Quantification of waste feed and fish faeces in sediments beneath yellowtail pens and possibility to reduce waste loading by co‐culturing with sea cucumbers: an isotopic study

To evaluate environmental impacts of yellowtail culture and to examine the efficiency of the sea cucumber Apostichopus japonicus in reduction of waste loading, surface sediments were sampled and A. japonicus were cultured at a yellowtail farm in Owase Bay, Japan. Waste feed‐ and faeces‐derived organic matter (WOM, FOM) in sedimentary organic matter (SOM) was estimated based on the δ¹³C and δ¹⁵N of the fish feed (?21.7‰, 9.2‰), yellowtail faeces (?20.6‰, 6.2‰) and SOM reference (?24.4‰, 4.4‰). Small WOM (0–22% in SOM) but substantial FOM (28–61%) loadings in the fish farm area and high sulphide concentrations in the sediments (1.0 mg g^?1) suggest that reduction in the fish stocks or mitigation of the faeces should be considered. A. japonicus juveniles were cultured in three cages deployed below a pen and growth was assessed after three different periods (62, 107, 181 days). A. japonicus grew well during the first 107 days (daily specific growth rate, 3.7%) and their survivorship was high (80–90%). Growth ceased after 107 days, probably due to fouling on the cages. The δ¹³C and δ¹⁵N of their hypothetical diet (–19.7‰, 5.5‰) were close to the FOM values, suggesting assimilation of FOM.     

Nutritional Contribution of Biofloc within the Diet of Growout and Broodstock of Litopenaeus vannamei,Determined by Stable Isotopes and Fatty Acids

The relative contributions of feed sources were determined through the isotopic signal (δ¹³C and δ¹⁵N) and fatty acid profile of feed items, shrimp muscle, and eggs of Litopenaeus vannamei reared in a biofloc system. In the growout phase, the isotope analysis showed the biofloc particle size class ≥250 μm contributed 55–100%; size ≥50 < 250 μm contributed 0–22%; and artificial feed contributed 0–45%. Principal component analysis applied to fatty acid profiles showed that biofloc ≥250 μm and artificial feed were the most important items in shrimp growout. For the egg production, isotope analysis suggested that the most important feed sources according to their relative contributions were polychaetes (0–100%), followed by artemia biomass (0–86%) and semi‐moist feed (0–66%), with lower contributions from squid, mussel, and the muscle of L. vannamei broodstock that had been cultured in biofloc. In terms of fatty acids, the most important items were artemia, polychaetes, and semi‐moist feeds. This work clarified the importance of feed sources for shrimp during culture in biofloc systems and during reproduction. Analysis of stable isotopes and fatty acids can be successfully used to trace the assimilation of nutrients during the nutrition of shrimp.     

Nutrient Assimilation by First‐Feeding African Catfish,Clarias gariepinus,Assessed Using Stable Isotope Analysis

Knowledge of ingredients assimilation and biomass contribution to recipient fish is important in feed formulation. The stable isotopes of ¹³C and ¹⁵N were used to investigate the assimilation and biomass contribution of bambaranut, Voandzeia subterranea, meal (BNM), corn, Zea mays, meal (CM) and fish meal (FM), in FM substituted diets of first feeding African catfish, Clarias gariepinus, during a 30‐d experiment. The catfish larvae were stocked at 40 fish/15 L three replicate glass aquaria. Larvae were fed with experimental diets varying FM, BNM, and CM. Proportions of FM : BNM : CM in the experimental diets were: feed 1 (F1) 60:20:20; feed 2 (F2) 40:40:20; feed 3 (F3) 20:60:20; and feed 4 (F4) 20:20:60. Feeds and larval stable isotopes of δ¹³C and δ¹⁵N were analyzed. Larval specific growth rates (SGRs) were high, enhancing incorporation of dietary δ¹³C and δ¹⁵N. The SGR was highest for catfish fed F1 and F2 but significantly lower for those on F3 and F4. Results of δ¹³C and δ¹⁵N analyses showed that the biomass contribution from FM was similar to BNM, which were better than CM at equal inclusion levels. The nitrogen contribution per ingredient to larval somatic crude protein was increasing with increases in ingredient inclusion and assimilation. Stable isotope analysis is useful for analyzing assimilation and utilization of nutrients.     

Ecological influences on the difference in δ15N and δ13C values between fish tissues: implications for studies of temporal diet variation

Temporal diet changes can be detected by comparing δ¹³C and δ¹⁵N values between tissues with different isotopic turnover times. However, other factors contribute to disparities in δ¹³C and δ¹⁵N signatures between tissues and could confound the interpretation of stable isotope data. We examined the effects of ecological factors on differences in muscle and liver δ¹³C and δ¹⁵N signatures of Arctic charr [Salvelinus alpinus, (L.)] to determine whether their effects were large enough to influence studies of diet change. In addition, we examined whether differences in lipid content accounted for a significant portion of the differences in δ¹³C values between tissues. Results indicated that life history, diet, reproductive status and gender had significant effects on the differences in δ¹³C values between tissues, while gender and diet significantly affected differences in δ¹⁵N values. Differences in % lipid content between tissues also explained approximately 74% of the observed variation in the difference in δ¹³C values between tissues. The differences in δ¹³C values observed between muscle and liver tissues may be partially explained by physiological differences in lipid storage and use between tissues associated with ecological factor effects. These results demonstrate that ecological factors other than a change in diet can influence the differences in δ¹³C and δ¹⁵N signatures between muscle and liver tissues. However, the impact of ecological factors on studies of diet change depends on how the magnitude of the factor effects compared with a biologically meaningful change in diet.     

Effect of feeding level on growth,body composition,fatty acid profile,and nutrient accumulation in shrimp (<Emphasis Type="Italic">Litopenaeus vannamei</Emphasis>)

A study was conducted in a recirculating aquaculture system (RAS) to assess the growth performance, body composition, fatty acid profile, and nutrient accumulation in different tissues of shrimp using two feeding levels (10 and 16 g/kg^0.8/day). The shrimp PL₂₅ was nursed in the nursing tank for 2 months and stocked in six grow out tanks (ten individuals in each 30 L tank). A commercial feed with 42.7% protein was used over a 16-h period per day. After 4 weeks, the final average weight when fed 16 g/kg^0.8/day was 9.7 g which was not significantly different from a final weight of 8.6 g achieved when feeding 10 g/kg^0.8/day. The protein and energy content increased in high feeding level whereas moisture, crude fat, and ash content decreased with feeding level (P?>?0.05). With the high feeding level, the fatty acid content increased in the eye, muscle, and whole body, with eye containing the highest concentration. The isotopic ratio of δ¹³C was enriched with feeding level in the muscle and whole shrimp whereas the ratio of δ¹³C was depleted in the viscera with feeding level. The fraction of δ¹⁵N was enriched with decreasing feeding level and highest value was found in the muscle than in the whole shrimp and viscera.     

A 13C/12C Tracer Study of the Utilization of Presented Feed by a Commercially Important Shrimp Penaeus vannamei in a Pond Growout System1

A study of added feed utilization by pond grown shrimp was done using seven feeds with different ¹3C/¹²C ratios, as expressed by δ¹³C. The pond shrimp did not track the δ¹³C value of the feed to the extent that tank grown shrimp did. For the seven pond experiments the δ¹³C data indicate that the added feed supplied between 23 and 47% of the growth carbon. This implies that between 53 and 77% of the growth is due to the grazing on pond biota. It is suggested that this experimental approach can be used to evaluate the relative contribution of presented dried feeds versus natural productivity in ponds as an aid to developing pond management strategies and optimum feeds.     

Stable carbon (δ13C) and nitrogen (δ15N) isotopes as natural indicators of live and dry food in Piaractus mesopotamicus (Holmberg, 1887) larval tissue

This study proposed the use of the stable isotope technique to track the type of food utilized by pacu Piaractus mesopotamicus larvae during their development, and to identify the moment when the larvae start using nutrients from the dry diet by retaining its carbon and nitrogen atoms in their body tissues. Five‐day‐old pacu larvae at the onset of exogenous feeding were fed Artemia nauplii or formulated diet exclusively; nauplii+formulated diet during the entire period; or were weaned from nauplii to a dry diet after 3, 6 or 12 days after the first feeding. δ¹³C and δ¹⁵N values for Artemia nauplii were ?15.1‰ and 4.7‰, respectively, and ?25.0‰ and 7.4‰ for the dry diet. The initial isotopic composition of the larval tissue was ?20.2‰ and 9.5‰ for δ¹³C and δ¹⁵N respectively. Later, at the end of a 42‐day feeding period, larvae fed Artemia nauplii alone reached values of ?12.7‰ and 7.0‰ for δ¹³C and δ¹⁵N respectively. Larvae that received the formulated diet alone showed values of ?22.7‰ for δ¹³C and 9.6‰ for δ¹⁵N. The stable isotope technique was precise, and the time at which the larvae utilized Artemia nauplii, and later dry diet as a food source could be clearly defined.     

Carbon and nitrogen dynamics in zero-water exchange shrimp culture as indicated by stable isotope tracers

The uptake and assimilation of nitrogen and carbon by shrimp were measured in 1200 L mesocosms using stable isotope enrichments. Labels were added via ¹⁵N‐, ¹³C‐glycine and amino acid mixtures in feeds or as ¹⁵NH₄⁺ to pond water. Label was incorporated into shrimp via algal growth indicating that up to 31% of nitrogen requirements were derived from pond ecosystem dynamics. This value is low in comparison with other shrimp aquaculture isotopic tracer studies but is probably due to differences in shrimp‐rearing conditions. Direct incorporation of the enriched feed label was low in shrimp muscle tissue (3.3% for ¹³C‐glycine, 5.9% for ¹⁵N‐glycine and 7.8% for ¹⁵N‐amino acid mixture). Mass balance calculations indicate the remaining shrimp biomass was derived from feed, but loss of label into solution during feeding led to underestimation based on tracers. Incorporation of isotopic labels into feed as large molecular weight proteinaceous or microencapsulated/fat‐coated compounds is recommended to prevent dissolution and loss.     

Contribution of bioflocs to the culture of <Emphasis Type="Italic">Litopenaeus vannamei</Emphasis> post-larvae determined using stable isotopes

The aim of this study was to use preselected quality indicators for Litopenaeus vannamei post-larvae and the stable isotopes technique with δ¹³C and δ¹⁵N to determine the influence of bioflocs in shrimp feeding during the nursery phase, between PL1 and PL30. A control treatment (CT) with water renewal was compared to a biofloc treatment (BT) that received organic carbon fertilizations. Different types of commercial feed (Stresspak and Flake-INVE?, PL40-GUABI?), microalgae (Chaetoceros muelleri) and Artemia sp. nauplii were used as food sources. The physical and chemical parameters of water and ammonia were monitored daily, and nitrite, nitrate and alkalinity were measured weekly. Suspended solids and the microorganisms of bioflocs were characterized. At the end of the experiment, fifteen shrimps of each replicate were collected to evaluate nine larvae quality indicators. Isotopic values of ¹³C and ¹⁵N of food sources and shrimp tissues were collected on days 10, 20 and 30, and a Bayesian model of isotopic mixture measured the contributions of these sources to the shrimp biomass. Salinity, alkalinity and nitrite differed significantly between the treatments but were appropriate for post-larvae production. The larvae quality conditions in the CT treatment were ranked as “good,” whereas the BT conditions were considered “excellent” according to the analysis of larval quality parameters. The stable isotopes analysis showed that the PL40 GUABI? feed was the most important food source throughout the experimental period in the CT. In the BT (10th and 20th days), the bioflocs and commercial feeds (Flake-INVE?, PL40-GUABI?) did not present significant variations in their contribution ratios; however, on the 30th day, the contribution of the PL40-GUABI? feed was higher when compared to bioflocs. In general, the commercial feed played a more important role as a food source for Litopenaeus vannamei larvae during the nursery phase, although the consumption of bioflocs led to a better larvae quality.     

Seasonal and spatial variations of the food web structure in a shallow eutrophic lake assessed by stable isotope analysis

Stable isotope analysis was used to investigate seasonal and spatial variations of the food web structure in a large eutrophic lake ecosystem (Lake Taihu, China). Basal food sources, invertebrates and fish were sampled in two lake regions with different environmental conditions and spatial variations in the isotopic composition of lake food webs were found. Overall, more depleted δ¹³C and enriched δ¹⁵N isotope values of organic matter sources and consumers were found in the phytoplankton-dominated lake region than in the macrophyte-dominated region. Wide seasonal variations in the isotopic ratios were also observed in the lake biota, with a general trend towards enriched δ¹³C and δ¹⁵N values in summer and depleted values in winter. This pattern could be explained by a combination of environmental (e.g., irradiance and nutrient inputs) and biotic (e.g., availability of food sources and plasticity in prey item choice) features. Results of isotope mass balance suggest that macrophytes provide some trophic support in the macrophyte-dominated area, but in both lake regions it is more likely that aquatic food webs are phytoplankton based rather than macrophyte based under eutrophic conditions.     

Non‐lethal sampling of walleye for stable isotope analysis: a comparison of three tissues

Abstract Stable isotope analysis of fishes is often performed using muscle or organ tissues that require sacrificing animals. Non‐lethal sampling provides an alternative for evaluating isotopic composition for species of concern or individuals of exceptional value. Stable isotope values of white muscle (lethal) were compared with those from fins and scales (non‐lethal) in walleye, Sander vitreus (Mitchill), from multiple systems, size classes and across a range of isotopic values. Isotopic variability was also compared among populations to determine the potential of non‐lethal tissues for diet‐variability analyses. Muscle‐derived isotope values were enriched compared with fins and depleted relative to scales. A split‐sample validation technique and linear regression found that isotopic composition of walleye fins and scales was significantly related to that in muscle tissue for both δ¹³C and δ¹⁵N (r² = 0.79–0.93). However, isotopic variability was significantly different between tissue types in two of six populations for δ¹⁵N and three of six populations for δ¹³C. Although species and population specific, these findings indicate that isotopic measures obtained from non‐lethal tissues are indicative of those obtained from muscle.     

Abundance and origins of plasma lipids in sexually maturing female coho salmon Oncorhynchus kisutch (Walbaum) in culture

Coho salmon Oncorhynchus kisutch are semelparous spawners with highly synchronized oocyte growth. Plasma was collected from maturing female fish and plasma lipids were quantified by lipoprotein class. Stable ¹³C isotopes were employed to investigate the origins of plasma lipids. Plasma lipoproteins were partitioned into the very low density (VLDL), low density (LDL), high density (HDL) and very high density (VHDL) lipoprotein classes. Lipids from all lipoprotein classes increased between the lipid droplet and early vitellogenesis stages of oocyte growth and VHDL lipids continued to increase through mid vitellogenesis (P < 0.05). During vitellogenesis, total plasma lipid concentrations were similar to the sum of lipoprotein lipids (P = 0.51). Plasma vitellogenin determined by an ELISA method was very well correlated with VHDL lipid during vitellogenesis (R² = 0.91, n = 15). In general, the δ¹³C values of plasma lipids reflected that of feed (exogenous) and muscle (endogenous) lipids when feed and muscle δ¹³C values were similar, and were intermediate when differences existed between the two lipid sources. With one exception, the δ¹³C values of lipids from all lipoprotein classes within a plasma sample were similar. Results indicate that during sexual maturity, lipids from all plasma lipoprotein classes originate from a common pool of exogenous and endogenous lipids.     

Dominant sources of dietary carbon and nitrogen for shrimp reared in extensive rice-shrimp ponds

Stable isotope analysis was used to determine the sources of dietary nitrogen (N) and carbon (C) for shrimp during the rearing phase in extensive rice‐shrimp ponds in My Xuyen and Gia Rai districts, Vietnam. Farm‐made feed was added as a food source in shrimp ponds in My Xuyen district, and based on stable isotope analyses, was generally a poor dietary source. The commercial formulated feed used in Gia Rai also appeared to contribute little directly to the nutritional needs of the shrimp. In contrast, the natural biota in all ponds appeared to contribute substantially. In particular, biota from beam trawls and benthic organic matter were the most likely sources of nutrition in My Xuyen ponds, while benthic organic matter was the main source in Gia Rai ponds. δ¹⁵N ratios in the natural biota in My Xuyen farms decreased over the growing season, suggesting increased N fixation in the case of the benthic organic matter reaching values as low as 1‰. This suggests N‐limitation in the ponds and that natural biota become increasingly dependent on N fixed by algae and/or other microorganisms. There is the potential to promote the growth of the plankton and hence, the other natural biota, by the judicious addition of fertilizer.     

Dietary protein level and C/N ratio manipulation in zero‐exchange culture of Litopenaeus vannamei: Evaluation of inorganic nitrogen control,biofloc composition and shrimp performance

A 30‐day experiment was conducted to evaluate inorganic nitrogen control, biofloc composition and shrimp performance in zero‐exchange culture tanks for juvenile L. vannamei offered a 35% (P35) or 25% (P25) crude protein feed, each feed supplemented with additional carbohydrate to increase the C/N ratio to 20:1 (CN20) or 15:1 (CN15). Sucrose was used as a carbohydrate to manipulate the two C/N ratios based on the carbon and nitrogen content of both the feeds and sucrose. The four treatments were referred to as: P35 + CN20, P35 + CN15, P25 + CN20 and P25 + CN15. Each treatment consisted of four replicate tanks (125 L), each stocked with 28 shrimp (equivalent to 224 shrimp m^?3). Bioflocs formed and developed based on initial inoculation in all four treatments; and monitored water quality parameters were maintained within acceptable ranges for shrimp culture throughout the experiment. No significant effects (P > 0.05) of dietary protein level, C/N ratio or their interaction were observed on biofloc development (BFV, TSS and BFVI) and inorganic nitrogen (TAN, NO₂^?‐N and NO₃^?‐N) concentrations. At the end of the experiment, proximate analysis of the bioflocs collected from the four treatments showed crude protein levels of 21.3% ~ 32.1%, crude lipid levels of 1.6% ~ 2.8% and ash levels of 43.4% ~ 61.4%. Extracellular protease and amylase activities of the bioflocs were 9.9 ~ 14.4 U g^?1 TSS and 293.5 ~ 403.8 U g^?1 TSS respectively. Biofloc composition and enzyme activity were both affected by dietary protein level (P < 0.01) and C/N ratio (P < 0.05). Survival, per cent weight gain and protein efficiency ratio of shrimp were not affected (P > 0.05) by dietary protein level, C/N ratio or their interaction; however, the feed conversion ratios were significantly lower (P < 0.05) in treatments with high dietary protein (P35) compared with those in treatments with low dietary protein (P25). The results from this study demonstrate that dietary protein level and C/N ratio manipulation can have important implications for water quality, biofloc composition and shrimp performance in intensive, zero‐exchange biofloc‐based culture systems.     

Contribution of the natural biota associated with substrates to the nutritional requirements of the post-larval shrimp, Penaeus esculentus (Haswell), inhigh-density rearing systems

The contribution of epiphytes associated with physical substrates to the nutritional requirements of post‐larval shrimp, Penaeus esculentus Haswell, was determined in high‐density rearing systems (3000, 6000 and 11 000 m^?3). Stable isotope signatures of epiphytes on polyethylene mesh substrate, AquaMats? and tank walls were compared with shrimp signatures. Two methods were used: the determination of carbon and nitrogen natural abundance ratios; and ¹⁵N‐nitrogen enrichment ratios after the addition of ¹⁵N‐ammonium to tanks. Using the natural abundance technique and a simple mixing model, epiphytes were found to contribute substantially to the carbon requirements of post‐larval shrimp (39–53%). This was despite the addition of formulated feed at satiation levels. There was no indication of a reduced contribution of carbon from epiphytes to shrimp nutrition at higher shrimp densities. The lack of a difference in the ¹⁵N/¹⁴N ratios of the two food sources meant that mixing models could not be used to calculate the contribution of nitrogen from epiphytes vs. artificial feed to shrimp nutrition. Using the ¹⁵N‐nitrogenenrichment method, the amount of nitrogen contributed by epiphytes to shrimp nutrition over 24 h could be determined. This method showed that nitrogen from epiphytes was assimilated by shrimp. ¹⁵N‐enrichment methods provided a more accurate alternative to natural abundance techniques, particularly when the stable isotope signals ofthe food sources are similar. This experiment hasshown the benefits in providing substrates for P.esculentus in high‐density rearing systems to provide an additional food source for shrimp.     

Trophic relationships between a native and a nonnative predator in a system of natural lakes

Abstract – Bull trout, a species of char listed as threatened under the US Endangered Species Act, have been displaced from portions of their historic range following the introduction of nonnative lake trout. It has been suggested that competitive exclusion as a result of trophic overlap between bull trout and lake trout may be the causal mechanism associated with displacement of bull trout. This study used stable isotope data to evaluate trophic relationships among native bull trout, nonnative lake trout and other fishes in seven lakes in Glacier National Park (GNP), Montana. Bull trout and lake trout had greater δ¹⁵N values relative to other fishes among lakes (δ¹⁵N ≥ 3.0‰). Lake trout had greater δ¹⁵N values relative to bull trout (δ¹⁵N = +1.0‰). Bull trout had greater δ¹³C values relative to lake trout in six of the seven lakes examined. Although both bull trout and lake trout had greater δ¹⁵N values relative to other fishes within lakes in GNP, differences in δ¹⁵N and δ¹³C between bull trout and lake trout suggest that they are consuming different prey species or similar prey species in different proportions. Therefore, displacement of bull trout as a direct result of complete overlap in food resource use is not anticipated unless diet shifts occur or food resources become limiting. Additionally, future studies should evaluate food habits to identify important prey species and sources of partial dietary overlap between bull trout and lake trout.     

东海中北部游泳动物稳定碳氮同位素研究

利用稳定同位素方法分析2009年夏季东海中北部23种游泳动物的类群差异、种间和种内变异。结果显示,东海北部游泳动物种间C、N同位素比值变化范围较大,δ15N值范围为6.9‰~13.4‰,1δ3C值为-20.7‰~-14.7‰。其中虾类生物1δ3C平均值为(-17.06±1.43)‰,δ15N平均值为(10.26±0.84)‰;头足类生物1δ3C平均值为(-16.54±0.97)‰,δ15N平均值为(11.68±0.62)‰;小型鱼类δ13C平均值为(-16.27±0.84)‰,δ15N平均值为(10.36±1.25)‰;大中型鱼类δ13C平均值为(-16.89±1.32)‰,δ15N平均值为(11.25±1.03)‰。方差分析结果显示,4个类群的稳定C、N同位素比值具有显著差异(P<0.01)。种内不同个体间也存在较大差异。线性回归表明鹰爪虾(Trachypenaeus curvirostris)、戴氏赤虾(M etapenaeopsisdalei)、脊腹褐虾(Crangon affinis)、小黄鱼(Larim ichthys polyactis)、蓝圆鲹(Decapterus maruadsi)和黄鮟鱇(Lophius litulon)6种游泳动物体长和1δ3C值之间存在显著的相关关系,脊腹褐虾、小黄鱼、银鲳(Pampusargenteus)和白姑鱼(Argyrosomus argentatus)4种动物1δ5N值随体长增加发生显著变化。此外,1δ5N值与种间大小格局也具有显著相关性。稳定同位素分析结果表明,东海游泳动物生态位重叠现象明显,种类摄食特化程度不高,营养结构正趋向简单化。     

Dietary map of Nile tilapia using stable isotopes in three tropical lakes,Ethiopia

Comprehensive analysis of food webs requires identifying dietary sources that support the production of all major organisms within the food web/food chain. Here, we use stable isotope ratios (δ¹³C, δ³⁴S and δ¹⁵N) to assess the relative contribution of different basal carbon sources to the diet of Nile tilapia (Oreochromis niloticus L.) in three tropical lakes Hawassa (also Awasa in literature), Ziway and Koka (Ethiopia). Computations were carried out with Stable Isotope Analysis in R (SIAR) model to quantify the dietary proportion of each prey for the tilapia fish. Basal food sources were distinguishable based on their δ¹³C, δ³⁴S and δ¹⁵N values. In Lake Ziway, macrophytes (64%) were the dominant assimilated diet of tilapia while particulate organic matter (POM) and zooplankton contributed only 20% and 16%, respectively. In parallel, Nile tilapia in Lake Hawassa assimilated macrophytes (35%), POM (33%) and zooplankton (32%) at comparatively equal proportion. The dietary sources of the fish in Lake Koka were POM (49%) and zooplankton (51%). In contrast with earlier studies based on gut content analysis, the present results reveal that macrophytes contributed more and phytoplankton less than previously reported especially in macrophyte‐dominated lakes Ziway and Hawassa. The ecological condition of the lakes might have been predominantly accountable for the diet change of the tilapia. As dietary data are prerequisite for food web/food chain analysis and aquaculture industry, re‐evaluating the diet of aquatic organisms appear relevant.     

δ13C-δ15N values as indicators of trophic position and competitive overlap for Pacific salmon (Oncorhynchus spp.)

Analysis of δ¹³C and δ¹⁵N isotope values for five species of Pacific salmon indicate that they form a trophic hierarchy on the high seas. On the basis of an analysis of these stable isotope ratios, chinook salmon feed at the upper end of the food chain and pink salmon at the lower end, in the sequence pink → sockeye → coho → chinook, with chinook and pink salmon separated by ca. 0.8 of a trophic level. Chum salmon occupy a peculiar position, with low δ¹³C values and high δ¹⁵N values, possibly reflecting an unusual diet that includes large amounts of gelatinous zooplankton (salps, ctenophoi;s, and medusae). Chum appear to occupy a unique trophic position, at a trophic level nearly as high as that of chinook but at the end of a different branch of the food chain. If true, the potential for trophodynamic competition with other salmon species is small. Our results suggest that the greatest potential for trophodynamic competition occurs within the pink-sockeye-coho grouping. A similar analysis, restricted to five stocks of North American sockeye, shows that four widely separated stocks have similar heavy isotope compositions but that the separation between these stocks is still statistically significant. The Chilko stock is strikingly different, however, suggesting that it is located in the central Gulf of Alaska, a region of intense upwelling. Circumstantial evidence supporting the possibility that the Chilko and other sockeye stocks are geographically separated within the Gulf of Alaska is reviewed.     

Uncertainties associated with trophic discrimination factor and body size complicate calculation of δ15N-derived trophic positions in Arapaima sp.

Stable isotope ratios of nitrogen (δ¹⁵N) have been used to estimate trophic position (TP) of organisms due to the predictable enrichment of nitrogen-15 in consumer tissues relative to their diet. We explored estimates of trophic position using liver and muscle δ¹⁵N and stomach content analysis in a broad size range of Arapaima sp. from Amazonian floodplain lakes. Estimates of TP based on liver δ¹⁵N were more closely related to the stomach content data than estimates based on muscle δ¹⁵N, possibly because of the higher turnover of nitrogen in liver. Total length and season explained most of the variation in TP values estimated from δ¹⁵N, showing that they have more effect than prey trophic position on δ¹⁵N values. The TP estimated by identification of stomach content was 3.6 and was unrelated to the size of the arapaima. This highlights the need for a better understanding of the factors that affect values of δ¹⁵N and stomach content analysis is still needed to unravel the trophic ecology of predatory fishes.