Trophic ecology of Mexican Pacific harbor seal colonies using carbon and nitrogen stable isotopes.
ABSTRACT: There is limited information that provides a comprehensive understanding of the trophic ecology of Mexican Pacific harbor seal (Phoca vitulina richardii) colonies. While scat analysis has been used to determine the diet of some colonies, the integrative characterization of its feeding habits on broader temporal and spatial scales remains limited. We examined potential feeding grounds, trophic niche width, and overlap, and inferred the degree of dietary specialization using stable carbon and nitrogen isotope ratios (?13C and ?15N) in this subspecies. We analyzed ?13C and ?15N on fur samples from pups collected at five sites along the western coast of the Baja California Peninsula, Mexico. Fur of natal coat of Pacific harbor seal pups begins to grow during the seventh month in utero until the last stage of gestation. Therefore pup fur is a good proxy for the mother's feeding habits in winter (~December to March), based on the timing of gestation for the subspecies in this region. Our results indicated that the ?13C and ?15N values differed significantly among sampling sites, with the highest mean ?15N value occurring at the southernmost site, reflecting a well-characterized north to south latitudinal 15N-enrichment in the food web. The tendency identified in ?13C values, in which the northern colonies showed the most enriched values, suggests nearshore and benthic-demersal feeding habits. A low variance in ?13C and ?15N values for each colony (<1‰) and relatively small standard ellipse areas suggest a specialized foraging behavior in adult female Pacific harbor seals in Mexican waters.
Project description:BACKGROUND:Resolving the preferred prey items and dietary proportions of leopard seals is central to understanding food-web dynamics in the rapidly-warming Antarctic Peninsula region. Previous studies have identified a wide range of prey items; however, due to anecdotal or otherwise limited information, leopard seal diets remain unresolved by seal sex, individual, body size, region, and season. Over the 2013, 2014, and 2017 field seasons we collected scat, tissue samples (red blood cells and plasma; n?=?23) for stable isotope analyses, and previously-reported animal-borne video from 19 adult leopard seals foraging near mesopredator breeding colonies at Cape Shirreff, Livingston Island. We summarized a priori diet information from scat and video analysis and applied a three-isotope (?13C, ?15N, ?34S), four-source (fish, fur seal, krill, penguin) Bayesian mixing model to examine temporal variability in both prey sources and leopard seal tissues. RESULTS:The austral spring diets of males and females focused on Antarctic krill (31.7-38.0%), notothen fish (31.6-36.5%), and penguin (24.4-26.9%) and were consistent across all 3 years. Several lines of evidence suggest the transition to summer foraging was distinct for males and females. Female diets transitioned rapidly to higher ?15N values (+2.1‰), indicating increased consumption of penguin (29.5-46.2%) and energy-dense Antarctic fur seal pup (21.3-37.6%). CONCLUSIONS:The seasonal increase in leopard seal ?15N values, and thus fur seal in their diet, was predictably related to larger body size; it may also be forcing reductions to the largest Antarctic fur seal colony in the Antarctic Peninsula. Our ensemble sampling approach reduces historical biases in monitoring marine apex predator diets. Further, our results are necessary to best inform regional fisheries management planning.
Project description:Ecological diversity has been reported for killer whales (Orcinus orca) throughout the North Atlantic but patterns of prey specialization have remained poorly understood. We quantify interindividual dietary variations in killer whales (n = 38) sampled throughout the year in 2017-2018 in northern Norway using stable isotopic nitrogen (?15N: 15N/14N) and carbon (?13C: 13C/12C) ratios. A Gaussian mixture model assigned sampled individuals to three differentiated clusters, characterized by disparate nonoverlapping isotopic niches, that were consistent with predatory field observations: seal-eaters, herring-eaters, and lumpfish-eaters. Seal-eaters showed higher ?15N values (mean ± SD: 12.6 ± 0.3‰, range = 12.3-13.2‰, n = 10) compared to herring-eaters (mean ± SD: 11.7 ± 0.2‰, range = 11.4-11.9‰, n = 19) and lumpfish-eaters (mean ± SD: 11.6 ± 0.2‰, range = 11.3-11.9, n = 9). Elevated ?15N values for seal-eaters, regardless of sampling season, confirmed feeding at high trophic levels throughout the year. However, a wide isotopic niche and low measured ?15N values in the seal-eaters, compared to that of whales that would eat solely seals (?N-measured = 12.6 vs. ?N-expected = 15.5), indicated a diverse diet that includes both fish and mammal prey. A narrow niche for killer whales sampled at herring and lumpfish seasonal grounds supported seasonal prey specialization reflective of local peaks in prey abundance for the two fish-eating groups. Our results, thus, show differences in prey specialization within this killer whale population in Norway and that the episodic observations of killer whales feeding on prey other than fish are a consistent behavior, as reflected in different isotopic niches between seal and fish-eating individuals.
Project description:The measurement of bulk tissue nitrogen (?15N) and carbon isotope values (?13C) chronologically along biologically inert tissues sampled from offspring can provide a longitudinal record of their mothers' foraging habits. This study tested the important assumption that mother-offspring stable isotope values are positively and linearly correlated. In addition, any change in the mother-offspring bulk tissues and individual amino acids that occurred during gestation was investigated. Whiskers sampled from southern elephant seal pups (Mirounga leonina) and temporally overlapping whiskers from their mothers were analyzed. This included n?=?1895 chronologically subsampled whisker segments for bulk tissue ?15N and ?13C in total and n?=?20 whisker segments for amino acid ?15N values, sampled from recently weaned pups (n?=?17), juvenile southern elephant seals (SES)?<?2 years old (n?=?23) and adult female SES (n?=?17), which included nine mother-offspring pairs. In contrast to previous studies, the mother-offspring pairs were not in isotopic equilibrium or linearly correlated during gestation: the ?15N and ?13C mother-offspring offsets increased by 0.8 and 1.2‰, respectively, during gestation. The foetal bulk ?15N values were 1.7?±?0.5‰ (0.9-2.7‰) higher than mothers' ?15N values before birth, while the foetal ?13C increased by ~1.7‰ during gestation and were 1.0?±?0.5‰ (0.0-1.9‰) higher than their mothers' ?13C at the end of pregnancy. The mother-offspring serine and glycine ?15N differed by ~4.3‰, while the foetal alanine ?15N values were 1.4‰ lower than that of their mothers during the third trimester of pregnancy. The observed mother-offspring ?15N differences are likely explained by shuttling of glutamate-glutamine and glycine-serine amongst skeletal muscle, liver, placenta and foetal tissue. Foetal development relies primarily on remobilized endogenous maternal proteinaceous sources. Researchers should consider foetal physiology when using offspring bulk tissue isotope values as biomarkers for the mother's isotopic composition as part of monitoring programmes.
Project description:While sexual segregation is expected in highly dimorphic species, the local environment is a major factor driving the degree of resource partitioning within a population. Sexual and individual niche segregation was investigated in the Australian fur seal (Arctocephalus pusillus doriferus), which is a benthic foraging species restricted to the shallow continental shelf region of south-eastern Australia. Tracking data and the isotopic values of plasma, red blood cells and whiskers were combined to document spatial and dietary niche segregation throughout the year. Tracking data indicated that, in winter, males and females overlapped in their foraging habitat. All individuals stayed within central Bass Strait, relatively close (< 220 km) to the breeding colony. Accordingly, both genders exhibited similar plasma and red cell ?13C values. However, males exhibited greater ?13C intra-individual variation along the length of their whisker than females. This suggests that males exploited a greater diversity of foraging habitats throughout the year than their female counterparts, which are restricted in their foraging grounds by the need to regularly return to the breeding colony to suckle their pup. The degree of dietary sexual segregation was also surprisingly low, both sexes exhibiting a great overlap in their ?15N values. Yet, males displayed higher ?15N values than females, suggesting they fed upon a higher proportion of higher trophic level prey. Given that males and females exploit different resources (mainly foraging habitats), the degree of individual specialisation might differ between the sexes. Higher degrees of individual specialisation would be expected in males which exploit a greater range of resources. However, comparable levels of inter-individual variation in ?15N whisker values were found in the sampled males and females, and, surprisingly, all males exhibited similar seasonal and inter-annual variation in their ?13C whisker values, suggesting they all followed the same general dispersion pattern throughout the year.
Project description:Diet is a primary driver of the composition of gut microbiota and is considered one of the main routes of microbial colonization. Prey identification is fundamental for correlating the diet with the presence of particular microbial groups. The present study examined how diet influenced the composition and function of the gut microbiota of the Pacific harbor seal (Phoca vitulina richardii) in order to better understand the role of prey consumption in shaping its microbiota. This species is a good indicator of the quality of the local environment due to both its foraging and haul-out site fidelity. DNA was extracted from 20 fecal samples collected from five harbor seal colonies located in Baja California, Mexico. The V4 region of 16S rRNA gene was amplified and sequenced using the Illumina technology. Results showed that the gut microbiota of the harbor seals was dominated by the phyla Firmicutes (37%), Bacteroidetes (26%) and Fusobacteria (26%) and revealed significant differences in its composition among the colonies. Funtional analysis using the PICRUSt software suggests a high number of pathways involved in the basal metabolism, such as those for carbohydrates (22%) and amino acids (20%), and those related to the degradation of persistent environmental pollutants. In addition, a DNA metabarcoding analysis of the same samples, via the amplification and sequencing of the mtRNA 16S and rRNA 18S genes, was used to identify the prey consumed by harbor seals revealing the consumption of prey with mainly demersal habits. Functional redundancy in the seal gut microbiota was observed, irrespective of diet or location. Our results indicate that the frequency of occurrence of specific prey in the harbor seal diet plays an important role in shaping the composition of the gut microbiota of harbor seals by influencing the relative abundance of specific groups of gut microorganisms. A significant relationship was found among diet, gut microbiota composition and OTUs assigned to a particular metabolic pathway.
Project description:Stable isotope analysis (SIA) is an important tool for investigation of animal dietary habits for determination of feeding niche. Ideally, fresh samples should be used for isotopic analysis, but logistics frequently demands preservation of organisms for analysis at a later time. The goal of this study was to establish the best methodology for preserving forest litter-dwelling crickets for later SIA analysis without altering results. We collected two cricket species, Phoremia sp. and Mellopsis doucasae, from which we prepared 70 samples per species, divided among seven treatments: (i) freshly processed (control); preserved in fuel ethanol for (ii) 15 and (iii) 60 days; preserved in commercial ethanol for (iv) 15 and (v) 60 days; fresh material frozen for (vi) 15 and (vii) 60 days. After oven drying, samples were analyzed for ?15N, ?13C values, N(%), C(%) and C/N atomic values using continuous flow isotope ratio mass spectrometry. All preservation methods tested, significantly impacted ?13C and ?15N and C/N atomic values. Chemical preservatives caused ?13C enrichment as great as 1.5‰, and ?15N enrichment as great as 0.9‰; the one exception was M. doucasae stored in ethanol for 15 days, which had ?15N depletion up to 1.8‰. Freezing depleted ?13C and ?15N by up to 0.7 and 2.2‰, respectively. C/N atomic values decreased when stored in ethanol, and increased when frozen for 60 days for both cricket species. Our results indicate that all preservation methods tested in this study altered at least one of the tested isotope values when compared to fresh material (controls). We conclude that only freshly processed material provides adequate SIA results for litter-dwelling crickets.
Project description:There is great concern about the future of sharks in Ecuador because of the lack of biological knowledge of most species that inhabit the region. This paper analyzes the feeding behavior of the pelagic thresher shark (Alopias pelagicus), the blue shark (Prionace glauca) and the silky shark (Carcharhinus falciformis) through the use of stable isotopes of carbon and nitrogen (?13C and ?15N), with the aim of determining the degree of interaction between these species in the Galapagos Marine Reserve. No interspecific differences were found in use of oceanic vs. inshore feeding areas (?13C: Kruskal-Wallis test, p = 0.09). The position in the hierarchy of the food web where A. pelagicus feeds differed from that of the other species (?15N: Kruskal-Wallis test, p = 0.01). There were no significant differences in ?13C and ?15N values between males and females of the three species (Student's t-test, p > 0.05), which suggests that both sexes have a similar feeding behavior. A specialist strategy was observed in P. glauca (trophic niche breadth TNB = 0.69), while the other species were found to be generalist (A. pelagicus TNB = 1.50 and C. falciformis TNB = 1.09). The estimated trophic level (TL) varied between the three species. C. falciformis occupied the highest trophic level (TL = 4.4), making it a quaternary predator in the region. The results of this study coincide with the identified behavior in these predators in other areas of the tropical Pacific (Colombia and Mexico), and suggest a pelagic foraging strategy with differential consumption of prey between the three species. These ecological aspects can provide timely information when implementing in conservation measures for these shark species in the Tropical Pacific and Galapagos Marine Reserve.
Project description:Competition for resources within a population can lead to niche partitioning between sexes, throughout ontogeny and among individuals, allowing con-specifics to co-exist. We aimed to quantify such partitioning in Antarctic fur seals, Arctocephalus gazella, breeding at South Georgia, which hosts ~95% of the world's population. Whiskers were collected from 20 adult males and 20 adult females and stable isotope ratios were quantified every 5?mm along the length of each whisker. Nitrogen isotope ratios (?15N) were used as proxies for trophic position and carbon isotope ratios (?13C) indicated foraging habitat. Sexual segregation was evident: ?13C values were significantly lower in males than females, indicating males spent more time foraging south of the Polar Front in maritime Antarctica. In males ?13C values declined with age, suggesting males spent more time foraging south throughout ontogeny. In females ?13C values revealed two main foraging strategies: 70% of females spent most time foraging south of the Polar Front and had similar ?15N values to males, while 30% of females spent most time foraging north of the Polar Front and had significantly higher ?15N values. This niche partitioning may relax competition and ultimately elevate population carrying capacity with implications for ecology, evolution and conservation.
Project description:Knowing the migratory movements and behaviour of baleen whales is fundamental to understanding their ecology. We compared ?15N and ?13C values in the skin of blue (Balaenoptera musculus), fin (Balaenoptera physalus) and sei (Balaenoptera borealis) whales sighted in the Azores in spring with the values of potential prey from different regions within the North Atlantic using Bayesian mixing models to investigate their trophic ecology and migration patterns. Fin whale ?15N values were higher than those recorded in blue and sei whales, reflecting feeding at higher trophic levels. Whales' skin ?15N and ?13C values did not reflect prey from high-latitude summer foraging grounds; instead mixing models identified tropical or subtropical regions as the most likely feeding areas for all species during winter and spring. Yet, differences in ?13C values among whale species suggest use of different regions within this range. Blue and sei whales primarily used resources from the Northwest African upwelling and pelagic tropical/subtropical regions, while fin whales fed off Iberia. However, determining feeding habitats from stable isotope values remains difficult. In conclusion, winter feeding appears common among North Atlantic blue, fin and sei whales, and may play a crucial role in determining their winter distribution. A better understanding of winter feeding behaviour is therefore fundamental for the effective conservation of these species.
Project description:To complement literature-based historical knowledge of the eating habits of 17th- and 18th-century Japan, we analysed carbon and nitrogen isotope ratios (?13C and ?15N, respectively) of human hairs embedded in cover paper of Japanese books printed during 1690s-1890s, taking regional and temporal variations into consideration. We purchased 24 book sets from second-hand book markets. Twenty-three sets contained enough human hairs, which were non-destructively extracted from the thick, recycled paper of the book covers and used to measure the ?13C and ?15N values, found to be identical within each book set. Relatively low ?13C values and high ?15N values suggested that people depended on rice, C3 vegetables, and fish, more exclusively than contemporary Japanese people. The relatively high ?13C values found in Edo (Tokyo) might be associated with the preference for C4 millets by Edo people as a measure against beriberi (locally recognised as the Edo affliction). The ?15N values gradually increased over 200 years, indicating an increase in the contribution of marine fish both as food and fertiliser for rice fields as suggested by literature-based studies. Further collection of hairs from books will enable a thorough examination of regional and temporal variations to better understand the pre-globalised food culture.