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Estuarine, Coastal and Shelf Science 84 (2009) 91–98
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Estuarine, Coastal and Shelf Science
journal homepage: www.elsevier .com/locate/ecss
Resource partitioning by two large planktivorous fishes Micromesistius australisand Macruronus magellanicus in the Southwest Atlantic
Paul Brickle*, Alexander I. Arkhipkin, Vladimir Laptikhovsky, Andrew Stocks, Alison TaylorFalkland Islands Government Fisheries Department, P.O. Box 598, Stanley FIQQ 1ZZ, Falkland Islands
a r t i c l e i n f o
Article history:Received 28 July 2008Accepted 3 June 2009Available online 17 June 2009
Keywords:Southwest AtlanticMicromesistius australisMacruronus magellanicusresource partitioningtrophic relationsdiet
* Corresponding author.E-mail address: [email protected] (P. Brickl
0272-7714/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.ecss.2009.06.007
a b s t r a c t
The ontogenetic, seasonal, bathymetric and regional variations in the feeding spectrum of 922 specimensof southern blue whiting Micromesistius australis and 512 specimens of hoki Macruronus magellanicuswere studied on the Falkland Islands’ shelf (Southwest Atlantic) between November 1999 and April 2003.A total of 49 different prey items were found in the stomach contents of the two species, with thehyperiid Themisto gaudichaudii and Euphausiacea being amongst the most important prey. Although thespecies composition did not change over fish size, the proportions of individual prey items in their dietsdid, with an increase in T. gaudichaudii and Euphausiacea with increasing fish size in southern bluewhiting. The opposite occurred in hoki. Seasonal variations in the diet were found to mirror the seasonalabundance of prey around the Falkland Islands for the two species. Intra-specific differences in the diet ofboth predators reflected the distribution of prey, which in turn was determined by the water structure inthe two regions sampled, leading to very different diets. In the limited time that the two species occupiedthe same space there was little or no competition resulting in almost total segregation of their trophicniches in space and time.
� 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Southern blue whiting Micromesistius australis Norman, 1938and hoki Macruronus magellanicus Lonnberg, 1907 are two abun-dant planktivorous fishes inhabiting temperate shelf and slopewaters of the southern tip of South America. They migrate andutilize resources of the shelf around the Falklands Islands atdifferent times of the year. Southern blue whiting move from theirvast feeding grounds in South Atlantic and sub-Antarctic waters tothe south-western part of the Falkland Islands’ shelf to spawn inSeptember–October (Shubnikov et al., 1969). After spawning, thefish spread around the southwest Atlantic to forage and theirabundance around the Falklands Islands gradually declines towardsaustral summer (December–January).
Hoki has an alternative strategy within the shelf ecosystemaround the Falklands Islands. The fish spawns outside FalklandIslands’ waters. After spawning hoki migrate to the western part ofthe Falkland Islands’ shelf to forage in October–November, whichroughly coincides with the southern blue whiting migration fromthe area. In summer, hoki mainly occupy the warmer northern shelfwhereas southern blue whiting prefer cooler southern and eastern
e).
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parts of the Falkland Islands’ shelf. Some adult hoki and southernblue whiting remain in the Falkland Islands all year round (FalklandIslands Government unpublished data). Immature and sub-adulthoki occur on the Falkland Islands’ shelf all year round and havea slightly different migratory pattern to adult fish. They enter thesouthern part of the shelf (south of 51�30 S) in April, which coin-cides with a significant drop in the abundance of southern bluewhiting; and remain there until September, when the southernblue whiting return to spawn. The species are separated by depthwith the main commercial catch of hoki occurring between 100 and200 m, and the main commercial catch of southern blue whitingoccurring between 250 and 400 m (Falkland Islands Government,unpublished data). Although the greatest abundance of the speciesis segregated, there is some temporal and spatial overlap in theaustral summer in the southern and north-eastern parts of theFalkland Islands’ shelf.
It has been shown that southern blue whiting is a pelagiczooplankton feeder (Hart, 1946; Inada and Nakamura, 1975). In theScotia Sea, southern blue whiting mainly feed upon Antarctic krill(Euphausia superba) (Shubnikov et al., 1969). In the northern part ofits range, on the Patagonian shelf, the fish has a broader dietpreying primarily upon upon euphausiids and hyperiid amphipods(Lopez and Bellisio, 1973). Perrotta (1982) reported seasonalchanges in the diet of southern blue whiting on the PatagonianShelf, with euphausiids being the most important prey item
P. Brickle et al. / Estuarine, Coastal and Shelf Science 84 (2009) 91–9892
throughout the year and pelagic amphipods becoming moreimportant in summer. The above study analysed prey items bymajor taxa only and did not examine the variability in feedingspectra at a species level. Hoki diet is less well studied, no detailedanalysis has been carried out in the Falkland Islands’ waters. Janusz(1986) mentioned the diet and briefly described the feedingbehaviour stating that hoki consumed very little after spawning.The main prey items found in their stomach included euphausiids,amphipods, squid and juvenile Micromesistius australis. Bezzi(1984) found that hoki diet in the southwest Atlantic was highlyvariable depending on season and location. She found that theirfeeding spectrum included Chaetognatha, Ctenophora, Amphi-poda, Euphausiacea, Cephalopoda and different species of fish,particularly the Myctophidae. Bezzi (1984) also found that inspring hoki feed on juvenile southern blue whiting in thesouthern parts of the Falkland Islands’ shelf. Studies carried out onNew Zealand hoki (Macruronus novaezelandiae) suggested that it isa pelagic feeder consuming natant decopods, myctophids andamphipods (Clark, 1985; Bulman and Blaber, 1986).
In the present study, we examined ontogenetic, seasonal, depthand geographical variations in the feeding spectra of southern bluewhiting and hoki, and compared their diets in order to elucidatetheir place in the food web and identify possible competition forplanktonic resources within the ecosystems of the PatagonianShelf.
2. Materials and methods
The stomachs of 922 southern blue whiting and 512 hoki withcontents were collected onboard commercial vessels and theFalkland Islands Government Fisheries Department’s (FIFD)research and patrol vessel Dorada between November 1999 andApril 2003 on the Falkland Islands’ shelf (Fig. 1). The vesselemployed a panel bottom trawl (vertical opening 4.5 m, horizontalopening 30 m) equipped with a small mesh liner (40 mm stretchedmesh).
64°W 63°W 62°W 61°W 60°W 59°W 58°W 57°W 56°W 55°WLongitude
54°S
53°S
52°S
51°S
50°S
49°S
48°S
Latitu
de
Patagonian Shelf
200m
500m1000m
Burdwood Bank
Fig. 1. The distribution of Micromesistius australis and Macruronus magellanicussamples collected around the Falkland Islands. The diagonal dashed line dividesFalkland Islands waters into two ecological regions with the north-western areasdominated by temperate waters of the Argentine drift current. While the south-eastern area is dominated by the cold sub-Antarctic waters originating from theFalkland Current. (þ, southern blue whiting samples; B, hoki samples).
Total length (LT) (southern blue whiting) and pre-anal length(LPA) (hoki) were recorded to the nearest centimetre below, andtotal weight to the nearest gram. All stomachs with contents wereplaced in individual plastic bags and frozen onboard. Beforeexamination, the stomachs were thawed and their contents wereindividually analysed in Petri dishes under a dissecting SZX12Olympus microscope. Prey items were identified to the lowestpossible taxonomic level using keys and reference material (Clarke,1986; Gon and Heemstra, 1990; Boltovskoy, 1999; and the FIFDreference collection). Otoliths were identified with the aid of theFIFD reference collection and lower beaks were used to identifycephalopod species (Clarke, 1986). Prey items that were completelyundigested were considered to be ‘net feeding’ and were not usedin subsequent analyses. All prey items were separated, counted andweighed to the nearest 0.1 g (wet weight). If the degree of digestionprevented the ability to count individual prey items (mainly in caseof chaetognaths, euphausiids and Themisto gaudichaudii) then onlytheir weight was recorded.
After preliminary data analysis, the following LT intervals werechosen for studies of ontogenetic variations in the feeding spec-trum of southern blue whiting: less than 40 cm LT; between 40 and55 cm LT; and more than 55 cm LT. These sizes correspond to thejuvenile period, the maturing period and the adult period of thisfish respectively (Trella, 1998; Wohler et al., 2000). The followingsize ranges were chosen for hoki, less than 23 cm; between 23 and35 cm; and more than 35 cm LPA. Seasonal variations were exam-ined only for the most abundant middle size classes for the periodsranging from February to June (n ¼ 154), July to October (n ¼ 54)and November to January (n ¼ 235). Variations in the feedingspectra with depth were examined using the middle size classesand the depths were partitioned as <200 m; >¼200 m and<300 m; and >¼300 m.
Spatial differences in the diets were investigated only betweenNovember and January for southern blue whiting and February–June for hoki using the middle size classes, with samples being splitby a diagonal running from 64�W; 54�S to 58�W; 49�300S. Thisapproximately divides the Falkland Islands’ waters into twodifferent ecological regions. The north-western area is dominatedby warmer temperate waters of the Argentinean Shelf, whereas thesouth-eastern area is dominated by colder sub-Antarctic waters ofthe Falkland Current (Arkhipkin et al., 2004). The difference in thesampling period for the species was used because it providedsufficient sample sizes for spatial analysis.
Changes in diet were examined by calculating the percentnumber (%N), percent mass (%M) and frequency of occurrence (%F)of prey items. The index of relative importance IRI was calculatedfor the three length groups, seasons, depths and geographicallocations as:
IRI ¼ ð%N þ %MÞ � ð%FÞ
(Cortes, 1997). Levin’s measure of niche breadth (B) was calculatedaccording to Krebs (1989) as:
B ¼�X
p2j
��1
where pj is a fraction of items in the diet that are of the foodcategory j (S pj ¼ 1).
The differences in the diets of southern blue whiting and hokibetween regions were tested with a linear discriminant functionanalysis (LDA) using the weight of prey items in individualsbetween different groups. Previous studies have used the numberof prey items within individuals when using LDA (Arkhipkin et al.,2003; Nyegaard et al., 2004; Laptikhovsky, 2004) to examine diet,but due to the fact that occasionally advanced states of digestion
0
1
2
3
4
5
6
7
8
9
10
Jan
Feb
Mar Apr
May Jun
Jul
Aug
Sep
Oct
Nov
Dec
Mean
d
aily catch
, M
T (1999-2006)
HokiBluewhiting
Fig. 2. The mean daily catches from commercial vessels of Micromesistius australis andMacruronus magellanicus within Falkland Islands’ waters between 1999 and 2006.
P. Brickle et al. / Estuarine, Coastal and Shelf Science 84 (2009) 91–98 93
made it difficult to count individual prey items, the log transformed(log(n þ 1)) weight of prey items per animals was used instead. Thedata were log transformed as they had an over dispersed frequencydistribution within their predators as indicated by a high varianceto mean ratio. Further analyses examined individual prey itemsbetween adjacent samples using the c2 test based on the weightoccurrence.
Feeding strategy plots were used to gauge whether southernblue whiting and hoki had specific or generalist feeding strategies.The graphic method proposed by Amundsen et al. (1996) was used,which incorporated the prey-specific abundance into Costello’s.(1990) analysis. This parameter is defined ‘as the percentage a preytaxon comprises of all prey items in only those predators in whichthe actual prey occurs, or in mathematical terms:
Pi ¼�X
Si=X
Sti
�� 100
where Pi is the prey-specific abundance of prey i, Si the stomach(volume, weight or number) comprised of prey i, and Sti the totalstomach content in only those predators with prey i in theirstomachs.’ This value is then plotted against frequency of occur-rence on a two dimensional graph.
To compare the diets of southern blue whiting and hoki, 50individuals of each species were chosen randomly from animalsweighing between 600 and 1500 g (¼46–64 cm LT and 23–34 cmLPA for southern blue whiting and hoki respectively), that weresampled between November and January at depths between 150and 400 m. November to January were chosen as these monthshave the greatest temporal overlap of both species on the FalklandIslands Shelf. The samples were collected at the same time of theyear but in different years, with hoki collected in 1999/2000 andsouthern blue whiting collected in 2001/2002. Initially a clusteranalysis was used to see if the data contained any structure throughclassification using an average linkage and a Euclidean measure ofsimilarity and using the most commonly shared prey items utilised,namely Euphausiacea, undetermined Amphipoda, Munida sp.,Themisto gaudichaudii and the myctophid Gymnoscopelus nicholsi.As the cluster analysis indicated that individuals of the samespecies tended to cluster together, the groups were then consideredto be known and a LDA was performed on the same dataset. As withall the LDAs, cross validation was not used as it is said to havelimited value because it is based on the same data to which the LDAis fitted. Therefore there is a tendency to overestimate thediscriminant power relative to a different set of data (Venables andRipley, 1998).
As above the weight of specific prey items per individual werelog transformed prior to both the cluster and the LDA.
0%
20%
40%
60%
80%
100%
100-149 150-199 200-249 250-299 300-349 350-399Depth, m
% o
f to
ta
l c
atc
h
Bluewhiting Hoki
Entire Falkland Shelf
Fig. 3. The relative proportion of southern blue whiting to hoki in catches ofcommercial vessels operating on the Falkland Islands’ shelf between 1999 and 2006.
3. Results
Southern blue whiting and hoki inhabit the waters of the Falk-land Islands shelf all year round but their aggregations are quitewell separated in space and time. The maximum abundance (asindicated by catch with consistent yearly effort and finfish vesselsusing similar fishing gear) of southern blue whiting occurs inSeptember and the beginning of October. In contrast, hoki adultsenter Falkland Islands waters during late spring and summer ata time when southern blue whiting begins to leave to forage else-where (Fig. 2). Even though they overlap in time they are also quitewell separated in their bathymetric distributions (Fig. 3).
A total of 28 and 21 different prey items were found in thestomach contents of southern blue whiting and hoki respectivelywith the pelagic amphipod Themisto gaudichaudii and theEuphausiacea being among the most important prey (Table 1).
3.1. Ontogenetic variations in the diet
3.1.1. Micromesistius australisIndividual prey varied in importance ontogenetically. Smaller
fish had a deeper distribution than their adults in our samples.The most important prey items in the diet of Micromesistius
australis, at all stages, were Themisto gaudichaudii and Euphausia-cea. These prey items increased (%M) with increasing fish size withsignificant differences between each size group (c[2]
2 ¼ 72.84,P < 0.0001; c[2]
2 ¼ 41.60, P < 0.0001 for the euphausiids andT. gaudichaudii respectively). Myctophids were more important insmaller fish and their %M decreased with increasing fish size. Thiswas particularly highlighted by Gymnoscopelus nicholsi(c[2]
2 ¼ 79.32, P < 0.0001) but also evident in Electrona subaspera(c[2]
2 ¼ 14.44, P < 0.0007) and unidentified Myctophidae (c[2]2 ¼
209.4, P < 0.0001). Chaetognaths were of secondary importanceand increased to the middle size group and then decreased(c[2]
2 ¼ 11.12, P < 0.0038) (Table 1).Their niche breadth decreased with increasing fish size sug-
gesting that larger fish were more specialized in their feeding.
3.1.2. Macruronus magellanicusIn contrast to southern blue whiting the importance of Euphau-
siacea and Themisto gaudichaudii decreased with increasing fishlength (c[2]
2 ¼ 381.7, P < 0.0001; c[2]2 ¼ 381, P < 0.0001 respectively).
However, the galatheid Munida sp. increased in the diet withincreasing fish length (c[2]
2 ¼ 261.8, P < 0.0001). Gymnoscopelus
Table 1Variations in the diet of Micromesistius australis and Macruronus magellanicus in relation to allocated size classes of samples collected around the Falkland Islands.
Micromesistius australis Macruronus magellanicus
Total length/pre-anallength (cm)
<40 cm >¼40 and <55 cm >¼55 cm <23 cm >¼23 and <35 cm >¼35 cm
Number of stomachanalysed
73 443 406 45 349 118
Prey items/indices %F %N %M IRI %F %N %M IRI %F %N %M IRI %F %N %M IRI %F %N %M IRI %F %N %M IRI
CrustaceaEuphausia lucens 2.74 12.19 3.05 42 3.39 5.30 3.42 30 2.71 2.49 0.00 7Euphausiacea (indet) 26.03 32.72 21.66 1415 53.05 64.98 59.17 6586 63.30 63.85 57.40 7675 40.00 64.00 17.77 3271 23.21 36.76 9.26 1068 7.63 15.00 0.84 121Thysanoessa gregaria 0.00 0.00 0.00 0 0.68 0.07 0.02 0 0.74 0.02 0.01 0Nematoscelis megalops 0.00 0.00 0.00 0 0.23 0.00 0.00 0 0.00 0.00 0.00 0Themisto gaudichaudii 31.51 47.83 16.63 2031 44.70 22.55 26.83 2207 54.43 30.28 33.23 3457 37.78 32.52 15.69 1821 38.11 36.97 6.48 1656 19.49 9.24 0.48 189Phronima spp. 0.00 0.00 0.00 0 0.90 0.05 0.07 0 2.46 0.03 0.10 0Amphipoda (indet) 5.48 1.59 0.15 10 4.06 0.37 0.30 3 2.96 0.12 0.09 1Munida spp. 0.00 0.00 0.00 0 0.90 0.05 0.02 0 0.25 0.00 0.01 0 0.00 0.00 0.00 0 7.16 19.33 16.46 256 22.03 53.37 28.88 1812Mysidacae (indet) 0.00 0.00 0.00 0 0.90 0.05 0.02 0 0.49 0.01 0.00 0CephalopodaLoligo gahi 0.00 0.00 0.00 0 0.45 0.01 0.05 0 1.72 0.02 1.95 3 0.00 0.00 0.00 0 4.30 0.23 4.01 18 4.24 0.34 0.73 5Gonatus antarcticus 0.00 0.00 0.00 0 1.13 0.08 0.20 0 0.74 0.01 0.01 0Semirossia patagonica 0.00 0.00 0.00 0 0.23 0.00 0.02 0 0.74 0.01 0.33 0 0.00 0.00 0.00 0 0.57 0.05 0.29 0 4.24 0.29 0.42 3Cephalopoda (indet) 0.00 0.00 0.00 0 1.35 0.02 0.08 0 2.71 0.03 0.07 0 0.00 0.00 0.00 0 0.00 0.00 0.00 0 0.00 0.00 0.49 0Moroteuthis ingens 2.22 0.09 3.51 8 7.16 0.33 3.85 30 13.56 1.56 10.81 168Invertebrata (other)Porifera 0.00 0.00 0.00 0 0.00 0.00 0.00 0 0.25 0.00 0.01 0Pteropoda 5.48 0.83 0.15 5 7.90 0.74 0.38 9 7.64 0.24 0.06 2Asteroidea 0.00 0.00 0.00 0 0.45 0.01 0.00 0 0.00 0.00 0.03 0Chaetognatha 10.96 2.67 0.61 36 36.34 5.44 2.30 281 25.37 2.70 1.24 100Salpa thompsoni 0.00 0.00 0.00 0 0.57 0.02 0.01 0 1.69 0.24 0.08 1Ctenophore 0.00 0.00 0.00 0 0.29 0.02 0.01 0 0.00 0.00 0.00 0AlgaeMacrocystis 0.00 0.00 0.00 0 0.00 0.00 0.00 0 0.85 0.05 0.03 0Algae (indet) 0.00 0.00 0.00 0 0.57 0.04 0.38 0 1.69 0.10 2.00 4PiscesMyxine spp. 8.22 0.50 7.70 67 0.23 0.00 0.15 0 0.00 0.00 0.00 0Electrona subaspera 4.11 0.42 7.93 34 0.45 0.01 0.33 0 0.00 0.00 1.72 0Gymnoscopelus bolini 0.00 0.00 0.00 0 0.23 0.00 0.18 0 0.00 0.00 0.00 0Gymnoscopelus nicholsi 13.70 0.83 36.08 506 3.84 0.10 5.39 21 0.74 0.02 1.08 1 17.78 2.00 40.62 758 30.95 3.79 32.28 1116 27.97 12.51 26.30 1085Notoscopelus resplendens 0.00 0.00 0.00 0 0.23 0.00 0.13 0 0.25 0.00 0.00 0Protomyctophum spp. 0.00 0.00 0.00 0 0.23 0.00 0.15 0 0.00 0.00 0.00 0Myctophidae (indet) 8.22 0.50 7.70 67 0.68 0.01 0.41 0 0.49 0.01 0.04 0 20.00 1.30 22.37 473 10.60 1.82 12.79 155 8.47 2.98 4.27 61Micromesistius australis 0.00 0.00 0.00 0 0.00 0.00 0.00 0 0.25 0.00 0.62 0 0.00 0.00 0.00 0 0.57 0.05 3.60 2 5.08 0.34 3.58Patagonotothen ramsayi 1.37 0.08 0.92 1 0.00 0.00 0.00 0 0.74 0.01 0.89 1 0.00 0.00 0.00 0 3.72 0.18 3.15 12 21.19 2.79 12.72 328Zoarcidae (indet) 1.37 0.08 0.00 0 0.00 0.00 0.00 0 0.00 0.00 0.00 0Teleostei (indet) 4.11 0.25 5.11 22 6.55 0.16 0.40 4 9.11 0.16 1.13 12 2.22 0.09 0.05 0 0.57 0.04 0.40 0 2.54 0.20 0.68 2larval Teleostei (indet) 0.00 0.00 0.00 0 0.86 0.11 0.02 0 0.85 0.05 0.00 0Sprattus fuegensis 0.00 0.00 0.00 0 2.58 0.27 4.38 12 3.39 0.83 3.00 13Cottoperca gobbio 0.00 0.00 0.00 0 0.00 0.00 0.00 0 0.85 0.05 0.18 0Merluccius australis 0.00 0.00 0.00 0 0.29 0.01 2.65 1 0.00 0.00 0.00 0Macruronus magellanicus 0.00 0.00 0.00 0 0.00 0.00 0.00 0 0.85 0.05 4.53 4
Total number ofprey items
1198 29,551 45,156 1150 8567 2046
Total weight of prey items 131 2608 3834 211 4442 4534Niche width 2.84 2.09 2.00 1.94 3.22 3.00
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P. Brickle et al. / Estuarine, Coastal and Shelf Science 84 (2009) 91–98 95
nicholsi was reasonably important in the diet throughout the sizeclasses and decreased slightly with increasing fish size with respectto %M (c[2]
2 ¼ 51.84, P < 0.0001). The notothenid fish Patagonotothenramsayi was of secondary importance and increased in the diet withincreasing fish size (c[2]
2 ¼ 304.1, P < 0.0001) (Table 1).The niche breadth was lowest for fish <23 cm (1.94) and then
increased to 3.22 and 3.00 for fish >¼23, <35 and >¼35 cm LPA
respectively suggesting that smaller fish had a narrow feedingspectrum which increased with increasing fish size.
3.2. Seasonal variations in the diet
3.2.1. Micromesistius australisThe two most important prey items, Euphausiacea and Themisto
gaudichaudii, had higher %M values in February–June andNovember–January but lower values between July and October(c[2]
2 ¼ 157.0, P < 0.0001; c[2]2 ¼ 54.57, P < 0.0001, respectively).
Seasonally the niche breadth was highest in February–June(2.63) and lowest on July–October (1.38), but somewhat interme-diate during November–January.
3.2.2. Macruronus magellanicusAs with southern blue whiting the most important prey items,
Euphausiacea and Themisto gaudichaudii, show a similar pattern inthe diet with higher %M in February–June and November–Januarybut lower values between July and October (c[2]
2 ¼ 138.6,P < 0.0001; c[2]
2 ¼ 27.93, P < 0.0001, respectively). Munida sp. alsoshowed a similar pattern to the latter (c[2]
2 ¼ 590.3, P < 0.0001)(Table 2).
Seasonally niche breadth was highest in November–January(3.26) but was almost similar for both February–June (2.71) andJuly–October (2.66).
3.3. Variations in diet with depth
3.3.1. Micromesistius australisSpecifically, Euphausiacea and Themisto gaudichaudii decreased
in importance and %M with increasing depth (c[2]2 ¼ 24.66,
P < 0.0001; c[2]2 ¼ 30.02, P < 0.0001, respectively). The myctophid
Gymnoscopelus nicholsi had increased in terms of IRI and %M withincreasing depth (c[2]
2 ¼ 241.0, P < 0.0001).The 200–300 m depth range had the highest niche breadth
(B ¼ 2.39) suggesting that southern blue whiting consumed a largerdiversity of prey species at this depth.
Table 2Results of a linear discriminant function analysis for the separation of Micromesistiusaustralis (a) and Macruronus magellanicus (b) based on region.
Assigned region Actual region
Northwest Southeast
(a) Micromesistius australisNorthwest 33 5Southeast 19 178Total number 52 183Number correct 33 178Proportion 0.635 0.973
(b) Macruronus magellanicusNorthwest 64 82Southeast 9 65Total number 73 147Number correct 64 65Proportion 0.877 0.442
3.3.2. Macruronus magellanicusThe Euphausiacea showed no significant difference between
depth ranges in terms of %M (c[2]2 ¼ 0.968, P ¼ 0.616) but did show
a reduction in IRI. Themisto gaudichaudii and Munida sp. showeda decrease in %M with increasing depth (c[2]
2 ¼ 170.9, P < 0.0001;c[2]
2 ¼ 603.5, P < 0.0001, respectively). As expected the myctophidGymnoscopelus nicholsi increased in %M with increasing depth(c[2]
2 ¼ 861.1, P < 0.0001) and also increased in IRI.Niche breadth was highest in the intermediate depth range
indicating that hoki consumed a slightly higher diversity of preyitems.
3.4. Variations in between region
3.4.1. Micromesistius australisThe results of the LDA indicated limited overlap between the
two regions with an overall correct classification of 89.8%. Thesoutheast region showed the greatest correct classification at97.3% whereas the northwest region was less well refined at 63.5%(Table 2).
There were some notable changes in specific prey itemsbetween the two regions. There was a significantly lower %M ofEuphausiacea and Themisto gaudichaudii to the northwest of theecotone compared with that to the southeast (c[1]
2 ¼ 20.93,P < 0.0001; c[1]
2 ¼ 5.84, P < 0.05, respectively). Conversely, andalthough not overly important in the diet at this time of the year(November–January), the Chaetognatha had a slightly higher %M intheir diet in the southeast region (c[1]
2 ¼ 9.56, P < 0.01).Levin’s niche breadth was 1.5 times more in the north-western
region (B ¼ 2.42) than in the south-eastern region suggestinga larger diversity of prey items in the former region.
3.4.2. Macruronus magellanicusThe results of the LDA were somewhat different to that of
southern blue whiting. There was a lower overall correct classifi-cation (58.6%) and the north-western region exhibited a greaterrate of correct classification than the south-eastern region (Table 2).
The regional analyses for hoki were conducted betweenFebruary and June. In contrast to southern blue whiting both theEuphausiacea and Themisto gaudichaudii were significantly greaterin terms of %M in the north west as opposed to the southeast(c[1]
2 ¼ 14.68, P < 0.001; c[1]2 ¼ 49.79, P < 0.0001, respectively).
However, the myctophid Gymnoscopelus nicholsi was a significantlygreater component of the diet in the south east with regard to %M(c[1]
2 ¼ 28.13, P < 0.0001).The niche breadths for both regions were similar with the south
east being slightly larger indicating a greater diversity of prey itemsconsumed.
3.5. Comparison between M. australis and M. magellanicus
3.5.1. Feeding strategy plotsThe feeding strategy plots suggest that southern blue whiting is
a specialised euphausiid and Themisto gaudichaudii feeder (Fig. 4a)whereas hoki were more generalist feeders with a mixed feedingstrategy (Fig. 4b).
3.5.2. Cluster analysisThe results of the cluster analysis are illustrated as a dendogram
showing the classification of individual fish based on the log-transformed weight of their prey items. As illustrated in Fig. 5,clustering is obvious where individuals of the same species tend tocluster together indicating a similar diet between individuals of thesame species.
%FO
0 20 40 60 80 100
0 20 40 60 80 100
Prey-sp
ecific ab
un
dan
ce (%
)
0
20
40
60
80
100
Euphausiacea
Themisto gaudichaudii
Chaetognatha
Gymnoscopelus niclosi
Euphausia lucens
Electrona subaspera
Notoscopelus resplendens
Loligo gahi
Rare species
Phronimidae
Gonatus antarcticus
n = 358a
%FO
Prey-sp
ecific ab
un
dan
ce (%
)
0
20
40
60
80
100 n = 87
Euphausiacea
Gymnoscopelus nicholsi
Themisto gaudichaudii
Munida sp.
Moroteuthis ingens
Finfish (indet) Patagonotothen ramsayi
Loligo gahi
b
Fig. 4. Feeding strategy plots for southern blue whiting (a) and hoki (b).
0.0 0.4 0.8 1.2Similarity
Fig. 5. A dendogram illustrating the results of a cluster analysis using the log trans-formed weight of prey items within individual Micromesistius australis (grey star) and
P. Brickle et al. / Estuarine, Coastal and Shelf Science 84 (2009) 91–9896
3.5.3. Linear discriminant function analysisAs the cluster analysis indicated that species tended to group
together in two main clusters, the groups were then considered tobe known and a LDA was performed on the same dataset. Asillustrated in the table, 84 of the 99 individuals examined werecorrectly identified (84.8%). Hoki had the highest correct classifi-cation with only three individuals being misclassified as southernblue whiting. Southern blue whiting was a little less resolved with12 individuals being misclassified as Macruronus magellanicus(Table 3).
Macruronus magellanicus (black square).
Table 3Results of a linear discriminant function analysis for the separation of Micromesistiusaustralis and Macruronus magellanicus individuals based on their diet. Overallcorrect classification ¼ 84.8% (84/99).
Assigned species Actual species
Micromesistius australis Macruronus magellanicus
Micromesistius australis 37 3Macruronus magellanicus 12 47Total number 49 50Number correct 37 47Proportion 0.755 0.940
4. Discussion
The results of this study provide new data and confirm thefindings of Perrotta (1982) who revealed that among the threetypes of prey analysed (euphausiids, pelagic amphipods and fish)the euphausiids were the most important in the diet of southernblue whiting throughout the year. This study also provides newdata on the diet of Patagonian hoki (Macruronus magellanicus) aspreviously only the work of Bezzi (1984) and Janusz (1986) indi-cated that this was a pelagic feeder with the former study touchingon seasonal and geographical variations in their diet. For the twospecies our attempts to identify all of the prey items to the lowesttaxonomic level helped illustrate the considerable variability in
P. Brickle et al. / Estuarine, Coastal and Shelf Science 84 (2009) 91–98 97
their feeding spectra ontogenically, seasonally, bathymetricallyand spatially.
In southern blue whiting and hoki the differences in diet betweensize groups were not evident in the composition of prey itemsconsumed. However, there was some significant changes in %M ofindividual prey types between size groups with opposing patterns inthe diet of the predators with respect to euphausiids, Themistogaudichaudii and myctophids, particularly Gymnoscopelus nicholsi.Clark (1985) found a similar pattern in the New Zealand hokiwith decreasing pelagic amphipods (particularly T. gaudichaudii)and increasing natant decapods and fish with increasing predatorsize. The niche breadth also had a contrasting pattern indicatinga more specialised diet in southern blue whiting and a moregeneralised one in hoki with increasing size.
Around the Falkland Islands Themisto gaudichaudii is mostabundant in the austral autumn (Sabatini et al., 1999, 2001) andthis is also the case for another sub-Antarctic island of similarlatitude, Kerguelen, where there is a maximum biomass at thebeginning of austral autumn (Labat et al., 2005). This is also theperiod when the importance of T. gaudichaudii is the highest inthe diet of both hoki and southern blue whiting. Other pelagicfeeders also take advantage of this increased biomass over theautumn months and include the Argentine short finned squidIllex argentinus (Mouat et al., 2001) and the slender tuna Allo-thunnus fallai (PB, VL, AA, pers. obs.). The Euphausiacea remainedimportant in the diet of the predators throughout the year butwere most important in spring and summer/autumn. Perrotta(1982) found a similar pattern in southern blue whiting in thenorthern part of the Patagonian Shelf whereas Bezzi (1984) didnot find this pattern in hoki because she did not sample inautumn. Themisto gaudichaudii is an important component of themacrozooplankton community in the sub-Antarctic and in certainareas it is has a trophic role similar to that of Antarctic krillEuphausia superba (Bocher et al., 2001). The abundance of krillaround South Georgia shows some considerable inter annualvariation and many predators consume T. gaudichaudii in krillpoor years (Everson et al., 1999; Collins et al., 2008).
As expected for the predators, myctophids started to appear intheir diets in the intermediate and deeper depth ranges, which is incontrast to Euphausiacea and Themisto gaudichaudii. Althoughsmall, there was an increase in the importance Chaetognatha in thediet of southern blue whiting with increasing depth, which is likelyto be a reflection of their increased abundance in an offshoredirection (Sabatini and Gustavo, 2001).
Regional differences in the diet of the species correspond wellwith the water structure in the two regions studied. Themistogaudichaudii were more common in the diet in the temperatewaters of the north-western region. Euphausiacea were morecommon in the diet in the sub-Antarctic south-eastern region forsouthern blue whiting but less important in hoki in the northwestzone which is likely to be a reflection of the sampling period forboth species; November–January and February–June for southernblue whiting and hoki, respectively.
Feeding strategy plots indicated that southern blue whitingwere specialized feeders of euphausiids and the hyperiid Themistogaudichaudii but consumed other prey in smaller amounts whenavailable. Conversely, hoki is a generalist feeder taking advantageof different prey items. The feeding strategy plot for hoki alsodemonstrated that there was a high individual contribution toniche width with a number of prey points, particularly the fishand the squid Moroteuthis ingens, in the top left hand corner ofthe plot.
Although the two species were studied in subsequent years wemade the assumption, based on the oceanographic conditions(Arkhipkin et al., 2004), that the abundance of prey around the
Falkland Islands was similar for the 2 years studied thereforeallowing us to examine the potential for resource partitioning.
One of the costs of interspecific competition is that twospecies cannot occupy the same eco-niche without exertingstrong negative effects on each other. When resources arelimiting, species can coexist if they differ in their behaviour or aremorphologically different. They subsequently use differentresources, or use them in different regions or in differenttemporal periods (Brown and Wilson, 1956; Davies et al., 2007).Resource partitioning may occur by segregation within one of thethree main resources axes: food, space and time (Schoener, 1974;Ross, 1986). There are numerous examples where resources canbe used by ecologically similar species that are separated spatiallyin the marine environment, e.g. fish species feeding at differentdepths or even in different geographical locations. Time may alsoseparate access to resources where they become available atdifferent times of the day or in different seasons (Townsend et al.,2008). These resource axes have been taken advantage of in orderto avoid competition and have been reported in a number of fishspecies including atherinids (Menidia menidia and Menidia ber-yllina) (Bengston, 1984) where two species coexist because theystrongly overlap in both food and habitat utilization only whenfood is abundant. When studying zooplanktivorous fishes (M.menidia, Anchoa mitichilli, Ancho hepsetus and Membras martinica)in salt marsh creeks in North Carolina, Allen et al. (1995) notedthat co-occurrence and possible competition amongst dominantzooplanktivorous fish were minimised by seasonal adjustments inlateral and vertical distributions as well as dietary preference. Intropical seagrass fish communities Nagelkerken et al. (2006)found that herbivores partitioned their dietary resources byspecialising on seagrass leaves, seagrass epiphytes or macroalgaefrom seagrass beds. Fish in the next trophic level exhibitedchronological segregation in feeding habits between fish families,while species within families showed separation in food type andsource. Guedes and Araujo (2008) studied fives species of Pleu-ronectiforms in southern Brazil and found resource partitioningalong spatial and size dimensions allowed them to co-exist inSepetiba Bay.
This study has shown that although southern blue whiting andhoki aggregations are quite well separated in space and time, theyoverlap on the Falkland Islands shelf in late winter/early spring butare largely separated bathymetrically. However, they do overlap atthe peripheries of their respective bathymetric distributions. At thistime of the year prey abundance is low and therefore resources arelimiting. Here they avoid competition by utilizing, similar sized,common prey items differently. Southern blue whiting isa euphausiid and amphipod specialist while hoki is more ofa generalist and takes advantage of fish and squid in its diet. As faras we are aware there are no comparative studies on the trophicrelations of large planktivorous fish in temperate regions. Thereforethis study illustrates that, at least on the Falkland Islands shelf,these large plantivorous fish avoid competition along the threemajor resource axes; food, space and time.
Acknowledgements
We thank the scientific observers of the Falkland IslandsGovernment Fisheries Department for their help in collecting thismaterial. We thank John Barton, Director of Fisheries, for sup-porting this work and we are grateful to the Masters and crew ofthe RV Dorada and the commercial vessels involved in thisproject. Finally, we thank the editor and two anonymous refereesfor their valuable comments that helped to significantly improvethe manuscript.
P. Brickle et al. / Estuarine, Coastal and Shelf Science 84 (2009) 91–9898
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