Aquaculture 370–371 (2012) 19–25

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The absorption efficiency of the suspension-feeding sea cucumber, Cucumariafrondosa, and its potential as an extractive integrated multi-trophic aquaculture(IMTA) species

E.J. Nelson a, B.A. MacDonald a,⁎, S.M.C. Robinson b

a Biology Department, University of New Brunswick, P.O. Box 5050, Saint John, NB, Canada E2L 4L5b Department of Fisheries and Oceans, St. Andrews Biological Station, 531 Brandy Cove Rd., St. Andrew's, NB, Canada E5B 2L9

⁎ Corresponding author. Tel.: +1 506 648 5620; fax:E-mail address: bmacdon@unb.ca (B.A. MacDonald)

0044-8486/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.aquaculture.2012.09.029

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 April 2012Received in revised form 22 September 2012Accepted 25 September 2012Available online 3 October 2012

Keywords:Absorption efficiencyIntegrated multi-trophic aquacultureSea cucumbersCucumaria frondosa

Finfish aquaculture commonly releases waste material in the form of excess feed and faeces, which can im-pact the surrounding environment, often through increased oxygen demand in the benthos as a result of abuildup of organic matter. Integrated multi-trophic aquaculture (IMTA) in the Bay of Fundy co-cultures ex-tractive species such as mussels (Mytilus edulis) and kelps (Saccharina latissima) alongside of the fed finfishto partially mitigate the impacts associated with excess inorganic and organic nutrients. The orange-footedsea cucumber (Cucumaria frondosa) is being examined as a potential extractive species to remove additionalparticulate organic waste in some of the larger particle size categories. Sea cucumbers were exposed to nat-ural (IMTA sites and natural seston) particles and enhanced laboratory diets where the organic content (OC)of the food and faeces were determined to estimate absorption efficiency (AE). AE ranged between 68 and85% for all the experimental trials but averaged 70±3% when evaluating their response to only the naturaldiets. Sea cucumbers were capable of consuming aquaculture waste material when exposed to it in the lab-oratory and when deployed at an IMTA site, feeding directly upon the particulates released. There was astrong positive relationship (R2=0.82) between food and faeces OC, making it possible to predict the faecalOC from the food supply OC. AE was not as readily predictable from the food supply OC although there was asignificant positive relationship between food OC and AE. Sea cucumbers are efficient in absorbing organicmaterial (70±3%) within the range (>30 and b50% OC) they are typically exposed to in their natural envi-ronment. When challenged with particulate material of higher organic content (>60% OC), such as culturedmicroalgae or salmon food and faeces they exhibit equal or enhanced (>80%) AE's. Our results show thatC. frondosa has a great deal of potential to become an effective organic extractive IMTA species and aid inthe reduction of organic loading occurring at aquaculture sites.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

In marine aquaculture, excess food and faeces can lead to organicloading on site beneath the cages (Buschmann et al., 2008; Troell etal., 2003). This can greatly impact the chemical and biological oxygendemand of the substrate leading to a decline of some oxygen-sensitivespecies and the increase in the abundance of opportunistic species(Buschmann et al., 2008; Edgar et al., 2010; Troell et al., 2003). Inte-grated multi-trophic aquaculture (IMTA) is one technique that hasthe potential to help reduce some of the environmental impactsand has been steadily gaining momentum in Canada (Chopin et al.,2001; Ridler et al., 2007) and internationally (e.g. Troell et al., 2003).IMTA involves the culture of traditional finfish (e.g. salmon), butuses the waste products (excess feed and faeces) produced by the

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finfish as a food source for other commercially viable extractive spe-cies (MacDonald et al., 2011; Reid et al., 2010; Troell et al., 2003).Both inorganic extractive species (e.g. seaweeds) and organic extrac-tive species (e.g. mussels) are grown in close proximity to the site tohelp absorb excess nutrients produced, while creating an additionalcash crop for farmers (Chopin et al., 2001; Troell et al., 2003). Thismultispecies approach appears to beworking, in that it has successful-ly created an additional cash crop, although there still appears to besome controversy on the demonstration of the mitigation of aquacul-ture wastes (Navarrete-Mier et al., 2010; Reid et al., 2010). Thesechallenges may be due to the fact that some of the feeding niches forthe larger organic particles are currently not being filled. As a result,there is a growing interest in increasing the variety of species usedfor extraction, with the commercially important orange-footed seacucumber (Cucumaria frondosa) being considered as an additionalorganic extractive species in open-water IMTA systems.

Some species of sea cucumbers have been observed consuming andreducing aquaculture wastes, such as the California sea cucumber,

20 E.J. Nelson et al. / Aquaculture 370–371 (2012) 19–25

Parastichopus californicus, which has been found to reduce both oysterwaste (Paltzat et al., 2008) and salmon pen net fouling (Ahlgren,1998). Australostichopus (Stichopus) mollis can effectively consumemussel aquaculture waste, leading to an increase in biomass and afaster growth rate for A. mollis (Slater and Carton, 2007). However,only deposit-feeding sea cucumbers, which feed by ingesting sedi-ment (Lopez and Levinton, 1987), have been used in aquaculture.Suspension-feeding sea cucumbers, such as C. frondosa, which feedon particles suspended within the water column, have not yet beenassessed for their potential value in aquaculture systems. C. frondosais already a commercially important species with an establishedmarket as a fishery commodity referred to as “beche-de-mer” or“trepang” which is sold to Asia and some European countries (Keet al., 1987).

C. frondosa is a benthic dendrochirotic echinoderm that primarily in-habits the sub-tidal zone (Singh et al., 1998). It is one of themost abun-dant and wide spread species of holothurians within the North AtlanticOcean and the Barents Sea (Russia) (Gudimova et al., 2004). Its geo-graphic range includes the coasts of New England, eastern Canada,southern Iceland, Greenland, northern Europe and Scandinavia to theFaroe Islands (Hyman, 1955; Jordan, 1972; Klugh, 1923; Singh et al.,1998). It is the largest sea cucumber along the eastern coast of NorthAmerica (Hyman, 1955; Jordan, 1972; Ke et al., 1987) and is known tocover vast areas of the rocky substrate at depths of less than 30 m(Jordan, 1972; Singh et al., 1998), but can be found at depths up to300 m (Hamel and Mercier, 1999).

C. frondosa is a passive suspension feeder that utilises its ten tenta-cles to capture particles within thewater column and is known to feedmainly during the spring and summer when food appears to be moreavailable (Hamel and Mercier, 1998; Singh et al., 1999). C. frondosa isalso known to be capable of adjusting its food consumption rate,feeding faster when food concentrations are higher (Singh et al.,1998). Additional studies that investigated seasonal and tidal feedingactivity of C. frondosa, confirmed that feeding increased as the quality(chloropigment concentration) of the suspended particulate materialincreased (Singh et al., 1999). Tentacle insertion rate (TIR) has beendetermined to be a good indicator of feeding activity and ingestion,and flow rates have been shown to have a significant effect on thefeeding activity of C. frondosa within the natural environment (Holtzand MacDonald, 2009; Singh et al., 1999). Beyond this however, verylittle is known. Absorption efficiency describes the efficiency atwhich an animal absorbs organic material as it is transferred throughthe gut (Calow and Fletcher, 1972; Conover, 1966; Penry, 1998;Wang and Fisher, 1999). It was originally termed assimilation efficien-cy (Conover, 1966) but was later amended to absorption efficiencyas it measures the proportion of organic material absorbed fromthe food within the gut, rather than the incorporation of the materialinto the animal's tissue (Calow and Fletcher, 1972; Penry, 1998; Wangand Fisher, 1999). While there is a little information on the feedingactivities of C. frondosa, there is nothing known about its absorption ef-ficiency and how it is likely to vary under different environmentalconditions. Absorption efficiency is an important variable to deter-mine within the IMTA infrastructure in order to quantify the overallreduction in quality (organic content) of the suspended particulatewaste material as it is converted and re-deposited by extractivespecies. With this information it will be possible to estimate thepotential for reduction in organic loading by sea cucumbers occur-ring at IMTA sites compared to that of traditional aquacultureoperations.

The purpose of this study is to determine the potential of the seacucumber, C. frondosa, as an effective IMTA organic extractive species.Specifically the objectives are to: (1) quantify the absorption efficien-cy of C. frondosa; (2) verify that C. frondosa is capable of consumingaquaculture waste; and (3) to determine if the absorption efficiencyof C. frondosa can be predicted based on the quality (organic content)of the material available to feed upon.

2. Methods

2.1. Species collection

All sampling and species collection took place within the Bay ofFundy, South-west New Brunswick, Canada from June 16th –July4th, 2010 and June 16th –September 4th, 2011. Adult sea cucumbers,C. frondosa, of a similar size were collected by divers (depth of12–18 m) at Tongue Shoal (45°03′47″ N, 67°00′47″ W) and held for1–2 weeks in wire mesh cages suspended off a raft (depth of8–10 m) at the St. Andrews Biological Station in New Brunswick.

2.2. Laboratory trials

Sea cucumbers were housed at the Huntsman Marine ScienceCentre (St. Andrews, New Brunswick) in an Aquatic Habitats Bench-top (AHAB) system (Aquatic Habitats™ Inc., Florida, USA; Model No.B25C-1; Fig. 1). The AHAB system was equipped with ten 10 L tanks(one sea cucumber per tank) with a screen covering each outflow tokeep faeces within the individual tank. The AHAB system was run asa recirculation system using ambient seawater (8–11 °C). Flow rateswere sufficient to ensure food did not settle out in the experimentalchambers, but remained in suspension. Partial water changes wereperformed following any water sampling activity to replace the vol-ume of water removed from the system. Sea cucumbers (n=10)were transferred to the laboratory and placed in the AHAB systemfor a period of 24–48 h, prior to experimentation, to acclimate andto void any faeces produced when feeding in the natural environ-ment. Following acclimation, different groups of sea cucumbers(n=10 per group) were exposed to one of the three diet treatments:(1) a commercial algae diet (Instant Algae®, Reed Mariculture,Campbell, CA, USA) of either T-iso (Isochrysis galbana) or ShellfishDiet® which is made up of a mix of 30% Isochrysis, 20% Pavlova, 20%Tetraselmis, and 30% Thalassiosire weissflogii; (2) a modified algaediet consisting of a mixture of diatomaceous earth (Agrogreen®Canada, Ontario) and T-iso used to create a diet of reduced organiccontent compared to regular algae; or (3) a mixture of particlesfound at local salmon farms. The mixture of farm particles consistedof ground salmon feed (Skretting® Optiline Microbalance WinterExtruded salmon feed) and dried and ground faeces collected fromsalmon (Salmo salar) held in a tank and feeding upon this diet. Theconcentrated mixture of feed and faeces was filtered through a100 μm sieve and added directly to the AHAB system to distributefood evenly to all individual sea cucumbers. All diet mixtures wereadded to seawater that was filtered to less than 5 μm, to achieve afinal concentration of 20–30×103 cells ml−1. Particle concentrationswere verified and monitored throughout the experiments using aCoulter Counter® Multisizer II.

The organic content (OC) of the algal diets (T-iso and Shellfish Diet)was measured by filtering water samples from the AHAB system, how-ever the weight of the algal diets was too low (b1 mg L−1) at the dietconcentrations to reliably estimate food OC (Table 1). The food OC ofthe algal diets was instead measured by pipetting a small volume(b2 mL) of the concentrated commercial algae solution onto apre-weighed filter to determine weight after drying and ashing. Watersamples (2 L) for the modified algae diet (n=9) and mixture of farmparticles (n=5) were taken 20 min after the respective diet mixtureswere added to the reservoir of the AHAB system to facilitate uniformdistribution of the particles. The amount of diatomaceous earth addedto the modified algae diet varied over a 3 day period (June 27–29,2011) to obtain a range of 40–50% OC, so water samples were takendaily (n=3) and averaged for the exposure period. All diet sampleswere processed at the St. Andrews Biological Station immediatelyafter collection to determine food OC (Table 1).

Any faeces produced by individuals in the 48–72 h following theaddition of either of the 3 diets were collected with a plastic bulb

Fig. 1. Schematic (front and side view) of the Aquatic Habitats Benchtop System (AHAB) used within the laboratory. Letters correspond to the various parts: rack (A), water supply(B), vertical drain (C), sump with preconditioned bioballs for biological filtration (D), spillway (E), tanks (F), lower manifold (G), upper manifold (H), pump (I), filters (J) and UVlamp (K).

21E.J. Nelson et al. / Aquaculture 370–371 (2012) 19–25

pipette and frozen separately for future analyses of OC. Faeces werecollected as soon as possible to prevent the loss of minerals throughleaching; however faeces egested overnight could have remainedfor up to 12 h. Tanks were siphoned clean immediately followingsampling.

2.3. Field trials

Two types of field trials were performed in order to determine ab-sorption efficiency (AE) for sea cucumbers that had been feeding onnatural material: one where sea cucumbers from the natural popula-tion, already exposed to natural suspended material, were collectedand the other where animals were placed at specific field locations.For the first field trial, resident sea cucumbers (n=10) were collectedfrom Tongue Shoal (45°03′47″ N, 67°00′47″ W) along with watersamples (n=2, 2 L each) containing the natural particles and trans-ferred to the laboratory at the Huntsman Marine Science Centre forfaeces collection. For the second field trial, sea cucumbers were heldin wire mesh cages and exposed to different assemblages of naturalparticles for a period of 48–72 h in 3 locations: 1) the wharf at theSt. Andrews Biological station (45°04′56″ N, 67°05′05″ W) to be ex-posed to a different set of conditions in the local water column on

Table 1Experimental conditions (mean±S.E.) for laboratory and field treatments.

Treatment Date Particle type T

Tongue Shoal June 16–18, 2011 NaturalWharf 1 June 16–18, 2010 NaturalWharf 2 June 25–27, 2010 NaturalWharf 3 July 2–4, 2010 NaturalIMTA 1 July 26–31, 2011IMTA 2 August 31–Sept 4, 2011 Natural+aquacultureIMTA 3 August 2–7, 2011Shellfish Diet June 21–23, 2010 3 cultured algal species 0T-iso June 29–July 2, 2010 Monoculture algae 0Modified Algae June 27–July 1, 2011 T-iso+diatomaceous earthFarm Particles June 20–24, 2011 Salmon feed+faeces

a TPM is the weight of the total particulate material deposited on the filter.b POM is the weight of the particulate organic material determined by loss on ignition.c Weights are from algae sampled with background seawater.

three different dates; 2) Fairhaven Aquaculture Site (44°57′ 901″N,067°00′ 810″W) (IMTA 1 and 2); and 3) Charlie Cove AquacultureSite (45°01′ 807″N, 066°51′ 985″W) (IMTA 3). Both aquaculturesites were integrated multi-trophic aquaculture (IMTA) sites stockedwith Atlantic salmon (Salmo salar), blue mussels (Mytilus edulis) andkelp (Saccharina lattisima), which allowed the sea cucumbers to feedupon a combination of natural seawater and particulate aquaculturewaste material while they were suspended from the salmon cages(depth=3 m). Water samples (2 L) were collected at the wharflocation (n=3) on two of the three experimental dates. For theremaining experimental date, an average of the two collections wasused as the OC of the suspended particulate material did not differsignificantly between dates (t=1.55, p=0.20, df=4). Water samples(n=5, 2 L each) from the IMTA sites were taken at deployment andretrieval of the cages using a Niskin Sampler next to the salmon pen(depth=3 m). All water samples were stored on ice in coolers andtransported to the St. Andrews Biological Station for analysis of OC(Table 1). The OC of the suspended particulate material at IMTAsites can fluctuate widely with the presence or absence of a wasteplume, the state of the salmon on the site (age, weight, feedingactivity, etc.), the feeding technique used (manual versus automaticfeeders), temperature, algal blooms, etc. Therefore the food OC

PMa (mg L−1) POMb (mg L−1) Mean food % OC Sample size (n)

2.18 (0.08) 0.74 (0.13) 33.93 (4.57) 22.52 (0.35) 0.90 (0.10) 36.96 (1.60) 62.36 (0.77) 0.89 (0.21) 39.95 (2.18) 32.68 (0.04) 0.91 (0.03) 33.98 (0.75) 3

3.21 (0.12) 1.43 (0.05) 46.63 (1.30) 126

.55c (0.05) 0.48c (0.04) 67.07 (0.39) 8

.29c (0.02) 0.29c (0.02) 70.21 (0.41) 61.12 (0.17) 0.48 (0.05) 45.76 (2.44) 93.25 (0.20) 2.71 (0.19) 83.49 (2.42) 5

Fig. 2. The mean (±S.E.) organic content (percentage) of food and faeces and their cor-responding mean (±S.E.) absorption efficiencies (percentage) for sea cucumbers(Cucumaria frondosa) fed natural and laboratory diets. Lower case a, b, and c indicatea significant difference between groups in laboratory trials. Upper case A, B, and C in-dicate a significant difference between groups in field trials. Diets with an asterisk(*) represent an average of 3 trials for faeces organic content and absorption efficiency.

22 E.J. Nelson et al. / Aquaculture 370–371 (2012) 19–25

value for IMTA sites (47±1% (S.E.)) was calculated by averaging theOC of the suspended particulate material (n=126) from previousstudies of IMTA sites throughout Passamoquoddy Bay, Bay of Fundyfrom 2009 to 2010 (Arsenault unpublished data) (Table 1). Sea cu-cumbers (n=10) from each field trial were transported in coolersfrom the natural environment to the Huntsman Marine ScienceCentre where they were placed in the AHAB system to collect faecesfrom individuals. Any faeces produced over a period of 72 h were col-lected with a plastic bulb pipette and frozen for analysis of OC. Faeceswere collected utilizing the same methodology as in the laboratorystudies.

2.4. Determining organic content

The foodOC of each diet (Table 1)was determined at the St. AndrewsBiological Station and OC of faeces was determined in the laboratory atthe University of New Brunswick. Our earlier trials (Nelson unpublisheddata) have shown that to obtain a reliable estimation of absorptionefficiency, a minimum of 5 mg of sample material is needed, thereforefaeces collected over more than one time interval were pooled for eachindividualwhennecessary. All samples (water or pooled faeces samples)were suction filtered onto pre-weighed, pre-ashed glass fiber filters(Whatman® GF/C) and rinsed with a 3% solution of ammonium formateto remove any salt residue. Filters were dried at 80 °C for at least 12 h,cooled in a desiccator for 15 min and weighed. The filters were thenplaced in aluminum weigh pans and ashed in a muffle furnace at450 °C for 3 h, cooled in a desiccator for 15 min and re-weighed. Totalorganic material was determined as the loss of weight on ignition(Strickland and Parsons, 1972).

2.5. Absorption efficiency

Absorption efficiency (AE) was determined by comparing the OCof the food supply to the reduced OC of the faeces using the Conoverequation (Conover, 1966)

AE ¼ F−Eð Þ= 1−Eð ÞF½ �⋅100

where F is the OC fraction of the food and E is the OC fraction of theexcreta (faeces). Estimation of AE using the Conover equation doesnot require the collection of all food and faeces, just a sample ofeach. This equation assumes that inorganic material is not digestedand therefore can be used as an inert tracer (Conover, 1966). TheAE values for laboratory and field trials were calculated usingmean OC fraction of the food and individual OC fraction of the faeces.Individual AE values were then averaged for each diet.

2.6. Statistical analyses

All data was tested for homogeneity of variances and normalityusing Levene's and Anderson–Darling tests respectively. In the caseswhere the criteria were not met, then the data was transformed(log, arcsine). The following data however, could not achieve homogene-ity even after transformation: foodOC (lab andfield),field faeces OC, andfield AE values. Differences inmeans between food OC, faeces OC and AEwere tested using a one-way ANOVA and a Tukey comparison to deter-mine significant differences between diets (for this test the wharf values(n=3)were averaged across dates, as were the IMTA site (n=3) valuesfor simplicity). Where variances were homogeneous but non-normal, aKruskal–Wallis test (H statistic) was performed to test differencesamong diets, followed by a Mann–Whitney test for paired comparisonswhen necessary. Randomisations (n=10,000) were done using themean food OC values of the natural diets to address the unbalanced de-sign (sample sizes). Diet data was randomly arranged and the resultingmeans were compared using an ANOVA, and differences between dietswere examined using t-tests, in order to determine if the original values

were achievable by chance or if there was a significant difference be-tween diets. The coefficient of determination (R2) and an ANOVA wereused to test the relationship between food OC and AE and between OCof food and faeces. An alpha value of 0.05 was used throughout thestudy. All statistical analyseswere performed usingMinitab® 16 Statisti-cal Software.

3. Results

There was a significant difference in organic content (OC) of theexperimental diets in the laboratory (F=113.4, df=3, pb0.001;Fig. 2A). Food OC values for the experimental diets ranged from ap-proximately 50 to 80% in the microalgae and enhanced diets, withthe farm particles being significantly higher, the modified algae dietbeing lower and the microalgae diets being equal (Fig. 2A). OC rangedbetween approximately 35 and 45% for the natural diets of the fieldtrials and there was no significant difference between diets (F=24.5, df=2, p=0.055), although the difference between the naturalparticles collected at the wharf and the IMTA diets (t-test) was onlyslightly non-significant (p=0.057) (Fig. 2A). The OC for faeces weregreatly reduced compared to the food (~20–50%) and followed a sim-ilar pattern as the food OC for the diets in the laboratory trials, exceptthe Shellfish Diet which was not significantly different from the mod-ified algae (F=16.8, df=3, pb0.001; Fig. 2B). There were howeversignificant differences observed in the OC of the faeces produced

Fig. 4. The relationship between the mean (±S.E.) absorption efficiency and the corre-sponding organic content of the food for sea cucumbers (Cucumaria frondosa) exposedto natural and enhanced diets. The solid line represents a linear regression through allpoints (R2=0.38, p=0.045).

23E.J. Nelson et al. / Aquaculture 370–371 (2012) 19–25

from the field trials with IMTA>Wharf>Tongue Shoal (F=21.1, df=2, pb0.001; Fig. 2B). Significant differences existed among the meanAE values of the laboratory trials (H=10.2, df=3, p=0.02). Themean AE's for the microalgae and farm particles diets ranged around79–85% with the exception of the sea cucumbers exposed to the mod-ified algae diet where significantly lower values of 69% were recorded(Fig. 2C). There was no significant difference between AE valuesamong the field trials which ranged in values from 69 to 76% (F=2.61, df=2, p=0.08; Fig. 2C).

The OC of the faeces produced by sea cucumbers significantly in-creased with the OC of the diet to which they were exposed, i.e. higherquality food produced higher quality faeces (R2=0.82, pb0.001; Fig. 3).The farm particle mixture produced faeces much richer than expected,but it was still reduced from the original food OC (Fig. 3). There wasmuch greater variability observed for the relationships between AEand OC of the diet (Fig. 4). As food OC is used in the calculation of AE,these terms (AE and food OC) are not independent. A correlation be-tween AE and food OC would be more appropriate, but the regressionequation is provided for comparison within the literature. There was asignificant relationship between OC of the food supply and AE (R2=0.38, p=0.045), with AE being 73±3% on average, however the foodOC only explained 38% of the variation in AE (Fig. 4). When AE valueswere split between natural (AE 67–76%) and enhanced diets (AE 70–85%) the relationship between AE and OC was very different. Whenonly natural diets were considered, the relationship between AE andfood OC was non-significant (R2=0.01, p=0.84) and AE averaged70±3% (Fig. 5). When we consider the enhanced diets, includingthe modified algae diet, the relationship between AE and food OC waspositive, but non-significant (R2=0.48, p=0.31), however AE washigher, averaging 79±3% (Fig. 6). Food OC values of the enhanceddiets (approximately 45–85%) were typically higher than particleswithin the natural environment.

4. Discussion

C. frondosa has been found to have high potential as an IMTA ex-tractive species absorbing approximately 70% of organic materialwhen feeding in the natural environment, with the potential to in-crease this efficiency when exposed to higher quality material. Seacucumbers within this study have been found to be capable of captur-ing and eating excess salmon feed and faeces and as such C. frondosahas the potential to reduce the organic loading at aquaculture sites.

This is the first time that absorption efficiency (AE) has beendetermined in a suspension-feeding sea cucumber. The AE's ofC. frondosa (69–85%) were similar to or higher than those recorded formost sea cucumber species. Their AE's were higher than P. californicus(Ahlgren, 1998), Stichopus japonicas (Zhou et al., 2006), and various spe-cies of deep-sea holothurians (Hammond, 1983; Khripounoff and Sibuet,1980). The range of AE's displayed by Stichopus mollis (Maxwell et al.,

Fig. 3. The relationship between the organic content (mean±S.E.) of food and faecesfrom sea cucumbers (Cucumaria frondosa) fed diets of varying organic content. Thesolid line represents a linear regression through the means (R2=0.82, pb0.001).

2009) and A. mollis (Zamora and Jeffs, 2011) were similar to those ofC. frondosa. The food organic content values (OC; approximately 30–50%) that the sea cucumbers were exposed to for the natural dietswere reflective of the range of natural particles within the Bay of Fundy(Lander, 2006). This suggests that C. frondosa meets one of the primarycriterion to be a good organic extractive IMTA species.

Suspension-feeding sea cucumbers may have higher AE's thandeposit-feeding species because they are processing a suspendedfood supply that may be higher in quality than the composition ofbenthic sediments. It is quite possible that the sediment OC may besignificantly reduced through the action of benthic microorganisms.In comparison, mussels (M. edulis), currently being used as an IMTAorganic extractive species, have a wide range (43–90%) of extractionefficiencies related to the range of food qualities it experiences(Bayne et al., 1987, 1988; Reid et al., 2010). For instance, when feed-ing in the natural environment M. edulis have an average AE of ap-proximately 70% when consuming particulate material between 45and 90% OC (Cranford and Hill, 1999). Thus, C. frondosa has a compa-rable mean AE to one of the current IMTA extractive speciesM. edulis.

The sea cucumbers in this study were found to be quite efficient atabsorbing particulate organic material with the AE ranging from 68 to85%. Some of the factors that can impact AE include: feeding physiol-ogy such the selective feeding observed in deposit-feeding sea cu-cumbers (Billett et al., 1987; Ginger et al., 2001; Hammond, 1983;Khripounoff and Sibuet, 1980; Paltzat et al., 2008; Zamora and Jeffs,2011); the structure and function of the gut (Farmanfarmaian,1969; Kozloff, 1990; Robison and Bailey, 1982); the addition of

Fig. 5. The relationship between the mean (±S.E.) absorption efficiency and the corre-sponding organic content of the food for sea cucumbers (Cucumaria frondosa) exposedto natural diets. The solid line represents a linear regression through all points (R2=0.01, p=0.84).

Fig. 6. The relationship between the mean (±S.E.) absorption efficiency and the corre-sponding organic content of the food for sea cucumbers (Cucumaria frondosa) exposedto enhanced diets. The solid line represents a linear regression through all points (R2=0.48, p=0.31).

24 E.J. Nelson et al. / Aquaculture 370–371 (2012) 19–25

mucous to the faeces (Filimonova and Tokin, 1980; Hollertz, 2002;Könnecker and Keegan, 1973; Massin, 1980); and gut passage time(Bayne et al., 1987, 1988). C. frondosa is known to have a long convo-luted gut, which could aid in nutrient absorption, a long gut passagetime (Nelson 2011 unpublished data) and high individual variation(Holtz and MacDonald, 2009), but in this study a fairly narrowrange of AE's were observed indicated that these impacts did notgreatly affect C. frondosa.

The weak predictability of AE from the food OC may be due to thefact that the sea cucumbers are removing a similar proportion of or-ganics regardless of the quality of the diet, and this is reflected inthe strong relationship between the OC of the food and faeces. Thissame relationship is exhibited in mussels (M. edulis) with faeces OCincreasing with food OC; however AE is more dependent on foodquality in mussels than in sea cucumbers (Bayne et al., 1987; Reidet al., 2010). In contrast, scallops (Placopecten magellanicus) have aninverse relationship, producing faeces of similar OC regardless offood quality and decreasing clearance rates (an indicator of feeding)with increasing food quality (Bacon et al., 1998). This inverse rela-tionship between AE and ingestion rate is common in suspensionfeeders (Bayne et al., 1988), thus it may be useful to examine inges-tion rate in addition to food OC as a possible predictor of AE forthese sea cucumbers. Tentacle insertion rate is a useful indicator of in-gestion in C. frondosa and is known to increase with quality of thesuspended particulate material (Holtz and MacDonald, 2009; Singhet al., 1998, 1999). It is possible that exposure to higher qualitydiets like those associated with salmon farms could increase ingestionand lower AE by facilitating a reduced gut passage time. In order toassess the full potential of this species to extract organic particulatematter at salmon farms, particularly any plumes of high quality mate-rial, future studies could be designed to establish the relationshipsbetween ingestion rate (via tentacle insertion rate), gut passagetime and AE.

Unlike many species that seem to behave and respond equally wellin the laboratory environment as they do in the field C. frondosa doesnot appear to be oneof them.Wenoticed that their behavior, particular-ly opening and extending their tentacles, did not seem to be as pro-nounced as it is in the field environment. It is unknown whether thischange in behaviorwould have any impact on their physiological condi-tion and absorption efficiency. Previous studies have found that thisspecies of sea cucumber does not acclimate well to the laboratory andbehaves abnormally with extended periods of tentacle withdraw andreduced feeding activity (Holtz and MacDonald, 2009; Sutterlin andWaddy, 1976). Reduced feeding activity in the laboratory may explainthe long gut passage time observed in our preliminary laboratorystudies (at least 24 h and possibly longer, Nelson unpublished data).Deposit-feeding sea cucumbers have been reported to have gut passage

times of 9–13 h (mean 12) for Stichopus tremulus and 19–28 h (mean23) for Mesothuria intestinalis, a deep-sea holothurian (Hudson et al.,2005). In contrast, the filter-feeding mussel, M. edulis has a minimumgut passage time of 2 h when fed algal diets (Bricelj et al., 1984), and3 h when feeding on natural particulate material (Hawkins et al.,1990). Future studies to assess gut passage time of C. frondosa in thefield could be very beneficial to our understanding of how this speciesfunctions in its natural environment. While behavior within the labora-tory may be different from the natural environment, the purpose of thelaboratory studies was to show that C. frondosa can absorb higherquality material efficiently and that they can capture and eat the excessfeed and faeces produced at aquaculture sites.

C. frondosa are well suited as an organic extractive IMTA species asthey are capable of consuming aquaculture waste both within the lab-oratory environment and when feeding directly on IMTA sites. Theenriched diets within the laboratory have shown that C. frondosa arecapable of a higher AE if exposed to high quality material at IMTAsites. When they are exposed to lower quality diets they still havethe capability of absorbing approximately 70% of the available organicmaterial. Assuming that sufficient biomass can be obtained by thefarmer, C. frondosa appears to have the potential to effectively reducesome of the organic loading to the natural environment occurring atsalmon aquaculture sites. Feeding and ingestion rates of C. frondosahowever, should be examined to further determine its potential asan IMTA organic extractive species. Further proof that sea cucumbersare capable of consuming and absorbing organic aquaculture wastecould be tested by quantifying assimilation efficiencies using radio-tracer techniques (such as 14C or 51Cr) (Calow and Fletcher, 1972;Cammen, 1977; Wightman, 1975). Future studies should examinethe growth rates of C. frondosa within an IMTA system to determine ifgrowth rates will be higher when exposed to aquaculture wastes, as iscommon for some deposit-feeding sea cucumbers (Slater and Carton,2007). It would also be useful to compare particle retention efficienciesof sea cucumbers to M. edulis to determine if C. frondosa is capable ofconsuming particulate waste material that mussels cannot.

Acknowledgements

We thank T. Lander, C. Smith, J. Arsenault, A. Hamer, D. Scott,L. Orr, P. Fitzgerald, B. Knight and S. Leadbeater for their technical sup-port in the field and in the laboratory. We would like to thank the crewof the Viola Davidson: P. Smith, D. Loveless, and W. Johnston. We re-ceived substantial support from the St. Andrews Biological Station,Huntsman Marine Science Centre, University of New BrunswickSaint John, as well as Cooke Aquaculture Inc. and the site managersRoy Branford and JodyHanley. Drs G. Reid, H. Hunt and C. Gray providedvaluable input into the project and comments on an earlier manuscript.We greatly appreciate the support this work received from the NaturalSciences and Engineering Research Council of Canada (NSERC) strategicCanadian Integrated Multi-Trophic Aquaculture Network (CIMTAN)in collaboration with its partners, Fisheries and Oceans Canada, theUniversity of NewBrunswick, Cooke Aquaculture Inc., Kyuquot SEAfoodsLtd. and Marine Harvest Canada Ltd.

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