Dual‐tracer‐based isotope turnover rates in a highly invasive mysid Limnomysis benedeni from Lake Constance

Understanding the ecological patterns of invasive species and their habitats require an understanding of the species’ foraging ecology. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values provide useful information into the study of animal ecology and evolution, since the isotope ratios of consumers reflect consumer's dietary patterns. Nevertheless, the lack of species‐ and element‐specific laboratory‐derived turnover rates could limit their application. Using a laboratory‐based dual stable isotope tracer approach (Na¹⁵NO₃ and NaH¹³CO₃), we evaluated the δ¹⁵N and δ¹³C isotope turnover rates in full‐grown adult invasive Limnomysis benedeni from Lake Constance. We provide δ¹⁵N and δ¹³C turnover rates based on nonlinear least‐squares regression and posterior linear regression models. Model precisions and fit were evaluated using Akaike's information criterion. Within a couple of days, the δ¹⁵N and δ¹³C of mysids began to change. Nevertheless, after about 14 days, L. benedeni did not reach equilibrium with their new isotope values. Since the experiment was conducted on adult subjects, it is evident that turnover was mainly influenced by metabolism (in contrast to growth). Unlike traditional dietary shifts, our laboratory‐based dual stable isotope tracer approach does not shift the experimental organisms into a new diet and avoids dietary effects on isotope values. Results confirm the application of isotopic tracers to label mysid subpopulations and could be used to reflect assimilation and turnover from the labeled dietary sources. Field‐based stable isotope studies often use isotopic mixing models commonly assuming diet‐tissue steady state. Unfortunately, in cases where the isotopic composition of the animal is not in equilibrium with its diet, this can lead to highly misleading conclusions. Thus, our laboratory‐based isotopic incorporation rates assist interpretation of the isotopic values from the field and provide a foundation for future research into using isotopic tracers to investigate invasion ecology.     

Annual variability in dentin δ15N and δ13C reveal sex differences in weaning age and feeding habits in Risso's dolphins (Grampus griseus)

Teeth of odontocetes accumulate annual dentinal growth layer groups (GLGs) that record isotope ratios, which reflect the time of their synthesis. Collectively, they provide lifetime records of individual feeding patterns from which life history traits can be inferred. We subsampled the prenatal dentin and postnatal GLGs in Risso's dolphins (Grampus griseus) (n = 65) that stranded or were collected as bycatch in Taiwan (1994–2014) and analyzed them for δ¹⁵N and δ¹³C. Age‐specific δ¹⁵N and δ¹³C values were corrected for effects of calendar year, stranding site, C/N, and sex. δ¹⁵N values were higher in prenatal layers (14.94‰ ± 0.74‰) than in adult female GLGs (12.58‰ ± 0.20‰), suggesting fetal enrichment during gestation. Decreasing δ¹⁵N values in early GLGs suggested changes in dietary protein sources during transition to complete weaning. Weaning age was earlier in males (1.09 yr) than in females (1.81 yr). Significant differences in δ¹⁵N values between weaned males and females suggest potential sexual segregation in feeding habits. δ¹³C values increased from the prenatal to the 4th GLG by ~1.0‰, indicative of a diet shift from ¹³C‐depleted milk to prey items. Our results provide novel insights into the sex‐specific ontogenetic changes in feeding patterns and some life history traits of Risso's dolphins.     

Diet segregation between two colonies of little penguins Eudyptula minor in southeast Australia

We studied foraging segregation between two different sized colonies of little penguins Eudyptula minor with overlapping foraging areas in pre‐laying and incubation. We used stomach contents and stable isotope measurements of nitrogen (δ¹⁵N) and carbon (δ¹³C) in blood to examine differences in trophic position, prey‐size and nutritional values between the two colonies. Diet of little penguins at St Kilda (small colony) relied heavily on anchovy while at Phillip Island (large colony), the diet was more diverse and anchovies were larger than those consumed by St Kilda penguins. Higher δ¹⁵N values at St Kilda, differences in δ¹³C values and the prey composition provided further evidence of diet segregation between colonies. Penguins from each colony took anchovies from different cohorts and probably different stocks, although these sites are only 70 km apart. Differences in diet were not reflected in protein levels in the blood of penguins, suggesting that variation in prey between colonies was not related to differences in nutritional value of the diet. Anchovy is currently the only available prey to penguins throughout the year and its absence could have a negative impact on penguin food supply, particularly at St Kilda where the diet is dominated by this species. While it is difficult to establish whether diet segregation is caused by inter‐ or intra‐colony competition or spatial differences in foraging areas, we have shown that colonies with broadly overlapping foraging ranges could have significant differences in trophic position, diet composition and prey size while maintaining a diet of similar nutritional value.     

Carbon (δ13C) and nitrogen (δ15N) stable isotope composition in plant and soil in Southern Patagonia's native forests

Stable isotope natural abundance measurements integrate across several biogeochemical processes in ecosystem N and C dynamics. Here, we report trends in natural isotope abundance (δ¹³C and δ¹⁵N in plant and soil) along a climosequence of 33 Nothofagus forest stands located within Patagonia, Southern Argentina. We measured 28 different abiotic variables (both climatic variables and soil properties) to characterize environmental conditions at each of the 33 sites. Foliar δ¹³C values ranged from ?35.4‰ to ?27.7‰, and correlated positively with foliar δ¹⁵N values, ranging from ?3.7‰ to 5.2‰. Soil δ¹³C and δ¹⁵N values reflected the isotopic trends of the foliar tissues and ranged from ?29.8‰ to ?25.3‰, and ?4.8‰ to 6.4‰, respectively, with no significant differences between Nothofagus species (Nothofagus pumilio, Nothofagus antarctica, Nothofagus betuloides). Principal component analysis and multiple regressions suggested that mainly water availability variables (mean annual precipitation), but not soil properties, explained between 42% and 79% of the variations in foliar and soil δ¹³C and δ¹⁵N natural abundance, which declined with increased moisture supply. We conclude that a decline in water use efficiency at wetter sites promotes both the depletion of heavy C and N isotopes in soil and plant biomass. Soil δ¹³C values were higher than those of the plant tissues and this difference increased as annual precipitation increased. No such differences were apparent when δ¹⁵N values in soil and plant were compared, which indicates that climatic differences contributed more to the overall C balance than to the overall N balance in these forest ecosystems.     

Nitrogen and carbon isotope variability in the green‐algal lichen Xanthoria parietina and their implications on mycobiont–photobiont interactions

Stable isotope patterns in lichens are known to vary largely, but effects of substrate on carbon and nitrogen stable isotope signatures of lichens were previously not investigated systematically. N and C contents and stable isotope (δ¹⁵N, δ¹³C) patterns have been measured in 92 lichen specimens of Xanthoria parietina from southern Bavaria growing on different substrates (bark and stone). Photobiont and mycobiont were isolated from selected populations and isotopically analyzed. Molecular investigations of the internal transcribed spacer of the nuclear ribosomal DNA (ITS nrDNA) region have been conducted on a subset of the specimens of X. parietina. Phylogenetic analysis showed no correlation between the symbionts X. parietina and Trebouxia decolorans and the substrate, isotope composition, or geographic origin. Instead specimens grown on organic substrate significantly differ in isotope values from those on minerogenic substrate. This study documents that the lichens growing on bark use additional or different N sources than the lichens growing on stone. δ¹⁵N variation of X. parietina apparently is controlled predominantly by the mass fraction of the mycobiont and its nitrogen isotope composition. In contrast with mycobionts, photobionts of X. parietina are much more ¹⁵N‐depleted and show less isotopic variability than mycobionts, probably indicating a mycobiont‐independent nitrogen acquisition by uptake of atmospheric ammonia.     

Isotopic fractionation with morphological change and sexual specificity in the lappet moth Euthrix potatoria

The δ¹⁵N values of adult holometabolous insects exceed those of larvae, but otherwise little information on terrestrial invertebrates has been obtained in food‐web analyses using stable isotope ratios (δ¹⁵N, δ¹³C). Changes in δ¹³C during metamorphosis and differences between males and females have not been examined. We collected the larvae and cocoons of Euthrix potatoria (L.) (Lepidoptera: Lasiocampidae) in the field and used them to assess the species’ isotopic fractionation. Each emerged moth was divided into five body parts. We conducted stable N and C isotope analyses for each body part, as well as for cocoons and exuviae, and also compared stable isotope ratios between sexes. We confirmed δ¹⁵N enrichment through metamorphosis and estimated that δ¹⁵N enrichment is accomplished by the relative concentration of ¹⁵N due to the excretion of copious meconium, which contains abundant ¹⁴N. We also observed changes in δ¹³C values through metamorphosis. Both isotope values tended to change more in males than in females. The proportion of the whole‐adult weight represented by meconium was higher in males than in females, suggesting that high meconium secretion in males contributes to the sexual difference in δ¹⁵N. These phenomena may be common in Holometabola, which require a pupal stage. For more accurate food‐web assessments, it is important to consider stable isotope changes during different life cycles, as well as sexual differences.     

Stable isotope records of sei whale baleens from Chilean Patagonia as archives for feeding and migration behavior

Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope variations in baleen plates of sei whales (Balaenoptera borealis) stranded after a mass mortality event in Chilean Patagonia were investigated to assess potential dietary and migratory patterns. Carbon and nitrogen isotope ratios of seven baleens from six individuals were analyzed. The δ¹³C values ranged from ? 19.1 to ? 15.9‰ and the δ¹⁵N values from 8.7 to 15.4‰. Variations of up to 2.9‰ for δ¹³C and 5.3‰ for δ¹⁵N were observed within one baleen. Carbon and nitrogen isotope records of each baleen were significantly correlated and showed recurring oscillations confirmed by wavelet analyses. Oscillations slightly differed in periodicity indicating variable baleen growth rates between 10.0 and 16.5 cm/year. Food sources of the whales are discussed in terms of available isotope data for potential prey taxa and potential migratory behavior on the basis of latitudinal isotope gradients of particulate organic matter. Cyclicity could be explained by regular migrations of the sei whales from subtropical calving areas to high‐latitude foraging grounds. δ¹⁵N records of baleens differed between individuals eventually pointing to diverse feeding and migratory preferences among sei whale individuals.     

DIFFERENCES IN FORAGING LOCATION OF MEXICAN AND CALIFORNIA ELEPHANT SEALS: EVIDENCE FROM STABLE ISOTOPES IN PUPS

Female northern elephant seals, Mirounga angustirostris, from Año Nuevo (AN) in central California feed offshore in mid‐latitude waters (40°–55°N). Migratory patterns and foraging locations of seals from Mexico are unknown. Rookeries on San Benitos (SB) islands in Baja California Sur, Mexico, are ～1,170 km south of AN. Although the colonies are similar in size, seals from SB begin breeding earlier and have an earlier breeding birthing peak than seals from AN. To determine if the foraging location of seals from Mexico was similar to that of seals from California, we measured δ¹³C and δ¹⁵N values in the hair of 48 suckling pups at SB and 37 from AN, assuming that their isotopic signatures reflected those of mothers' milk, their exclusive diet. The mean δ¹³C and δ¹⁵N values for SB pups (?16.1‰± 0.9‰ and 17.7‰± 0.9‰, respectively) were significantly higher than those for AN pups (?17.6‰± 0.4‰ and 15.6‰± 1.0‰, respectively). From data on environmental isotope gradients and known behavior of SB and AN populations, we hypothesize that the isotope differences are due to females in the SB colony foraging ～8° south of seals from AN. This hypothesis can be tested by deployment of satellite tags on adult females from the SB colony.     

Examination of relationships between stable isotopes and cortisol concentrations along the length of phocid whiskers

Alaskan seals are found in remote and sometimes inaccessible locations, making it difficult to collect time‐series information. This study explores a novel method to examine temporal changes in diet and physiological status of ringed (Pusa hispida), spotted (Phoca largha), and harbor (Phoca vitulina) seals using cortisol concentrations and δ¹⁵N and δ¹³C stable isotopes (SIs) measured in serial sections of whiskers. As whiskers grow, whisker tissue is deposited sequentially making these measurements temporally aligned. Whisker cortisol presented in a distinct pattern with elevated concentrations at the root section followed by a curvilinear decline moving toward the tip of most whiskers. Comparing SIs at the root to the rest of the whiskers, δ¹³C values were slightly lower in ringed and harbor seal whiskers and δ¹⁵N values were slightly higher in harbor seal whiskers. The data were modeled controlling for the observed trends in cortisol concentrations and further associations between cortisol concentrations and SIs were detected in spotted and harbor seal whiskers. Additional research examining the source and stability of whisker cortisol is warranted. However, the methods presented here demonstrate that whiskers could prove valuable to gather long‐term and naturally aligned dietary and physiological information.     

Estimating tissue‐specific discrimination factors and turnover rates of stable isotopes of nitrogen and carbon in the smallnose fanskate Sympterygia bonapartii (Rajidae)

This study aimed to estimate trophic discrimination factors (TDFs) and metabolic turnover rates of nitrogen and carbon stable isotopes in blood and muscle of the smallnose fanskate Sympterygia bonapartii by feeding six adult individuals, maintained in captivity, with a constant diet for 365 days. TDFs were estimated as the difference between δ¹³C or δ¹⁵N values of the food and the tissues of S. bonapartii after they had reached equilibrium with their diet. The duration of the experiment was enough to reach the equilibrium condition in blood for both elements (estimated time to reach 95% of turnover: C t95%_blood = 150 days, N t95%_blood = 290 days), whilst turnover rates could not be estimated for muscle because of variation among samples. Estimates of Δ¹³C and Δ¹⁵N values in blood and muscle using all individuals were Δ¹³C_blood = 1·7‰, Δ¹³C_muscle = 1·3‰, Δ¹⁵N_blood = 2·5‰ and Δ¹⁵N_muscle = 1·5‰, but there was evidence of differences of c.0·4‰ in the Δ¹³C values between sexes. The present values for TDFs and turnover rates constitute the first evidence for dietary switching in batoids based on long‐term controlled feeding experiments. Overall, the results showed that S. bonapartii has relatively low turnover rates and isotopic measurements would not track seasonal movements adequately. The estimated Δ¹³C values in S. bonapartii blood and muscle were similar to previous estimations for elasmobranchs and to generally accepted values in bony fishes (Δ¹³C = 1·5‰). For Δ¹⁵N, the results were similar to published reports for blood but smaller than reports for muscle and notably smaller than the typical values used to estimate trophic position (Δ¹⁵N c. 3·4‰). Thus, trophic position estimations for elasmobranchs based on typical Δ¹⁵N values could lead to underestimates of actual trophic positions. Finally, the evidence of differences in TDFs between sexes reveals a need for more targeted research.     

Temporal variability in stable isotope ratios of C and N in the vibrissa of captive and wild adult South American sea lions Otaria byronia: More than just diet shifts

We analyzed the δ¹³C and δ¹⁵N values in the vibrissae of captive adult breeding South American sea lions (Otaria byronia) fed at a constant diet and then used this information to analyze the change in stable isotope values along the vibrissae from wild individuals. The overall diet‐to‐vibrissa discrimination factor of the captive animals was 3.0‰ ± 0.1‰ for δ¹³C and 3.6‰ ± 0.1‰ for δ¹⁵N, but the stable isotope ratios fluctuated periodically despite constant diet. The δ¹³C and δ¹⁵N values of the captive male declined at the end of the breeding season, whereas the δ¹³C values of the female increased during the central part of pregnancy and the δ¹⁵N values peaked during lactation. The δ¹³C and δ¹⁵N values of adult wild specimens also fluctuated periodically and vibrissae growth rate (0.15 mm/d in both sexes) was slightly lower than in captivity (0.17 mm/d), assuming an annual periodicity for oscillations. Similarities in the amplitude of the cycles of captive and wild males suggested that fasting was probably the main source of periodic variability in the δ¹⁵N of wild males, whereas pregnancy and lactation were probably the main source of periodic variability for the δ¹³C of wild females.     

Fueling phocids: Divergent exploitation of primary energy sources and parallel ontogenetic diet switches among three species of subarctic seals

Determining how marine predators partition resources is hindered by the difficulty in obtaining information on diet and distribution. Stable isotopes (SI) of carbon (¹³C/¹²C, δ¹³C) and nitrogen (¹⁵N/¹⁴N, δ¹⁵N) provide a two‐dimensional estimate of the dietary space of consumers; an animal's isotopic composition is directly influenced by what they consume and where they feed. Harp (Pagophilus groenlandicus) and hooded (Cystophora cristata) seals are abundant phocid species found in the North Atlantic. We measured and contrasted SI values between seals sampled at nearshore and offshore sites to test for effects of sampling location, sex, age‐class, and body size to gain insight into how these species partition space and prey resources. In addition we contrasted previously published results for gray seals (Halichoerus grypus). Isotope values differed significantly by age class and location in harp and hooded seals. We found significant differences in SI values (mean δ¹³C and δ¹⁵N ± SE) between all species. Hooded seals, a continental shelf‐edge, deep‐diving species, exhibited low SI values (juveniles: ?20.9‰ ± 0.03‰, 13.36‰ ± 0.05‰; adults: ?20.41‰ ± 0.03‰, 14.81‰ ± 0.04‰) characteristic of feeding on meso‐ to bathypelagic prey. Harp seals, which dive to moderate depths primarily on the shelf had intermediate SI values (juveniles: ?20.53‰ ± 0.01‰, 13.91‰ ± 0.01‰; adults: ?20.13‰ ± 0.01‰, 14.96‰ ± 0.01‰) characteristic of feeding on epipelagic prey, whereas gray seals, which feed on or near the sea floor in shallow shelf waters, had high SI values (juveniles: ?19.74‰ ± 0.04‰, 17.51‰ ± 0.05‰; adults: ?18.86‰ ± 0.01‰, 17.23‰ ± 0.02‰) characteristic of feeding on demersal prey. In all species, δ¹³C values increased with body size and age in the same manner, indicating that seals exploit or forage in deeper habitats as they get larger and older. We hypothesize that the consistent ontogenetic shift in foraging niche, despite large differences between species in their diving behavior, geographic range and habitat use, not only reflects increased access to different prey due to increased diving capacity, but a progressive adjustment to balance energy budgets by reducing foraging costs.     

Stable isotope enrichment in laboratory ant colonies: effects of colony age,metamorphosis, diet,and fat storage

Ecologists use stable isotopes to infer diets and trophic levels of animals in food webs, yet some assumptions underlying these inferences have not been thoroughly tested. We used laboratory‐reared colonies of Solenopsis invicta Buren (Hymenoptera: Formicidae: Solenopsidini) to test the effects of metamorphosis, diet, and lipid storage on carbon and nitrogen stable isotope ratios. Effects of metamorphosis were examined in ant colonies maintained on a control diet of domestic crickets and sucrose solution. Effects of a diet shift were evaluated by adding a tuna supplement to select colonies. Effects of lipid content on stable isotopes were tested by treating worker ants with polar and non‐polar solvents. δ¹³C and δ¹⁵N values of larvae, pupae, and workers were measured by mass spectrometry on whole‐animal preparations. We found a significant effect of colony age on δ¹³C, but not δ¹⁵N; larvae, pupae, and workers collected at 75 days were slightly depleted in ¹³C relative to collections at 15 days (Δδ¹³C = ?0.27‰). Metamorphosis had a significant effect on δ¹⁵N, but not δ¹³C; tissues of each successive developmental stage were increasingly enriched in ¹⁵N (pupae, +0.5‰; workers, +1.4‰). Availability of tuna resulted in further shifts of about +0.6‰ in isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on δ¹³C, but treatment with a non‐polar solvent resulted in enriched δ¹⁵N values of +0.37‰. Identifying regular patterns of isotopic enrichment as described here should improve the utility of stable isotopes in diet studies of insects. Our study suggests that researchers using ¹⁵N enrichment to assess trophic levels of an organism at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing with holometabolous insects at one site and spiders or hemimetabolous insects at another site.     

Multiyear dual nitrate isotope signatures suggest that N‐saturated subtropical forested catchments can act as robust N sinks

In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH₄⁺), causes significant nitrate (NO₃^?) leaching from well‐drained acid forest soils on hill slopes (HS), whereas significant retention of NO₃^? occurs in near‐stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δ¹⁵N and δ¹⁸O) of nitrate (NO₃^?) in soil pore water along a hydrological continuum in the N‐saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH₄⁺ and production of NO₃^? in soils on HS were associated with a significant decrease in δ¹⁵N‐NO₃^?, suggesting efficient nitrification despite low soil pH. The concentration of NO₃^? declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δ¹⁵N and δ¹⁸O of residual NO₃^?, providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent ¹⁵N enrichment factor (ε) of NO₃^? of about ?5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δ¹⁵N and δ¹⁸O‐NO₃^? in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δ¹⁵N‐NO₃^? values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N‐saturated, subtropical forests with important implications for regional N budgets.     

INVESTIGATING TROPHIC RELATIONSHIPS OF PINNIPEDS IN ALASKA AND WASHINGTON USING STABLE ISOTOPE RATIOS OF NITROGEN AND CARBON

We measured stable-nitrogen (δ¹⁵N) and stable-carbon (δ¹³C) isotope ratios in muscle and hair from 7 northern fur seals (Callorhinus ursinus) from the Pribilof Islands, Alaska, and 27 Steller sea lions (Eumetopias jubatus), and 14 harbor seals (Phoca vitulina) from the Gulf of Alaska and coast of Washington State, in order to contrast dietary information derived from isotopic vs. available conventional dietary studies. Stable-nitrogen-isotope analysis of muscle revealed that harbor seals were enriched over sea lions (mean δ¹⁵N = 18.6‰vs. 17.5‰) which were in turn enriched over northern fur seals (mean δ¹⁵N = 16.6‰). Trophic segregation among these species likely results primarily from differential reliance on herring (Clupea harengus), Atka mackerel (Pleurogrammus monopterygius), and large vs. small walleye pollock (Theregra chalcogramma). According to their δ¹⁵N values, adult male Steller sea lions showed a higher trophic position than adult females (mean δ¹⁵N: 18.0‰vs. 17.2‰), whereas adult female northern fur seals were trophically higher than juvenile male fur seals (mean δ¹⁵N: 16.5‰vs. 15.0‰). Each of these observed differences likely resulted from differential reliance on squid or differences in the size range of pollock consumed. Three northern fur seal pups showed higher δ¹⁵N enrichment over adults (mean 17.7‰vs. 15.8‰) due to their reliance on their mother's milk. Stable-carbon isotope measurements of hair revealed a cline toward more negative values with latitude. Segregation in hair δ¹³C between Steller sea lions and harbor seals off the coast of Washington (mean δ¹³C: ?13.6‰vs.?15.0‰) reflected the greater association of harbor seals with freshwater input from the Columbia River. Our study demonstrates the utility of the stable isotope approach to augment conventional dietary analyses of pinnipeds and other marine mammals.     

Changes in diets of individual Baltic ringed seals (Phoca hispida botnica) during their breeding season inferred from stable isotope analysis of multiple tissues

The stable isotope ratios (δ¹³C and δ¹⁵N) of three tissues with different metabolic rates (plasma, liver, and muscle) were used to investigate temporal variation in diet among nine individual Baltic ringed seals (Phoca hispida botnica Gmelin) from the Bothnian Bay, northeast Baltic Sea. The isotope values from plasma should reflect the most recent diet, values from liver the diet of the past weeks prior to sampling, and values from muscle should integrate diet over almost the entire breeding season of the ringed seals. In general, δ¹³C values of liver were more enriched in ¹³C than were those of either muscle or plasma, suggesting that the diet of the seals may have included a higher proportion of ¹³C‐enriched benthic prey in April. Females showed more variable δ¹³C values than males, suggesting possible gender differences in diet or in foraging locations. The differences that were apparent between females possibly reflect individual variation in the onset and duration of parturition and lactation, both of which likely restrict female foraging. Previous data from parasite infections and from alimentary tract contents of the same seals were linked to the isotope data to assist in drawing inferences about changes in the diets of individual seals.     

Use of a δ13C–δ15N relationship to determine animal trophic positions in a tropical Australian estuarine wetland

Stable isotope composition of organisms from different trophic groups collected from a semi‐isolated wetland pool in the Ross River estuary, northern Australia, was analysed to determine if there was a consistent relationship between δ¹³C, δ¹⁵N and trophic level that could be used to assign trophic positions. A strong linear negative relationship between δ¹³C and δ¹⁵N was detected for the three trophic levels considered (primary producers, primary consumers and secondary consumers). This relationship was consistent among trophic levels, differing only in height, that is, on δ¹⁵N values, which indicate trophic positions. A difference of 3.6–3.8‰ between trophic levels was present, suggesting a δ¹⁵N fractionation of approximately 3.7‰, a value slightly higher than the commonly assumed δ¹⁵N fractionation of approximately 3.4‰. The relationship between δ¹³C and δ¹⁵N was similar for invertebrate and fish primary consumers, indicating similar δ¹⁵N trophic fractionation for both groups, meaning trophic positions and trophic length could be reliably calculated based on either invertebrates or fish.     

Trophic shift in young‐of‐the‐year Mugilidae during salt‐marsh colonization

This study investigated the trophic shift of young‐of‐the‐year (YOY) thinlip grey mullet Liza ramada and golden grey mullet Liza aurata during their recruitment in a salt marsh located on the European Atlantic Ocean coast. Stable‐isotope signatures (δ¹³C and δ¹⁵N) of the fishes followed a pattern, having enrichments in ¹³C and ¹⁵N with increasing fork length (L_F): δ¹³C in fishes < 30 mm ranged from ?19.5 to ?15.0‰, whereas in fishes > 30 mm δ¹³C ranged from ?15.8 to ?12.7‰, closer to the level in salt‐marsh food resources. Large differences between the δ¹⁵N values of mugilids and those of food sources (6·0‰ on average) showed that YOY are secondary consumers, similar to older individuals, when feeding in the salt marsh. YOY mugilids shift from browsing on pelagic prey to grazing on benthic resources from the salt marsh before reaching 30 mm L_F. The results highlight the role of European salt marshes as nurseries for juvenile mugilids.     

Effects of decomposition and storage conditions on the δ13C and δ15N isotope values of killer whale (Orcinus orca) skin and blubber tissues

Several different factors in the collection and preservation of whale skin and blubber samples were examined to determine their effect on the results obtained by stable nitrogen and carbon isotope (δ¹⁵N and δ¹³C) analysis. Samples of wet killer whale skin retained their original stable isotope values for up to 14 d at 4°C or lower. However, decomposition significantly changed the δ¹⁵N value within 3 d at 20°C. Storage at ?20°C was as effective as ?80°C for the preservation of skin and blubber samples for stable isotope analysis for at least a year. By contrast, once a skin sample had been freeze‐dried and lipid extracted, the stable isotope values did not change significantly when it was stored dry at room temperature for at least 12 mo. Preservation of whale skin samples for a month in DMSO‐salt solution, frozen or at room temperature, did not significantly change the δ¹⁵N and δ¹³C values of lipid extracted tissues, although the slight changes seen could influence results of a study if only small changes are expected.     

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.