Predicting the contributions of novel marine prey resources from angling and anadromy to the diet of a freshwater apex predator

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Using Bayesian stable isotope mixing models to estimate wolf diet in a multi-prey ecosystem

Stable isotope analysis (SIA) of wolf (Canis lupus) tissues can be used to estimate diet and intra-population diet variability when potential prey have distinct δ¹³C and δ¹⁵N values. We tested this technique using guard hairs collected from 44 wolves in 12 northwestern Montana packs, summer 2009. We used hierarchical Bayesian stable isotope mixing models to determine diet and scales of diet variation from δ¹³C and δ¹⁵N of wolves and potential prey, white-tailed deer (Odocoileus virginianus), mule deer (Odocoileus hemionus), elk (Cervus canadensis), moose (Alces alces), snowshoe hare (Lepus americanus), and other prey. As a check on SIA results, we conducted a separate diet analysis with temporally matched scats (i.e., collected in summer 2008) from 4 of the same packs. Wolves were centered on the ungulate prey in the isotope mixing space. Both methods revealed differences among pack diets and that wolves may consume moose in greater proportions than predicted by available biomass. Stable isotope analysis, and scat results were not entirely concordant; assumptions related to tissues of use in SIA, hair growth period in wolves, and scat sampling may have contributed to a mismatch between methods. Incorrect fractionation values, insufficient separation of prey in the isotope mixing space, choice of prior information in the Bayesian mixing models, and unexplained factors may have distorted diet estimates. However, the consistently high proportion of moose in pack diets suggests that increased population monitoring would benefit management of moose and wolves. Our results also support suggestions of other researchers that species-specific fractionation values should be used whenever possible, and that SIA may sometimes only provide indices of use for general groups of prey (e.g., large ungulates). © 2012 The Wildlife Society.     

Application of non‐lethal stable isotope analysis to assess feeding patterns of juvenile pallid sturgeon Scaphirhynchus albus: a comparison of tissue types and sample preservation methods

Traditional techniques for stable isotope analysis (SIA) generally require sacrificing animals to collect tissue samples; this can be problematic when studying diets of endangered species such as the pallid sturgeon Scaphirhynchus albus. Our objectives were to (i) determine if pectoral fin tissue (non‐lethal) could be a substitute for muscle tissue (lethal) in SIA of juvenile pallid sturgeon, and (ii) evaluate the influence of preservation techniques on stable isotope values. In the laboratory, individual juvenile pallid sturgeon were held for up to 186 day and fed chironomids, fish, or a commercially available pellet diet. Significant, positive relationships (r² ≥ 0.8) were observed between fin and muscle tissues for both δ¹⁵N and δ¹³C; in all samples isotopes were enriched in fins compared to muscle tissue. Chironomid and fish based diets of juvenile pallid sturgeon were distinguishable for fast growing fish (0.3 mm day^?1) using stable δ¹⁵N and δ¹³C isotopes. Frozen and preserved fin tissue δ¹⁵N isotopes were strongly related (r² = 0.89) but δ¹³C isotopes were weakly related (r² = 0.16). Therefore, freezing is recommended for preservation of fin clips to avoid the confounding effect of enrichment by ethanol. This study demonstrates the utility of a non‐lethal technique to assess time integrated food habits of juvenile pallid sturgeon and should be applicable to other threatened or endangered species.     

Relationships between growth traits and scale stable isotopes (δ13C, δ15N) of adult chum salmon Oncorhynchus keta in Hokkaido,Japan

Stable isotope analysis (SIA) in combination with growth analysis using scales collected from adult chum salmon Oncorhynchus keta migrating back to Hokkaido, Japan, was performed to describe the variation of isotopic composition of carbon (δ¹³C) and nitrogen (δ¹⁵N) in scales and to examine relationships with growth traits [age, fork length (FL), and relative growth ratio in the last growth period [(RGR^last)]. Scale stable isotope (SI) values in 3‐ to 6‐year‐old fish were highly variable, ranging from ?17.6‰ to ?14.3‰ for δ¹³C and 9.5‰ to 13.4‰ for δ¹⁵N. The δ¹⁵N was positively correlated with FL, and this tendency may indicate changes in trophic level with growth. Significant effect was not detected between δ¹⁵N and RGR^last, it can be inferred that factors potentially yielding high δ¹⁵N may not necessarily result in higher growth rates. No trend found between FL and δ¹³C may imply that there is no clear segregation in feeding locations between the 3‐ to 6‐year groups. This study provided basic information for scale SI values of chum salmon adults and indicated that SIA using scales could be a new approach to elucidating the trophic ecology of chum salmon.     

Trophic interactions between native and introduced fish species in a littoral fish community

The trophic interactions between 15 native and two introduced fish species, silverside Odontesthes bonariensis and rainbow trout Oncorhynchus mykiss, collected in a major fishery area at Lake Titicaca were explored by integrating traditional ecological knowledge and stable‐isotope analyses (SIA). SIA suggested the existence of six trophic groups in this fish community based on δ¹³C and δ¹⁵N signatures. This was supported by ecological evidence illustrating marked spatial segregation between groups, but a similar trophic level for most of the native groups. Based on Bayesian ellipse analyses, niche overlap appeared to occur between small O. bonariensis (<90 mm) and benthopelagic native species (31·6%), and between the native pelagic killifish Orestias ispi and large O. bonariensis (39%) or O. mykiss (19·7%). In addition, Bayesian mixing models suggested that O. ispi and epipelagic species are likely to be the main prey items for the two introduced fish species. This study reveals a trophic link between native and introduced fish species, and demonstrates the utility of combining both SIA and traditional ecological knowledge to understand trophic relationships between fish species with similar feeding habits.     

Stable isotopic assessment of site fidelity of mummichogs, Fundulus heteroclitus, exposed to multiple anthropogenic inputs

The goals of the study were: (1) to evaluate stable isotopic analysis (SIA) in determining the site fidelity of mummichogs, Fundulus heteroclitus, along a smaller spatial scale (~10?km) in homogenous habitat type relative to previous SIA studies; and (2) to cross-validate SIA results with mark-recapture results from a study conducted concurrently at the same sites in the upper Miramichi River estuary (MRE), New Brunswick, Canada influenced by two pulp mills and three municipal wastewater facilities. Mummichogs sampled at 9 sites along the upper MRE (n?=?198) had overall mean (± SD) ratios of ?21.03?±?1.45 ‰ δ¹³C and 11.37?±?1.02 ‰ δ¹⁵N. Mean δ¹³C and δ¹⁵N ratios were significantly different among sites with mean δ¹³C increasing in a downstream direction and distinct δ¹⁵N group signatures along the northern and southern shores. Multivariate analyses detected seven distinct groups out of nine sites sampled and these differences appear to be related to wastewater treatment influences, thus demonstrating the utility of SIA as a method to determine the site-specificity of organisms on a relatively small spatial scale within homogenous habitat within an estuary. These results, in addition to the scarcity of statistical outliers (3?%) during examination of isotopic ratios within sites support the results of a previous mark-recapture study that demonstrated very few mummichogs (3.4?%) in the upper MRE move more than 200?m.     

Determining the appropriate pretreatment procedures and the utility of liver tissue for bulk stable isotope (δ13C and δ15N) studies in sharks

Stable-isotope analysis (SIA) provides a valuable tool to address complex questions pertaining to elasmobranch ecology. Liver, a metabolically active, high turnover tissue (~166 days for 95% turnover), has the potential to reveal novel insights into recent feeding/movement behaviours of this diverse group. To date, limited work has used this tissue, but ecological application of SIA in liver requires consideration of tissue preparation techniques given the potential for high concentrations of urea and lipid that could bias δ¹³C and δ¹⁵N values (i.e., result in artificially lower δ¹³C and δ¹⁵N values). Here we investigated the effectiveness of (a) deionized water washing (WW) for urea removal from liver tissue and (b) chloroform-methanol for extraction of lipids from this lipid rich tissue. We then (a) established C:N thresholds for deriving ecologically relevant liver isotopic values given complications of removing all lipid and (b) undertook a preliminary comparison of δ¹³C values between tissue pairs (muscle and liver) to test if observed isotopic differences correlated with known movement behaviour. Tests were conducted on four large shark species: the dusky (DUS, Carcharhinus obscurus), sand tiger (RAG, Carcharias taurus), scalloped hammerhead (SCA, Sphyrna lewini) and white shark (GRE, Carcharodon carcharias). There was no significant difference in δ¹⁵N values between lipid-extracted (LE) liver and lipid-extracted/water washed (WW) treatments, however, WW resulted in significant increases in %N, δ¹³C and %C. Following lipid extraction (repeated three times), some samples were still biased by lipids. Our species-specific “C:N thresholds” provide a method to derive ecologically viable isotope data given the complexities of this lipid rich tissue (C:N thresholds of 4.0, 3.6, 4.7 and 3.9 for DUS, RAG, SCA and GRE liver_LEWW tissue, respectively). The preliminary comparison of C:N threshold corrected liver and muscle δ¹³C values corresponded with movement/habitat behaviours for each shark; minor differences in δ¹³C values were observed for known regional movements of DUS and RAG (δ¹³C_Diffs = 0.24 ± 0.99‰ and 0.57 ± 0.38‰, respectively), while SCA and GRE showed greater differences (1.24 ± 0.63‰ and 1.08 ± 0.71‰, respectively) correlated to large-scale movements between temperate/tropical and pelagic/coastal environments. These data provide an approach for the successful application of liver δ¹³C and δ¹⁵N values to examine elasmobranch ecology.     

Individual variability in stable isotope turnover rates of epidermal mucus according to body size in an omnivorous fish

Epidermal mucus (‘mucus’) is increasingly applied to fish ecological studies based on stable isotope analysis (SIA) due to its non-invasive collection. However, knowledge on mucus SI turnover rates of individual fish remains limited, including uncertainty over how they are influenced by fish body sizes. Here, a diet switch experiment predicted mucus SI turnover rates (δ¹³C and δ¹⁵N) as a function of time using samples taken over 200 days from 10 individually tagged common carp Cyprinus carpio covering two size groups. Non-linear mixed effects models revealed rapid turnover of both δ¹³C and δ¹⁵N (T₅₀: 2–5 days; T₉₅: 9–22 days); δ¹⁵N turnover rates were slower for the larger cohort, while δ¹³C turnover rates were independent of body size. Within size groups, turnover rates were not expected to vary between individuals. These experimental results suggest that due to these fast turnover rates, epidermal mucus can provide insights into the diets of fish over very short timeframes, although for δ¹⁵N the body size of the fish needs consideration.

     

New insights into the trophic ecology of the scalloped hammerhead shark,Sphyrna lewini,in the eastern tropical Pacific Ocean

Scalloped hammerhead shark, Sphyrna lewini, is considered a generalist predator, showing individual specializations and different trophic levels. This species inhabits both oceanic islands and coastal nursery areas in the eastern tropical Pacific (ETP) Ocean. Existing trophic ecology studies on S. lewini have been focused on stomach contents. This study provides new insights into its foraging preferences, habitat use, and trophic position, using stable isotope analysis (SIA) of muscle tissue. Stable isotope signatures of δ¹³C and δ¹⁵N were determined in S. lewini muscles (n?=?29) from the Ecuadorian Pacific in 2013. Trophic position (TP), isotopic niches, and overlap of S. lewini were estimated by sex, age, and maturity stages to infer their dietary habits throughout life stages. SIA revealed complex movement patterns related to sex and age classes of S. lewini, highlighting high degrees of dietary plasticity and habitat use, with a stronger relation to coastal regions than previously reported. This study provides crucial information regarding essential areas and the related migration behavior of S. lewini, with important implications for their conservation and management in the ETP.

     

Niche overlap and habitat use at distinct temporal scales among the California sea lions (Zalophus californianus) and Guadalupe fur seals (Arctocephalus philippii townsendi)

The aim of this study was to determine the trophic level, trophic breadth, and dietary overlap of two species of otariids (Zalophus californianus and Arctocephalus philippii townsendi) at the San Benito Islands in Baja California, Mexico, using scat analysis, and stable isotope analyses (SIA) of carbon and nitrogen in pup hair and dental collagen. Scat samples were collected during summer and winter. The most important prey for both species during summer was squid, comprising 74% of the A. p. townsendi diet and 45% of that of Z. californianus. In winter, squid was 87% of the A. p. townsendi diet, whereas fish was the predominant food (76%) for Z. californianus. Both species were specialist feeders and reached maximum trophic overlap during the summer (C_H = 0.9). SIA of the hair of otariid pups showed significant differences in δ¹⁵N (P = 0.001), coinciding with A. p. townsendi's greater consumption of squid. The average δ¹³C value was significantly lower for A. p. townsendi (?16.3‰ ± 0.2‰), reflecting the species' consumption of squid from pelagic habitats, whereas that of Z. californianus (?15.9‰ ± 0.3‰) was significantly higher, reflecting their consumption of benthic fish. Difference in habitat use was confirmed in δ¹³C values of dental collagen.     

The diet of great tit nestlings: Comparing observation records and stable isotope analyses

The diet of wild animals has been studied using many different strategies, approaches and methods in recent decades. In this regard, stable isotopes analysis (SIA) is becoming a widespread tool, but no study has yet, to our knowledge, compared diet estimations from SIA with direct observations of the diet of passerine nestlings. Accordingly, our aim was to test the predictive power of SIA for this purpose and identify potential confounding factors such as habitat effects. To do this, we compared isotopic signatures of δ¹³C and δ¹⁵N in the feathers of great tit (Parus major) nestlings, and the corresponding estimates of their diet based on stable isotope mixing models, with prey proportions delivered by their parents obtained through video-recordings. Between-nest differences in isotopic signatures of δ¹⁵N were larger than within-nest differences. We found that δ¹⁵N signatures of nestling feathers correlated positively with the proportion of spiders and negatively with the proportion of caterpillars in the nestlings’ diet, the most important prey types. On the other hand, between-nest and within-nest differences in δ¹³C ratios were of similar magnitude and δ¹³C ratios correlated mainly with the proportion of trees surrounding nest-boxes that were Quercus spp. Estimates of diet composition based on mixing models correlated with the observed nestling diet, yet effect sizes were quite low. Although mixing models are commonly used to ascertain diets, our data show that they can provide valuable information on the relative intake of prey types from different trophic levels; but when complex dietary patterns are recorded (e.g. due to the confounding effects of habitat and/or temporal variation) it can be difficult to draw firm conclusions about diet composition.     

Biogenic methane contributes to the food web of a large,shallow lake

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The trophic niche of sculpins Cottus spp. in forage fish assemblages of boreal lakes

We compared the trophic niches of freshwater sculpins Cottus spp. with those of other co-habiting forage fishes in two groups of boreal lakes with distinct habitats and fish communities. Near North Lakes (45° 00′ to 47° 30′ N) were deeper, stratified and contained lake trout Salvelinus namaycush as the apex piscivore, whereas Far North Lakes (51° 10′ to 52° 20′ N) were shallower, did not stratify and contained pike Esox lucius and walleye Sander vitreus as the apex piscivores. Trophic niches of sculpins and other forage fishes were compared based on niche metrics calculated from muscle stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios. In Near North Lakes, sculpins were found almost exclusively in deep, offshore waters and their niche positions reflected a greater reliance on pelagic production (lower δ¹³C) and a higher trophic elevation (higher δ¹⁵N) compared with most other forage fishes. Furthermore, sculpins in Near North Lakes tended to have larger trophic niches (occupied greater area in δ¹³C– δ¹⁵N space), particularly in the food chain (δ¹⁵N) dimension, than other cohabiting forage fishes. In contrast, sculpins in Far North Lakes were commonly found in the nearshore and had trophic niche positions and sizes that were similar to those of the other cohabiting forage fishes. This study illustrates the flexibility in the realised trophic niches of sculpins in relation to habitat availability and fish community composition in boreal lakes.     

Non-lethal dorsal fin sampling for stable isotope analysis in seahorses

Sampling collection for stable isotope analysis has traditionally involved the sacrifice of the animal. Seahorses (Hippocampus spp.) are listed as threatened by the Convention on International Trade in Endangered Species (http://www.cites.org) and consequently lethal sampling is undesirable. We evaluated the adequacy of dorsal fin tissue of adult seahorses Hippocampus guttulatus for stable isotope analysis as an alternative to lethal tissue sampling. Three seahorse tissues (dorsal fin, muscle, and liver) were analyzed for comparisons of δ¹⁵N and δ¹³C values. Similarities found between δ¹⁵N and δ¹³C values in dorsal fin and muscle tissue of H. guttulatus suggest that both tissues are adequate for stable isotope analysis to understand feeding ecology of seahorses. However, considering the threatened status of the species, dorsal fin tissue would be recommended in adult seahorses as a non-lethal sampling. The effect of lipid extraction on carbon and nitrogen stable isotope values was also evaluated in each seahorse tissues. Significant effects of lipids extraction did only occur for δ¹³C values in muscle and liver. It was found that lipid removal was not necessary to perform SIA in dorsal fin tissues. Due to the limited availability of fin tissue obtained from fin-clipping in seahorses, the relationship between the mass/surface of dorsal fin clip and stable isotope values was analyzed. δ¹⁵N and δ¹³C values in fin samples were found to be independent of the size of fin analyzed. According to our study, the use of fin-clipping sampling, with a minimum surface analyzed of 12.74?mm², was found to be an adequate method for SIA in seahorses.     

Food sources and trophic structure of fishes and benthic macroinvertebrates in a tropical seagrass meadow revealed by stable isotope analysis

Stable carbon and nitrogen isotope analysis was used to examine the food sources and trophic structure of 17 fish species and six groups of benthic macroinvertebrates in a seagrass meadow in North Sulawesi, Indonesia. The seagrass, their associated epiphytes, sediment organic matter (SOM) and particulate organic matter (POM) were identified to be the food sources, with δ¹³C values ranging from ?19.49 (POM) to ?9.66‰ (seagrass). The δ¹³C of the 23 fauna taxa were between ?18.57 (Arothron manilensis) and ?11.62‰ (Protoreaster sp.). For five of the six groups of benthic macroinvertebrates, seagrass and their epiphytes contributed more than 69.4%. For 14 of the 17 fish species, seagrass and their epiphytes are the main contributors. For 15 of the 17 fishes, the trophic levels inferred from SIA are lower than those from the previously reported diet composition analysis. These findings show that seagrass and their epiphytes are consumed by most of the fish and benthic macroinvertebrates, and are important for a large portion of the food web in seagrass meadows in the Coral Triangle area.     

Refining trophic dynamics through multi‐factor Bayesian mixing models: A case study of subterranean beetles

Food web dynamics are vital in shaping the functional ecology of ecosystems. However, trophic ecology is still in its infancy in groundwater ecosystems due to the cryptic nature of these environments. To unravel trophic interactions between subterranean biota, we applied an interdisciplinary Bayesian mixing model design (multi‐factor BMM) based on the integration of faunal C and N bulk tissue stable isotope data (δ¹³C and δ¹⁵N) with radiocarbon data (Δ¹⁴C), and prior information from metagenomic analyses. We further compared outcomes from multi‐factor BMM with a conventional isotope double proxy mixing model (SIA BMM), triple proxy (δ¹³C, δ¹⁵N, and Δ¹⁴C, multi‐proxy BMM), and double proxy combined with DNA prior information (SIA + DNA BMM) designs. Three species of subterranean beetles (Paroster macrosturtensis, Paroster mesosturtensis, and Paroster microsturtensis) and their main prey items Chiltoniidae amphipods (AM1: Scutachiltonia axfordi and AM2: Yilgarniella sturtensis), cyclopoids and harpacticoids from a calcrete in Western Australia were targeted. Diet estimations from stable isotope only models (SIA BMM) indicated homogeneous patterns with modest preferences for amphipods as prey items. Multi‐proxy BMM suggested increased—and species‐specific—predatory pressures on amphipods coupled with high rates of scavenging/predation on sister species. SIA + DNA BMM showed marked preferences for amphipods AM1 and AM2, and reduced interspecific scavenging/predation on Paroster species. Multi‐factorial BMM revealed the most precise estimations (lower overall SD and very marginal beetles' interspecific interactions), indicating consistent preferences for amphipods AM1 in all the beetles' diets. Incorporation of genetic priors allowed crucial refining of the feeding preferences, while integration of more expensive radiocarbon data as a third proxy (when combined with genetic data) produced more precise outcomes but close dietary reconstruction to that from SIA + DNA BMM. Further multidisciplinary modeling from other groundwater environments will help elucidate the potential behind these designs and bring light to the feeding ecology of one the most vital ecosystems worldwide.     

Ecological implications of unprecedented warm water anomalies on interannual prey preferences and foraging areas of Guadalupe fur seals

The Guadalupe fur seal (GFS) currently is recovering from near extinction and prey availability is an important factor. Scat and stable isotope analyses (SIA) were used to assess GFS foraging on Guadalupe Island during 2013–2016. This period was characterized by normal (2013) ocean temperatures followed by warm conditions (2014–2016). Scat samples (~40/year) were collected for prey identification. Additionally, lanugo samples from 1-month-old pups (50/year) were processed for SIA (δ¹³C/δ¹⁵N). Interannual prey preferences were identified (ANOSIM, p < .05), including significant differences between 2013 and the anomalous years. The most important prey species among these years was the jumbo squid; followed by the neon flying squid (2014–2016). The GFS diet was more specialized in 2013–2015 and more generalized in 2016. Prey from higher trophic levels (determined by scat analysis) were consumed during 2014–2016, but with significantly lower δ¹³C/δ¹⁵N values than in 2013 (ANOVA, p < .05 for both isotope ratios). This pattern may indicate more northerly or offshore foraging areas, with the presence of oceanic prey (neon flying squid). The widest isotopic niche was observed in 2015 (2.2‰²), reflecting a broader foraging area. Our findings are an important step toward better understanding the impacts of climate change on the recovery of GFS.     

Individual specialization and trophic adaptability of northern pike (Esox lucius): an isotope and dietary analysis

Northern pike (Esox lucius) are often considered to be specialist piscivores, but under some circumstances will continue to eat invertebrates as adults. To examine effects of fish assemblage composition on the trophic ecology of pike, we combined stable isotope analysis (SIA) of carbon and nitrogen and stomach content analysis (SCA) on pike from five lakes in northern Alberta, three of which contain only pike (“pike-only”) and two that also contain yellow perch (Perca flavescens) or white sucker (Catostomus commersoni) (“pike-other”). Fish were more important as prey and empty stomachs, which often characterize piscivores, were significantly more frequent in pike-other than in pike-only lakes. However, even though invertebrates were more important for pike in pike-only lakes, SIA and SCA indicated that invertebrates were also an important component of pike diets in pike-other lakes. SIA and SCA also revealed considerable intrapopulation variation in trophic ecology, with individuals in some populations differing by as much as two trophic levels. Comparisons of stomach contents and isotope signatures of the same fish suggested that within these variable populations, specialization on invertebrates or fish was a long-term trait of some individuals. SIA indicated that trophic position increased and diets shifted to a greater importance of littoral prey as pike grew in pike-only lakes, but not in lakes with other fish present. Trophic adaptability in northern pike is expressed at both the population level, where the trophic ecology is sensitive to differences in prey regimes, and at the organismal level, in the form of intrapopulation variation and individual specialization. Received: 1 July 1998 / Accepted: 3 February 1999     

Stable isotope analysis as a tool to monitor dietary trends in little penguins Eudyptula minor

Long‐term dietary monitoring of seabirds can be used to relate population fluctuations to at‐sea events. Stomach flushing is a conventional dietary monitoring technique, but has a number of disadvantages. Stable isotope analysis (SIA) is a less invasive method that provides unbiased dietary information over a longer period. We evaluated stable isotope analysis as a potential tool for monitoring long‐term little penguin Eudyptula minor diet. We determined diet composition during the chick feeding stage using stomach flushing and SIA at three separate colonies, using spatial variation in diet as a surrogate for potential temporal variation. Bayesian isotopic mixing models were generated for blood and feathers to evaluate their ability to discriminate broad‐scale (fish, squid, crustaceans) and fine‐scale (individual prey species) diet composition. Differences in stable carbon and nitrogen isotope ratios were found between colonies: broad‐scale isotopic mixing models predicted different proportional contributions of broad taxa (fish, cephalopod, crustacean) to diet than was indicated by stomach samples, reflecting the bias incurred by one‐off stomach contents analysis. Fine‐scale isotopic mixing models predicted proportional contributions of prey items with less certainty. Blood isotopic mixing models had narrower confidence intervals than models for feathers, but trends in δ¹⁵N for feathers mirrored those for blood. Our results suggest that relying on stomach contents analysis to detect shifts in prey consumption in little penguins could be very misleading, resulting in a less‐than‐complete idea of total prey consumption. SIA of little penguin tissues could be used to monitor dietary shifts across dissimilar taxa that may affect population numbers, but would fail to detect shifts between fish species.     

Stable isotopes reveal contrasting trophic dynamics between host–parasite relationships: A case study of Atlantic salmon (Salmo salar) and parasitic lice (Lepeophtheirus salmonis and Argulus foliaceus)

Across existing fish host–parasite literature, endoparasites were depleted in δ¹⁵N compared to their hosts, while ectoparasitic values demonstrated enrichment, depletion and equivalence relative to their hosts. δ¹³C enrichment varied extensively for both endo- and ectoparasites across taxa and host tissues. In our case study, sea lice (Lepeophtheirus salmonis) were enriched in δ¹⁵N relative to their farmed Atlantic salmon (Salmo salar) hosts, although the value contradicted the average that is currently assumed across the animal kingdom. Common fish lice (Argulus foliaceus) did not show a consistent trend in δ¹⁵N compared to their wild S. salar hosts. Both parasitic species had a range of δ¹³C enrichment patterns relative to their hosts. Farmed and wild S. salar had contrasting δ¹³C and δ¹⁵N, and signals varied across muscle, fin and skin within both groups. L. salmonis and A. foliaceus subsequently had unique δ¹³C and δ¹⁵N, and L. salmonis from opposite US coasts differed in δ¹⁵N. Given the range of enrichment patterns that were exhibited across the literature and in our study system, trophic dynamics from host to parasite do not conform to traditional prey to predator standards. Furthermore, there does not appear to be a universal enrichment pathway for δ¹³C nor δ¹⁵N in parasitic relationships, which emphasizes the need to investigate host–parasite linkages across species.