Integrating stomach content and stable isotope analyses to quantify the diets of pygoscelid penguins

Stomach content analysis (SCA) and more recently stable isotope analysis (SIA) integrated with isotopic mixing models have become common methods for dietary studies and provide insight into the foraging ecology of seabirds. However, both methods have drawbacks and biases that may result in difficulties in quantifying inter-annual and species-specific differences in diets. We used these two methods to simultaneously quantify the chick-rearing diet of Chinstrap (Pygoscelis antarctica) and Gentoo (P. papua) penguins and highlight methods of integrating SCA data to increase accuracy of diet composition estimates using SIA. SCA biomass estimates were highly variable and underestimated the importance of soft-bodied prey such as fish. Two-source, isotopic mixing model predictions were less variable and identified inter-annual and species-specific differences in the relative amounts of fish and krill in penguin diets not readily apparent using SCA. In contrast, multi-source isotopic mixing models had difficulty estimating the dietary contribution of fish species occupying similar trophic levels without refinement using SCA-derived otolith data. Overall, our ability to track inter-annual and species-specific differences in penguin diets using SIA was enhanced by integrating SCA data to isotopic mixing modes in three ways: 1) selecting appropriate prey sources, 2) weighting combinations of isotopically similar prey in two-source mixing models and 3) refining predicted contributions of isotopically similar prey in multi-source models.     

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-isotope analysis of a deep-sea benthic-fish assemblage: evidence of an enriched benthic food web

In this study, fishes and invertebrates collected from the continental slope (1000 m) of the eastern North Pacific Ocean were analysed using stable-isotope analysis (SIA). Resulting trophic positions (T(P) ) were compared to known diets and habitats from the literature. Dual isotope plots indicated that most species groups (invertebrates and fishes) sorted as expected along the carbon and nitrogen axes, with less intraspecific variability than interspecific variability. Results also indicated an isotopically distinct benthic and pelagic food web, as the benthic food web was more enriched in both nitrogen and carbon isotopes. Trophic positions from SIA supported this finding, resulting in the assignment of fishes to different trophic positions from those expected based on published dietary information. These differences can be explained largely by the habitat of the prey and the percentage of the diet that was scavenged. A mixing model estimated dietary contributions of prey similar to those of the known diet of Bathyraja trachura from stomach-content analysis (SCA). Linear regressions indicated that trophic positions calculated from SIA and SCA, when plotted against B. trachura total length for 32 individuals, exhibited similar variation and patterns. Only the T(P) from SCA yielded significant results (stomach content: P < 0·05, stable isotope: P > 0·05).     

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     

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.     

Fine‐scale spatial differences in humpback whale diet composition near Kodiak,Alaska

On the North Pacific feeding grounds, humpback whales (Megaptera novaeangliae) are recovering from commercial whaling at a rapid rate (6.8%). The potential effect that this recovery will have on trophic dynamics involving these predators is currently unknown. To better elucidate complex trophic dynamics, variability in diet composition of apex predators on their respective feeding grounds needs to be understood. Thus, we explored the diet composition of two defined subaggregations of humpback whales of the Kodiak Archipelago population (“North,” “South”) using stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios of humpback whale skin and regional prey samples in Bayesian dietary mixing models. Humpback whales in the “North” region consumed proportionally more fish, dominated by capelin (Mallotus villosus), whereas, whales in the “South” region consumed predominantly krill. The difference in diet composition appears to reflect regional differences in prey availability. Thus, regional variability in diet composition by humpback whales may have disproportionate impacts on prey resources of sympatric predators depending on available prey biomass. As a result, we suggest fine‐scale studies of apex predator diets are needed to better model trophic dynamics with accuracy.     

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.     

Feeding ecology of Kelp Gulls (Larus dominicanus) in marine and limnetic environments

The diet of the Kelp Gull (Larus dominicanus) was investigated at the northern limit of its distribution along the South American Atlantic coast. We used two complementary methods, pellet analysis and stable isotope analysis (SIA), to describe and compare Kelp Gull feeding ecology in freshwater and marine environments. The assimilated diet over two different time scales was investigated via SIA of plasma and red blood cells, blood components with different turnover rates. Fish composed the bulk of the diet of Kelp Gulls in both marine (White Croaker Micropogonias furnieri and Banded Croaker Paralonchurus brasiliensis) and limnetic areas (Armoured Catfish Loricariidae and La Plata Croaker Pachyurus bonariensis), despite the importance of benthic prey from the intertidal zone in samples collected from the marine environment (Wedge Clam Donax hanleyanus and the Yellow Clam Mesodesma mactroides). The fish consumed by the gulls were common discards from fisheries in both environments, and marine bivalves were found at a high density at the marine beach. Diet varied between the different time scales analysed. Conventional diet data generally agreed with stable isotope model estimates, emphasising the importance of using complementary approaches in dietary studies.     

Evidence of two subaggregations of humpback whales on the Kodiak,Alaska, feeding ground revealed from stable isotope analysis

Knowledge of humpback whale (Megaptera novaeangliae) foraging on feeding grounds is becoming increasingly important as the growing North Pacific population recovers from commercial whaling and consumes more prey, including economically important fishes. We explored spatial and temporal (interannual, within‐season) variability in summer foraging by humpback whales along the eastern side of the Kodiak Archipelago as described by stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios of humpback whale skin (n = 118; 2004–2013). The trophic level (TL) of individual whales was calculated using basal food web δ¹⁵N values collected within the study area. We found evidence for the existence of two subaggregations of humpback whales (“North,” “South”) on the feeding ground that fed at different TLs throughout the study period. Linear mixed models suggest that within an average year, Kodiak humpback whales forage at a consistent TL during the feeding season. TL estimates support mixed consumption of fish and zooplankton species in the “North” (mean ± SE; 3.3 ± 0.1) and predominant foraging on zooplankton in the “South” (3.0 ± 0.1). This trend appears to reflect spatial differences in prey availability, and thus, our results suggest North Pacific humpback whales may segregate on feeding aggregations and target discrete prey species.     

脂类抽提对北太平洋柔鱼肌肉碳、氮稳定同位素测定结果的影响

稳定同位素技术可深层次地分析头足类的摄食习性和栖息地等方面的重要信息.稳定同位素分析多选取动物肌肉为研究组织,然而,肌肉中的脂类含量会影响对其稳定同位素信息的准确解析.北太平洋柔鱼是重要的大洋经济性头足类,在海洋生态系统中具有重要的生态地位.本研究选取了53尾北太平洋柔鱼,通过分析脂类抽提对胴体肌肉稳定同位素比值的影响,探讨脂类物质对稳定同位素比值的干扰机制,对比不同碳稳定同位素比值校正模型的适用性,并提出适用于北太平洋柔鱼胴体肌肉δ¹³-C校正模型.结果表明： 北太平洋柔鱼肌肉的脂类抽提会引起稳定同位素测定结果的显著变化,δ¹³-C和δ¹⁵-N分别平均升高0.71‰和0.47‰,在分析组织碳稳定同位素比值时,脂类抽提在样品预处理过程中十分必要,其能消除脂类对样品稳定同位素分析的干扰,从而更加准确地分析研究对象食性与栖息地的变化.在单独进行氮稳定同位素比值分析时,则不需要进行脂类抽提.     

生物考古学视野下人类的牙齿与饮食

饮食在人类文化的起源与演变中扮演着重要角色,而牙齿作为饮食信息的主要载体,从不同角度为复原古代人类饮食谱系提供了可能。作为人类咀嚼行为、饮食结构及生业模式的直接反映,口腔疾病和牙齿表面微观痕迹的体质人类学分析显得尤其重要。牙齿中微量元素和稳定同位素的检测,对于揭示古代人类的迁徙与流动、生命在不同阶段的摄食行为与营养状况、农业的起源与发展等问题具有着不可估量的作用。本文在回顾考古遗址中出土人类牙齿的重要性及研究方法的多样性基础上,结合牙齿的生长发育特征,绘制了不同牙位各生长序列所匹配的人类年龄图谱,并参照此图谱,提出了全新的牙本质连续切片取样方法。本文构想从体质人类学与碳、氮等稳定同位素分析的角度出发,以牙本质的连续切片、肢骨及肋骨等人体组织为关联式地研究材料,在探索古人类个体生命史的基础上展望“代际考古”。     

Experimental study on the effect of diet on fatty acid and stable isotope profiles of the squid Lolliguncula brevis

Fatty acid and stable isotope analyses have previously been used to investigate foraging patterns of fish, birds, marine mammals and most recently cephalopod species. To evaluate the application of these methods for dietary studies in squid, it is important to understand the degree to which fatty acid and stable isotope signatures of prey species are reflected in the squids' tissue. Four groups of Lolliguncula brevis were fed on prey species with distinctly different fatty acid and stable isotope profiles over 30 consecutive days. One group of squid were fed fish for fifteen days, followed by crustaceans for a further fifteen days. A second and third group were fed exclusively on fish or crustaceans for thirty days. And a fourth group was fed on a mixture of fish and crustaceans for thirty days. Analysis of squid tissue showed that, after 10 days of feeding, fatty acid profiles of squid tended to reflect those of their prey. Squid that fed on a single prey type, i.e. fish or crustacean, showed only minor modifications in fatty acid proportions after the initial change and fatty acid profiles were clearly distinguishable between the two feeding groups. Shifts in fatty acid proportions towards respective prey profiles could clearly be observed in squid the diet of which was swapped after 15 days. Clear differences could also be seen in fatty acid profiles of squid feeding on a mixed diet with trends towards either fish or crustacean fatty acid signatures. Stable isotope signatures of squid tissues clearly distinguished between animals feeding on different diets and supported findings from fatty acid analysis, thus indicating both methods to be viable tools in feeding studies on squid species.     

Merging resource availability with isotope mixing models: the role of neutral interaction assumptions

Background

Bayesian mixing models have allowed for the inclusion of uncertainty and prior information in the analysis of trophic interactions using stable isotopes. Formulating prior distributions is relatively straightforward when incorporating dietary data. However, the use of data that are related, but not directly proportional, to diet (such as prey availability data) is often problematic because such information is not necessarily predictive of diet, and the information required to build a reliable prior distribution for all prey species is often unavailable. Omitting prey availability data impacts the estimation of a predator''s diet and introduces the strong assumption of consumer ultrageneralism (where all prey are consumed in equal proportions), particularly when multiple prey have similar isotope values.

Methodology

We develop a procedure to incorporate prey availability data into Bayesian mixing models conditional on the similarity of isotope values between two prey. If a pair of prey have similar isotope values (resulting in highly uncertain mixing model results), our model increases the weight of availability data in estimating the contribution of prey to a predator''s diet. We test the utility of this method in an intertidal community against independently measured feeding rates.

Conclusions

Our results indicate that our weighting procedure increases the accuracy by which consumer diets can be inferred in situations where multiple prey have similar isotope values. This suggests that the exchange of formalism for predictive power is merited, particularly when the relationship between prey availability and a predator''s diet cannot be assumed for all species in a system.     

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.     

Comparing dietary preferences of Bufflehead ducklings in Western Canada through gut content and stable isotope analysis

Aquatic invertebrates are essential for duckling growth and development. We present results on the trophic status and dietary analysis of Bufflehead (Bucephala albeola) ducklings from the boreal breeding grounds of western Canada. We estimated dietary preference by comparing invertebrates found in Bufflehead diets to those identified in standardized dip net samples at their wetland feeding sites. Stable isotope ratios of Bufflehead and their prey were used as a second estimator of trophic position. Bufflehead ducklings preferentially foraged for larval Dytiscidae (predaceous diving beetles; 46% of total dietary biomass), Zygoptera larvae (damselflies; 14%) and non-Dytiscidae adult Coleoptera (5%; mainly Haliplidae). Results from stable isotope analyses supported these results; the separation between primary and secondary invertebrate consumers and ducklings was significant for all possible contrasts when considering nitrogen isotope ratios (Tukey HSD; P < 0.001). We iteratively explored all possible combinations of δ¹⁵N and δ¹³C data to generate a proportional range over which each food source may contribute to Bufflehead stable isotope signatures; these results suggested larval Zygoptera and larval Dytiscidae figure prominently in diets when accounting for isotope fractionation. The incorporation of prey availability into the metric of dietary preference, as opposed to the tabulation of ingested items alone, reduces the importance of invertebrate groups such as adult Dytiscidae as highlighted in previous studies.     

Ecotype and geographical variation in carbon and nitrogen stable isotope values in western North Pacific killer whales (Orcinus orca)

Killer whales are top predators in marine trophic chains, and therefore their feeding preferences can substantially affect the abundance of species on the lower trophic levels. Killer whales are known to feed on many different types of prey from small fish to large whales, but a given killer whale population usually focuses on a specific type of prey. Stable isotope analysis is widely used to study whale diets, because direct observations are often impossible. Killer whale feeding habits in the western North Pacific are poorly studied, and the large-scale stable isotope analysis provides a unique opportunity to gain insights into the trophic links of this top predator. In this study, we compare the δ¹³C and δ¹⁵N stable isotope values from killer whale skin samples obtained in different areas of the western North Pacific from fish-eating (R-type) and mammal-eating (T-type) killer whale ecotypes. The effect of ecotype was highly significant: both carbon and nitrogen stable isotope values were lower in R-type whales than in T-type whales. The geographical variation also affected killer whale stable isotope values due to both the differences in killer whale diet and the variation in baseline stable isotope values across the study areas.     

Foraging ecology and niche overlap in pygmy (Kogia breviceps) and dwarf (Kogia sima) sperm whales from waters of the U.S. mid‐Atlantic coast

A complementary approach of stomach content and stable isotope analyses was used to characterize the foraging ecology and evaluate niche overlap between pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales stranded on the U.S. mid‐Atlantic coast between 1998 and 2011. Food habits analysis demonstrated both species were primarily teuthophagous, with 35 species of cephalopods, and 2 species of mesopelagic fishes represented in their overall diets. Pianka's Index of niche overlap suggested high overlap between whale diets (O_n = 0.92), with squids from the families Histioteuthidae, Cranchidae, and Ommastrephidae serving as primary prey. Pygmy sperm whales consumed slightly larger prey sizes (mean mantle length [ML] = 10.8 cm) than dwarf sperm whales (mean ML = 7.8 cm). Mean prey sizes consumed by pygmy sperm whales increased with growth, but showed no trend in dwarf sperm whales. Significant differences were not detected in δ¹⁵N and δ¹³C values of muscle tissues from pygmy (10.8‰ ± 0.5‰, ?17.1‰ ± 0.6‰), and dwarf sperm whales (10.7‰ ± 0.5‰, ?17.0‰ ± 0.4‰), respectively. Isotopic niche widths also did not differ significantly and dietary overlap was high between the two species. Results suggest the feeding ecologies of the pygmy and dwarf sperm whales are similar and both species occupy equivalent trophic niches in the region.     

The carbon isotope ecology and diet of Australopithecus africanus at Sterkfontein,South Africa

The stable carbon isotope ratio of fossil tooth enamel carbonate is determined by the photosynthetic systems of plants at the base of the animal's foodweb. In subtropical Africa, grasses and many sedges have C(4)photosynthesis and transmit their characteristically enriched 13C/(12)C ratios (more positive delta13C values) along the foodchain to consumers. We report here a carbon isotope study of ten specimens of Australopithecus africanus from Member 4, Sterkfontein (ca. 2.5 to 2.0Ma), compared with other fossil mammals from the same deposit. This is the most extensive isotopic study of an early hominin species that has been achieved so far. The results show that this hominin was intensively engaged with the savanna foodweb and that the dietary variation between individuals was more pronounced than for any other early hominin or non-human primate species on record. Suggestions that more than one species have been incuded in this taxon are not supported by the isotopic evidence. We conclude that Australopithecus africanus was highly opportunistic and adaptable in its feeding habits.     

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.