and unregulated fishing practices (e.g. Petrossian 2014). Conservation measures: effective conservation measures put in place and adhered to regarding safeguarding habitats and species (e.g. Bergström et al. 2018; Grip and Blomqvist 2018). Ecosystem functioning: a more stable and resilient marine ecosystem, through e.g. trophic interactions (Donadi et al. 2017), together with improved understanding and application of marine ecological connectivity (e.g. Weeks 2017). Atlantic cod illustration by Karl Jilg, Swedish Species Information Centre (ArtDatabanken), SLU. Other images from the Integration and Application Network, University of Maryland, Center for Environmental Science

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This thesis has delved into the world of seascape ecology, a relatively new branch within the marine sciences, specifically focusing on fish. In temperate seas, seascape ecology is very much in its infancy and this research covered here is just scratching the surface of exploring patterns and processes throughout the marine ecosystem.

Tracking marine species is becoming more prevalent around the world, which reveals vital information at multiple scales on animal behaviour and connectivity (Hussey et al. 2015). As in paper III, further telemetry studies, including new transmitters enabling detection of predation (Halfyard et al. 2017), on more mobile species (e.g. sharks, rays, codfish, eels) inhabiting the seas around northern Europe would uncover a wealth of knowledge. This could be implemented into conserving species under threat and aid management in MPAs (see Lea et al. 2016 as an example), which might be able to offer better spatial protection to vulnerable species.

Through working at a higher resolution with acoustic telemetry, such as a more systematic receiver array (e.g. Vemco positioning system) has the potential to explore fine scale connectivity of mobile organisms and link them to seascape features (Grober-Dunsmore et al. 2009; Hitt et al. 2011). In temperate coastal waters this would be of particular importance providing evidence-based science that can be applied to marine spatial planning and conservation.

As Atlantic cod spawning is likely occurring in the Gullmar fjord (Svedäng et al. 2018) investigating the spatial distribution of different life stages (i.e. eggs, larvae, 0-group, age >1) in this region would be advantageous for conservation and fisheries management. Methods such as sampling for eggs, acoustic tracking and genetic analysis, could be used to determine local population structure and ecological connectivity. Specifically it would give a better understanding on local cod populations and how stationary or transient they are throughout the region, and perhaps to where management needs should be focused to ensure (and reach) sustainable populations for the future.

Furthermore, focusing at the policy level, the need to assess the true value of current MPAs in relation to their goals would be pivotal to achieve long-term national and international sustainability goals. This is especially relevant in light of the extremely high fishing pressures still carried out in European MPAs (Dureuil et al. 2018).

Finally, with improving technologies, geospatial data availability and processing and advancement in the exploration of seascape ecology tools and approaches, this knowledge can lead us to explore and ask questions that are at the forefront of global marine conservation.

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Acknowledgements I can’t believe I’ve finally got here – and the journey was far from lonely Firstly, thank you to The Fishes and The Sea. Martin, you gave me my dream PhD project (not that I knew it from the beginning!). Thank you for always believing in me, most importantly at times when I needed it the most. Thank you for understanding the importance of balancing work and personal life, and giving me the freedom to explore my own path. Your enthusiasm and motivation for the subject inspires me to strive forward and make me think anything is possible. Regina & Mats- many thanks for your support, feedback and being there when I needed you.

Thank you to Sofia, Monika, Kristoffer and Lina for feedback on various parts of the thesis.

Diana P, thanks for all the support, laughs and the early starts in the field ;-) Couldn’t have wished for a better PhD buddy!

Lina, well what is there to say? Your friendship and support has carried me through these years. Awesome office mate! Thanks for introducing me to the seagrass community.

Felix, my go to fish and tech guy. Thanks for all the help and fun on land and sea – not to mention documenting it with every type of camera available! So happy you knocked on my office door all those years ago J

Peter B, we have bounced back and forth being office mates, but most importantly become good friends. Thanks for the many fikas, talks and support. Marcus, great times over beers and the ‘feedback’! Oskar (el presidente) & Juanita, great sharing laughs and the office with you. There are so many others at the department to thank, so please accept this as a big thank you to you all.

The Seagrass Ecology and Physiology Research Group - all past and present members, nice to work with you and to get to know you all. To the many field assistants that have helped over the years – couldn’t have done it without you. Many thanks. Thank you to the administrative and technical staff at DEEP, particularly Leila, Erica and Kristina.

I have had the privilege of co-supervising and getting to know many young scientists. Nellie and Patrick- Master students extraordinaire! The halcyon ‘student days’ with Pontus, Charlie, Felix and Lina.

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All the staff at Kristineberg research station (University of Gothenburg). You always give a warm welcome! Could not have wished for a better place to be whilst doing fieldwork.

Ingvar and Mikael from Länsstyrelsen, Göteborg. Introducing me to the wonderful world of fish tracking – tusen tack! David J, my contact in London. Thank you for your support and helping me connect the dots. To my mentor Daniel thanks very much for the discussions and support. Thanks to the staff and fellow divers at Kungsholmens fire brigade. I never dreamt I could become a commercial diver - that was probably the toughest thing I have ever done!

All my family and friends (wherever you were in the world) for showing your encouragement and unconditional support over these years - it means so much. Thank you / tack så mycket!

My darling wife – you have supported me throughout this journey. Who knew that we would have two beautiful girls during these years. Aurelia and Isabella, you have changed my world more than you know. Thank you and love you.

Thank you nature for showing and sharing with me the wonders of this world.

As the sun rises over the fjord I ponder and look forward to what the future has in store.

Financial support This research was funded by The Swedish Research Council Formas (Dnr 2011-1640) (M. Gullström) and the Department of Ecology, Environment and Plant Sciences (DEEP), Stockholm University. Additional scholarships for equipment were received from Albert & Maria Bergströms stiftelse, Wilhelm & Martina Lundgrens vetenskapsfond and the W Pokora-Kulinskas donation fund. Board and lodgings during fieldwork funded through The Royal Swedish Academy of Sciences. Travel and participation in workshops and conferences was made possible by DEEP, the Knut & Alice Wallenberg Foundation, Liljevalchs stipendium, the Marine Biological Association UK and the World Seagrass Association (T. Staveley).

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