Larger Isotopic Niche Research Articles

Ontogenetic shifts in feeding habits of orangemouth weakfish (Cynoscion xanthulus): From estuarine benthic feeder to marine nektivore top predator

AbstractOrangemouth weakfish is a highly important commercial and game fish species in the Tropical Eastern Pacific. However, studies assessing changes in its trophic ecology relevant to size, habitat (estuarine or marine), sex, and season are non‐existent. We assessed its feeding habits and trophic position (TP) according to the aforementioned factors. Its dietary breadth was composed of 33 types of prey. The feeding habits and isotopic values (δ15N and δ13C) registered differences between sizes and habitat, indicating ontogenetic changes in habitat as well as TP, as small organisms (TP 3.1) inhabited the estuarine area, feeding on benthic and demersal organisms, whilst large individuals (TP 4.1) were found in the marine environment, preying on nektonic fauna. No differences were found according to sex or season. The isotopic niche showed that the different sizes of this species are generalist consumers with a clear separation between them and between habitats, as estuarine juvenile and preadult organisms have a larger isotopic niche than adult marine organisms, being an opportunist tertiary estuarine consumers during their early stages, and a top marine predator during the adult phase.

Isotopic (δ15N and δ13C) profiles in dentine indicate sex differences and individual variability in resource use among narwhals (Monodon monoceros)

AbstractWhile the impact of ecological changes on Arctic marine mammals remains largely unknown, quantifying variability in resource use among conspecifics can serve as an indication of adaptive plasticity and help assess species vulnerability to climate‐induced changes. We measured within‐tooth δ15N and δ13C profiles of narwhals (Monodon monoceros) sampled 35 years apart from Baffin Bay, Nunavut, Canada, to evaluate intrapopulation variation in narwhal resource use. Narwhal dentine δ15N and δ13C were lower post‐1983 (n = 7) by 0.83‰ and 0.52‰, respectively, compared to pre‐1983 (n = 13), but isotopic niche breadth increased from 0.95‰2 to 1.12‰2. Between sexes, the isotopic niche of males expanded from pre‐1983 to post‐1983 while that of females decreased, which may indicate increased flexibility in resource use for males and the opposite for females. Additional isotopic variation among narwhals (n = 23) suggests individual differences in diet composition and/or foraging location during both periods. Individual isotopic niches ranged between 0.08‰2 and 2.27‰2, where a larger isotopic niche likely indicates more variable foraging strategies over time. Narwhal as a species is considered vulnerable to ecosystem change; however, the variability in resource use between sexes and among individuals suggests narwhals may vary in their adaptability to changing Arctic conditions.

Size‐based differences in isotopic niche width (δ13C and δ15N) of green turtles (Chelonia mydas) nesting on Príncipe Island, Gulf of Guinea

AbstractWithin the same population, nesting green turtles (Chelonia mydas) might exploit different niches by exhibiting polymorphic foraging strategies and/or inhabiting geographically distinct foraging areas. This is crucial information for the conservation of this species. Here, we used stable carbon and nitrogen isotope ratios (δ13C and δ15N) to test for differences in a population of green turtles nesting on Príncipe Island (1°37′N; 7°24҆′E), Central Africa. A total of 60 nesting females were sampled on the two main nesting beaches of the island in December 2012. Minimum curved carapace length (CCL) was recorded, and δ13C and δ15N values were measured in the epidermis of each individual. Overall, CCL varied from 87.0 to 108.0 cm (mean ± SD =100.0 ± 5.1), δ13C values from −19.4 to −8.6‰ (−17.3 ± 1.8) and δ15N values from 7.9 to 17.3‰ (13.6 ± 1.5). Despite the large variation in both isotopic ratios, their distributions were unimodal, showing an absence of polymorphic foraging strategies and isotopically distinct foraging areas. However, smaller females (< median, 100.8 cm) occupied a much larger isotopic niche (i.e., four times greater) than larger females. These results suggest that nesting green turtles may forage opportunistically on the resources available in each of their foraging home ranges, with smaller females venturing to more isotopic‐diversified areas and/or exhibiting broader foraging strategies than larger females. In addition, and in accordance with other studies, findings suggest that the foraging grounds used by the Príncipe green turtle nesting females are distributed mainly throughout the Gulf of Guinea.

Isotopic evidence for intersexual foraging variation in northern elephant seals from Baja California, Mexico

Intersexual foraging variation is a strategy that is usually adopted by sexually dimorphic species. The northern elephant seal (Mirounga angustirostris) exhibits one of the most pronounced examples of sexual dimorphism in pinnipeds and segregation in this species’ migration toward foraging grounds in the North Pacific is well-documented for colonies in California; however, we lack comparable data from colonies in Mexico. The aim of this study was to use stable isotope analysis (δ13C and δ15N) to evaluate diet variation between adult males and females from a Mexican colony. Fur was collected from adult males (N = 15) and pups (N = 42) on the San Benito Archipelago during the 2012 molting season (summer) and the 2013 breeding season (winter). Values for adult females were inferred from their pups, based on previous studies. The resulting δ13C and δ15N values were significantly lower than those of their male counterparts. This difference may be due to the higher trophic position and a probable consumption of benthic prey (δ15N) by males, and the use of mostly oceanic habitats (δ13C) by adult females; the latter also had a larger isotopic niche than that of the former, as a probable consequence of more variable foraging grounds. Although our results are not the reflection of a simultaneous segregation, we obtained solid evidence on intersexual variation, which is common among dimorphic pinnipeds, particularly in terms of trophic position and habitat use, which tend to be the result of a marked difference in body mass and different energetic requirements.

Environmentally driven sexual segregation in a marine top predator

Sexual segregation in foraging occurs in many animal species, resulting in the partitioning of resources and reduction of competition between males and females, yet the patterns and drivers of such segregation are still poorly understood. We studied the foraging movements (GPS-tracking), habitat use (habitat modelling) and trophic ecology (stable isotope analysis) of female and male Cory’s shearwaters Calonectris borealis during the mid chick-rearing period of six consecutive breeding seasons (2010–2015). We found a clear sexual segregation in foraging in years of greater environmental stochasticity, likely years of lower food availability. When food became scarce, females undertook much longer foraging trips, exploited more homogeneous water masses, had a larger isotopic niche, fed on lower trophic level prey and exhibited a lower body condition, when compared to males. Sexual competition for trophic resources may be stronger when environmental conditions are poor. A greater foraging success of one sex may result in differential body condition of pair mates when enduring parental effort, and ultimately, in an increased probability of breeding failure.

The trophic structure of fish communities from streams in the Brazilian Cerrado under different land uses: an approach using stable isotopes

The aim of this study was to evaluate how distinct land uses can alter the trophic structure of fish communities in streams. For this purpose, nine streams under the influence of three distinct land uses (pasture, sugarcane, and natural cover) were evaluated. The structure and isotopic niche of the fish communities were investigated by calculating descriptive community-wide metrics based on stable isotopes of δ13C and δ15N. The largest isotopic niche was observed in fish communities in pasture streams, and the smallest in sugarcane streams. Pasture streams exhibited greater ranges of carbon sources exploited by fishes, higher trophic diversity, and lower trophic redundancy. In contrast, sugarcane streams had greater ranges of nitrogen exploited by fishes and showed the smaller trophic diversity, higher trophic redundancy, and uniformity. Sugarcane was also the only land use that exerted a negative influence on community isotopic niche width. Natural cover streams in turn, showed lower trophic uniformity and exhibited intermediate values for all remaining metrics. We conclude that fish communities residing in streams influenced by pastures displayed characteristics that led to greater trophic diversity, and fish communities influenced by sugarcane plantations were more negatively affected by this land use.

Feeding ecology and niche overlap of Lake Ontario offshore forage fish assessed with stable isotopes

The forage fish communities of the Laurentian Great Lakes continue to experience changes that have altered ecosystem structure, yet little is known about how they partition resources. Seasonal, spatial, and body size variation in δ13C and δ15N was used to assess isotopic niche overlap and resource and habitat partitioning among the five common offshore Lake Ontario forage fish species (n = 2037; alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), round goby (Neogobius melanostomus), and deepwater (Myoxocephalus thompsonii) and slimy (Cottus cognatus) sculpins). Round goby had the largest isotopic niche (6.1‰2, standard ellipse area), followed by alewife (3.4‰2), while rainbow smelt, slimy sculpin, and deepwater sculpin had the smallest and similar niche size (1.7‰2–1.8‰2), with only the sculpin species showing significant isotopic niche overlap (>63%). Stable isotopes in alewife, round goby, and rainbow smelt varied with location, season, and size, but did not vary in the sculpin species. Lake Ontario forage fish species have partitioned food and habitat resources, and non-native alewife and round goby have the largest isotopic niche, suggestive of a boarder ecological niche, and may contribute to their current high abundance.

Seasonal Fisheries Changes in Low-Rainfall Mangrove Ecosystems of Iran

To determine the role of mangroves for fisheries in the arid region of the Persian Gulf, we investigated fish community structure and trophic diversity in intertidal creeks with and without mangroves. Fish community abundances and biomass were compared across habitats and seasons. To identify variations in overall community trophic niches among habitats and seasons, we measured niches with size-corrected standard ellipse areas (SEAc) calculated from C and N stable isotope values. Although there was a slightly greater species richness occurred in mangrove creeks, we found a general similarity in the diversity patterns in creeks with and without mangroves. Also, there were no consistent differences in fish abundance or biomass for mangrove vs. non-mangrove fish collections. Community trophic diversity measured as SEAc also showed no significant difference between mangrove and non-mangrove sites. Instead, strong seasonal patterns were observed in the fish assemblages. Winter samples had consistently higher fish abundance and biomass than summer samples. Winter SEAc values were significantly higher, indicating that the fish community had a larger isotopic niche in winter than summer. Overall, we found that seasonality was much stronger than habitat in determining fish community structure and trophic diversity in the mangrove and non-mangrove ecosystems of Qeshm Island, Iran.