Trophic Overlap Research Articles

Trophic strategies of two sympatric endemic pigeons in insular ecosystems: a framework for understanding spatiotemporal frugivory interactions

Pigeons are considered to play key ecological roles in frugivory and seed dispersal. They have colonised numerous oceanic islands and diversified into several species in sympatry. How these species coexist in similar niches is poorly understood although dietary separation is among the mechanisms suggested to avoid trophic overlap. We investigated the trophic ecology of the two endemic Columba species co‐occurring in the laurel forest and thermosclerophyllous relicts of two of the Canary Islands. This study includes diet description in spatiotemporal terms, its relationship with fruit availability and seed treatment in 10 study areas established on La Palma and La Gomera. We used non‐invasive DNA analysis to identify the faeces of the two congeneric species and microhistological methods to examine their diets. The degree of trophic overlap was evaluated by niche similarity and breadth indices. Molecular faecal sampling determined the spatiotemporal distribution of both pigeons to identify their areas of coexistence. These frugivorous pigeons' diets did not differ concerning the main plant species, but they diverged quantitatively in the proportions and parts of plants consumed. Lauraceae fruits were their staple foods although Rhamnaceae and some Fabaceae and Solanaceae were also important. Both pigeons showed selective preferences for some fruits. Significant spatiotemporal variations in their diets were observed along with a general tendency to increase fruit intake at its ripening times. Our results suggest that different trophic strategies facilitate the coexistence of these frugivorous columbids. These pigeons act as seed dispersers and/or predators depending on seed features (size and hardness), and this may have valuable implications for their conservation.

Resource use and the impacts of fisheries on two sympatric sea snake species on the west coast of India

We investigated differences in resource use between two sympatric sea snake species from the west coast of India, Hydrophis curtus and H. schistosus, and described the impacts of fishing on the interactions of these species. We compared habitat use, diet, and isotopic niche width between species to determine resource overlap. We then compared trophic overlap of each species with the fisheries in the region and tested the effect of fishing intensity on their isotopic niche width. Hydrophis curtus used deeper habitats than H. schistosus, resulting in increased spatial overlap with fisheries. The two species also had distinct trophic niches and H. curtus prey formed a larger proportion of fishery catch on average than H. schistosus. This greater overlap could make H. curtus more vulnerable to the effects of fisheries. Both species exhibited expansion in short-term and long-term isotopic niche width along a gradient of fishing intensity which may indicate behavioural changes associated with the presence of fisheries. Hydrophis curtus is a trophic generalist, competes with syntopic species and is dominant in most assemblages. However, H. schistosus exhibits higher plasticity in resource use and may have an advantage over H. curtus. Sea snakes play an important role as mesopredators and as intermediate links in coastal marine food webs. Thus, fishing could alter the relative abundance of these mesopredators with cascading effects through coastal food webs.

Stable Isotope Analysis of Juvenile White Sharks Inside a Nursery Area Reveals Foraging in Demersal-Inshore Habitats and Trophic Overlap With Sympatric Sharks

Knowledge about top predators’ trophic ecology is crucial for defining their role in ecosystems, understanding habitat preferences, characterizing life stage-specific feeding habits, and evaluating their interaction with fisheries. In the northeastern Pacific, white sharks (Carcharodon carcharias) occupy coastal habitats during the early life stages, including Bahía Sebastián Vizcaíno (BSV) in Mexico, which is a known nursery area. Although BSV presumably provides high prey abundance, the trophic ecology of immature white sharks is poorly understood. Carbon and nitrogen bulk stable isotope analyses (SIA) were used to explore the trophic relationship of early life stages with their potential prey and to infer dietary overlap with sympatric sharks, while SIA of amino acids were used to estimate trophic position. Muscle samples from young white sharks and inshore demersal prey commonly found in their stomach contents were sampled. Demersal prey and literature-derived isotope ratios for pelagic and offshore species were incorporated into mixing models with a Bayesian framework to estimate their contribution to white shark tissues. Nearshore demersal prey had the highest contribution for all life stages (between 35 and 47%), consistent with previous reports based on gut content analysis. The contribution of pelagic (between 26 and 37%) and offshore (between 14 and 32%) prey was smaller and suggests potential periodic changes in foraging grounds and the presence of a maternal-derived isotopic signature. A high contribution of demersal prey indicates a high level of interaction with local fisheries that target those species and catch white sharks incidentally and is consistent with immature shark movement patterns. Isotope ratios of two sympatric sharks, smooth hammerhead Sphyrna zygaena and copper sharks Carcharhinus brachyurus, were used to estimate the overlap in isotopic niche space. Immature white sharks had the smallest isotopic niche, while the highest was for copper sharks. Overlap was greatest between white sharks and hammerheads (∼45%), while overlap with copper sharks was limited (<20%), suggesting less potential for competition. Trophic position estimates were similar to those previously reported for the species. These results highlight the importance of coastal demersal prey heavily targeted by local fisheries for the diet of young white sharks and support the importance of BSV as a nursery habitat.

Invasive fish keeps native feeding strategy despite high niche overlap with a congener species

Weakfish Cynoscion regalis (Bloch and Schneider, 1801) is one of the most recent invasive fish in the Iberian Peninsula (Europe). Weakfish has established in the Sado estuary (Portugal) since the early 2010s, and fishers and anglers have expressed concern about its impacts on native prize fish. However, almost a decade later, there is no information on the ecology of weakfish in the non-native area. So, we aimed to assess weakfish feeding strategy and feeding plasticity through stomach content analysis to evaluate if these factors may contribute to its invasiveness, as well as to determine the ecological overlap between weakfish and three native prize fish — European bass, white seabream, and particularly meagre (since they are taxonomically closer), through carbon and nitrogen stable isotopes. Our results demonstrate that Sado’s weakfish has a generalist feeding strategy and preys the same functional groups it targets in the native area, therefore feeding strategy may weigh on invasiveness but not feeding plasticity. Weakfish, meagre, and European bass were in the same trophic level and weakfish exhibited higher trophic overlap with meagre, suggesting that weakfish could directly impact meagre if food and habitat become limiting. This study is the first assessment about weakfish ecology in the non-native area and our findings are an excellent starting point to understand this invasion. It can also be useful for management programmes that promote weakfish consumption to minimize its impacts, alleviate fishing pressure on native species, and raise public awareness.

The best defence is not being there: avoidance of larger carnivores is not driven by risk intensity

AbstractSpecies interactions are key factors determining the distribution and structure of species assemblages. Owing to their central positions, mid‐ranking mammalian carnivores are involved in interactions with numerous species, including competition for resources and instances of killing by higher ranking predators. Lethal interactions can directly influence species’ demography. However, the fear of lethal interactions, competition or both may also affect when and where individuals are active (i.e. non‐lethal interactions). Although differences in body–size and trophic overlap are known predictors of the frequency of lethal interactions, their influence on non‐lethal interactions is uncertain. Through camera trapping, we studied non‐lethal interactions between a small mesocarnivore (pine marten), a potential killer and intense competitor (red fox) and a moderate competitor and unlikely killer (Eurasian badger). We determined overlap and differences in their diel activity patterns and the degree of spatial overlap in two seasons with contrasting resource availability. Additionally, we estimated the effect of larger carnivore detection rates on pine marten detection rates. We also compared time intervals between pine marten visits to baited stations in the absence and presence of either or both larger carnivores. Our results are consistent with pine martens distributing their daily activity to maximize overlap with prey and to minimize competition and risk of aggression over the spatial scale. Pine martens also responded to the immediate threat of larger carnivores irrespective of the threat they pose by taking 4–7 days longer to re‐visit a station. Small‐scale non‐lethal interactions such as these may enable pine martens to coexist closely with two larger carnivores, yet it remains uncertain whether their population incurs a demographic cost through restricted access to resources. Carnivore's risk‐avoidance strategies could be harnessed to protect prey species of interest. However, our results suggest avoidance is short‐lived and recurrent stimuli would be necessary.

New method of isotopic analysis: baseline‐standardized isotope vector analysis shows trophic partitioning in loricariids

AbstractStable isotope analyses have refined the study of trophic niche diversity within an ecosystem, yet traditional trophic partitioning methods may not be appropriate to identify variation among groups with similar dietary requirements. By building on vector‐based analyses, we introduce a baseline‐standardized isotopic vector analysis (BaSIVA) to visualize dietary variation while accounting for isotopic discrepancies between locations. To test the effectiveness of our new method, we collected muscle samples from eleven species of Loricarioidea in five assemblages in Northern Peru. Loricarioidea is a large, ecomorphologically diverse superfamily of scraping‐feeding fishes. Most feed on an indistinguishable mix of detritus and algae, but some lineages have specialized diets of wood, seeds, and macroinvertebrates, making them an excellent group to study trophic variation. Isotopic data were collected using mass spectrometric isotope analyses, and communities were standardized by calculating a mean baseline (algae and periphyton) for each location. The entire community was shifted by subtracting the baseline of 15N and 13C from the consumers at each location, which allowed for comparison between assemblages. Incremental differences of 15N and 13C from the baseline were found via vector analysis, and the azimuth and module of each consumer were calculated. Standardization resulted in a significant shift of assemblages within the isotopic biplot, and vector analysis shows three trophic groups primarily described by differences in carbon assimilation. Isotopic variation between species may account for some diversification in jaw shape within the Loricarioidea, but BaSIVA suggests several instances of trophic overlap in different jaw morphologies. Moreover, results from BaSIVA are better able to delineate trophic groups than traditional trophic positioning methods while accounting for variation in basal resources. We suggest a baseline‐standardized vector analysis should be the standard for vector‐based stable isotope analysis in riverine environments with similar baseline resources.

Ontogenetic shifts in natural diet, chelae, and mandibles of the omnivorous freshwater crab Aegla uruguayana: linking morphology and function

We analyze whether ontogenetic diet change in Aegla uruguayana Schmitt, 1942 is related to the morphological changes in chelae and mandibles. We use a combination of scanning electron microscope (SEM) observation, dietary composition analysis (stomach observation, trophic niche amplitude, trophic overlap, and feeding strategy), and geometric morphometric tools (discriminant analysis and two-block partial least squares analysis). The two structures analyzed by SEM show differences between juvenile and adult specimens, both in their cuticle and morphology specializations. In juveniles, the absence of the lobular tooth is the notable feature. However, in adult sizes, it is marked development. Both groups have simple setae with infracuticular articulation and denticles with different arrangements between groups. In the incisive mandible process, the teeth and spines present sharper points in juveniles and considerable wear in adults. Aegla uruguayana presented two trophic strategies: a profile of more predatory traits when younger and mostly detritivorous when adults. The analysis of geometric morphometry substantiates the patterns observed in SEM and stomach contents analysis. The morphology of smaller specimens is associated with the consumption of animal items (ephemeropteran larvae and cladocerans), while the morphology of larger specimens is related to the consumption of plant remains.

Foraging depth depicts resource partitioning and contamination level in a pelagic shark assemblage: Insights from mercury stable isotopes

The decline of shark populations in the world ocean is affecting ecosystem structure and function in an unpredictable way and new ecological information is today needed to better understand the role of sharks in their habitats. In particular, the characterization of foraging patterns is crucial to understand and foresee the evolution of dynamics between sharks and their prey. Many shark species use the mesopelagic area as a major foraging ground but the degree to which different pelagic sharks rely on this habitat remains overlooked. In order to depict the vertical dimension of their trophic ecology, we used mercury stable isotopes in the muscle of three pelagic shark species (the blue shark Prionace glauca, the shortfin mako shark Isurus oxyrinchus and the smooth hammerhead shark Sphyrna zygaena) from the northeastern Pacific region. The Δ199Hg values, ranging from 1.40 to 2.13‰ in sharks, suggested a diet mostly based on mesopelagic prey in oceanic habitats. We additionally used carbon and nitrogen stable isotopes (δ13C, δ15N) alone or in combination with Δ199Hg values, to assess resource partitioning between the three shark species. Adding Δ199Hg resulted in a decrease in trophic overlap estimates compared to those based on δ13C/δ15N alone, demonstrating that multi-isotope modeling is needed for accurate trophic description of the three species. Mainly, it reveals that they forage at different average depths and that resource partitioning is mostly expressed through the vertical dimension within pelagic shark assemblages. Concomitantly, muscle total mercury concentration (THg) differed between species and increased with feeding depth. Overall, this study highlights the key role of the mesopelagic zone for shark species foraging among important depth gradients and reports new ecological information on trophic competition using mercury isotopes. It also suggests that foraging depth may play a pivotal role in the differences between muscle THg from co-occurring high trophic level shark species.

Niche overlap among anglers, fishers and cormorants and their removals of fish biomass: A case from brackish lagoon ecosystems in the southern Baltic Sea

We used time series, diet studies and angler surveys to examine the potential for conflict in brackish lagoon fisheries of the southern Baltic Sea in Germany, specifically focusing on interactions among commercial and recreational fisheries as well as fisheries and cormorants (Phalacrocorax carbo sinensis). For the time period between 2011 and 2015, commercial fisheries were responsible for the largest total fish biomass extraction (5,300 t per year), followed by cormorants (2,394 t per year) and recreational fishers (966 t per year). Commercial fishing dominated the removals of most marine and diadromous fish, specifically herring (Clupea harengus), while cormorants dominated the biomass extraction of smaller-bodied coastal freshwater fish, specifically perch (Perca fluviatilis) and roach (Rutilus rutilus). Pike (Esox lucius) as large-bodied freshwater fish was the only species where recreational fisheries were responsible for the major fraction of the annual biomass extraction. A strong trophic overlap and hence a similar foraging niche was documented among commercial fishers and recreational anglers and among non-resident and resident anglers, indicating that the aversion expressed by anglers against commercial fisheries in a survey had an objective underpinning related to resource competition. By contrast, the foraging niches of cormorants and of both fishers and anglers differed strongly as evidenced by largely non-overlapping sets of species that were caught and removed by cormorants and by commercial as well as recreational fishers. However, for individual species of commercial and recreational interest, specifically perch, cormorants were responsible for a major fraction of total biomass extraction, suggesting that at the individual fish species level competition with fishers and anglers may still occur. In an angler survey, respondents expressed a preference for cormorant control, indicating the existence of conflict between fisheries and cormorants. We recommend that conflicts in the lagoon fisheries be proactively managed, e.g., through improved communication, zoning, predator control and outreach. Further research should clarify the population-level impacts of cormorants on target species of commercial and recreational fisheries as well as the relative impact of commercial and recreational fisheries on selected species of joint interest.

Unexpected effects of coastal storms on trophic ecology of two rocky reef fish species

Coastal storm risks have been increasing over the last couple of decades, affecting both marine infrastructure and human security. Nevertheless, the ecological impact on intertidal marine fishes has not been addressed yet. We carried out a field experiment during austral summer 2018, using statistical wave parameters, characterizing coastal storms as a factor, and the gut content of two sympatric intertidal triplefins, Helcogrammoides chilensis and Helcogrammoides cunninghami, as a proxy for trophic interactions. Three locations were sampled before and after the arrival of three consecutive summer coastal storm events. From all gut contents, a total of 16,597 prey items were found. Both species are micro-carnivorous, predating mostly on harpacticoid copepods, amphipods and chironomid larvae, without significant differences in prey composition with the passing of the summer coastal storms. However, after coastal storms, heavier-at-length fish (0.05–0.5 g weight gain) were found in the intertidal. Indeed, after the impact of the coastal storms, H. chilensis increased their ingestion of larger prey (amphipods, chironomid larvae), while H. cunninghami decreased the number of prey ingested. When compared between species, the feeding success after the passing of coastal storms was greater for H. chilensis than H. cunninghami. Finally, trophic overlap between species was high, but after the coastal storms passed, it decreased noticeably due to small changes in proportion of large chironomid and gammarid amphipods in the diet. Therefore, summer coastal storms affected the foraging behavior, increasing the weight of two recurrent cryptobenthic rocky reef fish from central Chile. In a broader ecological context, the shift from pelagic (i.e., copepods) to benthic prey (i.e., amphipods, polychaetes) may change the relative contribution and the subsidy production in nearshore habitats via pelagic-benthic coupling.

Trophic overlap of lionfish (Pterois volitans) and two native predators (Lutjanus apodus and Cephalopholis cruentata) in the western Caribbean

Abstract Impact of invasive species on native biota may be due to predation, competition for space or food, or indirect effects. Lionfish (Pterois volitans), invasive in the western Atlantic, is a voracious generalist predator, so it is expected to have a significant trophic overlap with native fishes of comparable size and habits. The goal of this study was to determine the diets of potential competitors of the lionfish, in particular a grouper, Cephalopholis cruentata (Graysby), and a snapper, Lutjanus apodus (Schoolmaster), and to compare them to the diet of lionfish in Xcalak, southern Mexican Caribbean. Stomach contents were analyzed and electivity and diet overlap were estimated. The trophic overlap between the lionfish and the two putative competitors, especially the grouper, was high, including prey that was consumed by the predators in higher proportion than the relative abundance of the prey in the environment, and probably at the same time of day. Lionfish and grouper shared as important diet items Stegastes sp., Halichoeres sp., Brachyura, and Palaemonidae, and most full stomachs were found during the early morning. The hypothesis of competition between them for particular prey is supported, so we advise to continue the culling programs of lionfish and also to monitor the abundance of the possible native competitors.

Ontogenetic diet shifts: an additional mechanism for successful invasion of a piranha species in a Neotropical floodplain.

Ontogenetic shifts in food preference reduces intraspecific competition as immature individuals eat different food types than adults. This diet plasticity could facilitate species' ability to successfully invade and establish itself in a new environment, even when co-occurring with phylogenetically close species. Here, ontogenetic diet shifts of a non-native piranha species (Serrasalmus marginatus) was tested by analyzing the relationship between its body length and the carbon and nitrogen stable isotope signatures. Carbon stable isotope was not correlated to fish length, but positive significant correlation between δ15N values and fish length was found for the non-native piranha. Also, immature and adult S. marginatus showed low isotopic niche overlap. The correlation between fish length and δ15N, and the low trophic overlap between immature and adult, indicate that the non-native S. marginatus had ontogenetic shifts in food preference, which may be viewed as an additional mechanism underlying its successful establishment in the upper Paraná River. Our findings indicate that ontogenetic shifts in food preference may be an invasive trait that facilitates the establishment of non-native fish species in tropical aquatic systems.

Trophic niche overlap and abundance reveal potential impact of interspecific interactions on a reintroduced fish

Conceptually, trophic niche overlap and species abundance can describe the strength and number of interspecific trophic interactions to determine the competitive impact on reintroduced species or other ecosystem changes. We use an example with young-of-year (YOY) Atlantic salmon (Salmo salar) reintroductions to determine whether trophic niche overlaps and abundances limit restoration success. Using seasonal stable isotopes and abundance estimates for invertivorous fishes in three Lake Ontario tributaries, we assessed community isotopic structure, trophic niche overlap, and the impact of the niche overlap by incorporating relative abundance. Brown trout (Salmo trutta) YOY could be a strong competitor with a high trophic niche overlap with Atlantic salmon YOY, but at lower abundances relative to Atlantic salmon there is minimal impact. Stream resident fish communities appeared to partition resources across seasons such that abundant species had low trophic niche overlap to minimize overall competition with Atlantic salmon YOY given available resources, indicating niche complementarity. Through joint consideration of trophic overlap and abundance using our conceptual model, the competitive impact of community composition on a reintroduced species could be assessed.

Trophic ecology of detritivorous fish along a reservoir cascade in a tropical semi‐arid region

AbstractThe objectives of this study were to investigate the food resources consumed by two species of detritivorous fishes along the Apodi‐Mossoró River reservoir cascade system, located in the tropical semi‐arid region of Brazil. The hypotheses tested were as follows: (a) the diet, niche breadth and niche overlap of the species in the reservoirs downstream of the cascade are different due to the blockage of food resources upstream, and (b) the two species coexist in the reservoir cascade system due to resource partitioning. The diets of the species were analysed based on the volumetric data of the food resources using NMDS ordination, and possible differences in diet were tested with PERMANOVA. The contribution of each food resource was analysed by SIMPER analysis. Trophic niche breadth was evaluated by PERMDISP analysis, and the trophic niche overlap was calculated by the Pianka index. The difference in niche overlap between the reservoirs was evaluated by Welch's t test. Plant material, detritus and sediment were the most important food resources in the diets of the two species, but they differed along the reservoir cascade. Plant material, detritus and sediments were the predominant resources in their diets, which showed differences in feeding, a narrow trophic niche and high dietary overlap. The differences in diet observed along the cascade did not follow the expected pattern, and the narrow trophic niche and high trophic overlap indicate that the sharing of food resources was not the main cause of the coexistence of these species in the reservoirs.

Feeding habits variability of Lutjanus synagris and Lutjanus griseus in the littoral of Campeche, Mexico: an approach of food web trophic interactions between two snapper species

The study of the feeding habits variability (spatial and temporal scales) allows us to evaluate the trophic interactions between species, thus, the short and long-term effects of the removal of different species by the presence of different phenomena. In this study, we carried out stable isotope (δ13C and δ15N) and stomach content analyses to infer the trophic linkages between two snapper species (Lutjanus synagris and Lutjanus griseus) captured in the littoral of Campeche, Mexico. In total, 781 specimens were collected (528 L. synagris and 253 L. griseus) and based on relative importance index [%PSIRI] both snapper species consumed based on the prey-specific index of relative importance (PSIRI), Penaeidae family (PSIRI = 23.41%) was the most important species in the diet of Lutjanus synagris, while Callinectes sapidus (PSIRI = 21.45%) was the primary prey of Lutjanus griseus. The isotopic analyses indicated that both snapper species feed in the coastal-marine (δ15N: 10.6 to 12.1‰ and δ13C: -15.7 to -12.7‰); however, according to PERMANOVA, low diet similarity was found between snapper species (R = 0.07, P < 0.01), also, significant differences was detected in the δ15N and δ13C values between L. synagris, and L. griseus. Therefore, despite both snapper species presents similar trophic positions (L. synagris: 4.2 ± 0.2; L. griseus: 3.9 ± 0.1) and feeding behavior (according to SIBER = opportunistic predators), a low trophic overlap was observed, probably associated with the differential use of habitat in the coastal zone; where L. synagris is ecologically fed in areas of seagrass, while L. griseus is probably intermittent between pasture and mangrove areas, therefore, the role of each species is crucial in the dynamics of coastal-marine ecosystems as predators and potential structuring of the populations of their prey.

High Trophic Niche Overlap between a Native and Invasive Mink Does Not Drive Trophic Displacement of the Native Mink during an Invasion Process.

Simple SummaryInvasive species are widely recognized to negatively affect native species through both direct and indirect interactions. When diet overlap between the native and invasive species increases, their competitive interaction is expected to increase too. This in turn may lead to displacement of one of the species. However, the specific mechanisms of the diet displacement are still unclear. In this study, we analysed the diet and diet overlap between the critically endangered European mink and the invasive American mink during the invasion process of the latter species by means of stable isotope analyses. We found a significant diet overlap between the native and invasive mink when they co-occur, an important individual variation of diet, and no significant change of diet of the native species in response to the arrival of the invasive mink. These results suggest significant competitive pressure imposed on the native European mink by the invasive American mink. As such, urgent implementation of control measures of invasive species is needed to ensure the viability and conservation of endangered European mink populations.The pressure elicited by invasive species on native species significantly increases with the increase of the overlap of their ecological niches. Still, the specific mechanisms of the trophic displacement of native species during the invasion process are unclear. The effects of the invasive American mink (Neovison vison) on the critically endangered European mink (Mustela lutreola) was assessed by analyses of diet and niche overlap during the invasion process. To do this, the isotopic composition (δ13C and δ15N) of both species of mink and their four main types of prey was analysed. Significant trophic overlap between the native European mink and invasive American mink was found when they coexisted in sympatry. Furthermore, both mink species were characterised by significant individual variation in diet and no obvious change in diet of the native species in response to the arrival of the introduced species was observed. High niche overlap registered between both species in sympatry with no displacement in diet of the native mink in response to the arrival of the invasive mink is expected to have important consequences for the viability and conservation of the native mink populations, as it suggests high competitive pressure.

Feeding habits of the fishes Euthynnus lineatus and Scomberomorus sierra (Perciformes: Scombridae) in the Eastern Tropical Pacific

ABSTRACT. Introduction: Black skipjack, Euthynnus lineatus, and Pacific sierra, Scomberomorus sierra, are of great economic importance in the small-scale fishery of the Tropical Eastern Pacific and in particular in the study area. Objectives: The objectives of the present study were to assess the diet components of E. lineatus and S. sierra, as well as to evaluate the niche width, trophic level, and trophic overlap between the two species, by size and season. Methods: Biological samples were obtained weekly from small-scale fishery catches in Bahía de Acapulco, Mexico. The E. lineatus sampling period occurred from October 2016 to October 2017, whereas S. sierra was sampled from October 2016 to June 2018. The Prey-specific Index of Relative Importance (% PSIRI) was used to evaluate the importance of each prey item in the diet of the predator. Levin´s index (Bi) was used as a measure of niche width. The Morisita-Horn index was used to evaluate diet overlap by size (juveniles or adults), by season (dry or wet), and between the two species. The trophic level (TL) of predators was calculated using the Cortés´ method. Results: A total of 262 E. lineatus stomachs were analyzed (42 % were empty and 58 % contained food) and a total of 209 S. sierra stomachs were analyzed (74.6 % were empty and 25.3 % contained food). Of the total E. lineatus specimens sampled, 107 were juveniles and 155 were adults. Black skipjack juveniles fed on five food items and adults fed on 13 food items. Of the total S. sierra specimens sampled, 75 were juveniles and 134 were adults. Pacific sierra juveniles and adults fed on five food items. The diet of the two species comprised fish, mollusks, and crustaceans. The two species showed narrow niche width. The diet of juvenile and adult E. lineatus presented differences with growth, contrary to what was observed for juveniles and adults of S. sierra. There were no significant differences in diet by season for both species. The trophic overlap obtained between the two species was low. The trophic level calculated for both species indicated that are tertiary consumers. Conclusions: Black skipjack and Pacific sierra are tertiary predators and present a specialist behavior, feeding mainly on fish from the families Engraulidae and Clupeidae. Although a degree of similarity in the dietary food components of these species was observed, there was not a high degree of interspecific competition for food.

Diet shifts of Rhizoprionodon terraenovae from the southern Gulf of Mexico: possible scenario by temperature changes

The current study examined the stomach contents of the Atlantic sharpnose shark (Rhizoprionodon terraenovae) in the southern Gulf of Mexico during 2015 to understand the relationship between diet and changes in sea surface temperature (SST). Prey-specific index of relative importance (%PSIRI), diet breadth (Bi), trophic level (TrL), and trophic overlap (PERMANOVA) were calculated between sexes, body size, and climatic seasons (dry, rainy and winter storm). The lowest temperature recorded in the area was during February (23.9°C), and the highest was during August (29.1°C). A total of 124 stomachs were analyzed, with 54.84% containing food. The trophic spectrum was composed of 32 identified prey, with demersal fish (Haemulon plumierii; %PSIRI = 22.82) and pelagic fish (Sardinella aurita; %PSIRI = 12.83) being the most important. According to the diet breadth (Bi = 0.002), Costello's graph, and trophic level (TrL = 4.2), R. terraenovae is a specialist tertiary consumer. PERMANOVA indicated significant trophic differences between sexes (F = 32.22; P < 0.05), body size (F = 13.68; P < 0.05), and among climatic seasons (F = 23.86; P < 0.05). Spearman's correlation indicated a negative relationship between the diversity of prey consumed by R. terraenovae and sea surface temperature (r = -0.75; P < 0.05). Therefore, diet for R. terraenovae is associated with SST, allowing for the development of possible scenarios related to climatic phenomena like climate change.

Ontogenetic trophic segregation between two threatened smooth-hound sharks in the Central Mediterranean Sea

Elasmobranchs are among the species most threatened by overfishing and a large body of evidence reports their decline around the world. As they are large predators occupying the highest levels of marine food webs, their removal can alter the trophic web dynamic through predatory release effects and trophic cascade. Suitable management of threatened shark species requires a good understanding of their behaviour and feeding ecology. In this study we provide one of the first assessments of the trophic ecology of the “vulnerable” smooth-hounds Mustelus mustelus and M. punctulatus in the Central Mediterranean Sea, based on stomach contents and stable isotope analyses. Ontogenetic diet changes were addressed by comparing the feeding habits of three groups of individuals: juveniles, maturing and adults. Our results highlighted that the two species share a similar diet based mostly on the consumption of benthic crustaceans (e.g. hermit crabs). Their trophic level increases during ontogeny, with adults increasing their consumption of large-sized crustaceans (e.g. Calappa granulata, Palinurus elephas), cephalopods (e.g. Octopus vulgaris) and fish (e.g. Trachurus trachurus). Our results provide also evidence of ontogenetic shifts in diet for both species showing a progressive reduction of interspecific trophic overlap during growth. The results of this study contribute to improve the current knowledge on the trophic ecology of these two threatened sharks in the Strait of Sicily, thus providing a better understanding of their role in the food web.

Trophic niche of the Pacific Sierra (Scomberomorus sierra) in the southeastern Gulf of California: Assessing its importance as a predator and prey (Mesopredator) in the food web

The identification of interspecific links (trophic niche) is important to characterize resource use of a predator, and to know its trophic role (for example, mesopredator or top predator) in the food web. In this study, we examined: a) the trophic ecology of Scomberomorus sierra as a predator (niche breadth, trophic overlap, and trophic position) and b) its presence as a prey in the diets of the region´s top predators, to evaluate the critical link of S. sierra as a probable mesopredator in the food web of the southeastern Gulf of California (GC). Based on %PSIRI, the dominant diet of S. sierra were engraulids and cephalopods. The diet was similar between sexes and among size-classes. However, the isotopic niche breadth values and δ15N variance (>1) reflect a broad niche for young adults likely related to a) changes in morphology (e.g., size of the mouth), b) development of the visual system, and c) changes in the energy requirements of the species reproductive stages. Seasonal changes in prey species’ availability and abundance resulted in isotopic variations, indicating that S. sierra is an opportunistic predator. A wide range in trophic position value (from 3.8 to 4.2) indicated that it also is an intermediary carnivore, with a high degree of trophic plasticity. Although S. sierra has not a dominant role in top predators’ diets, they share some prey species such as anchovies and other fish, depending on predator size. Therefore, S. sierra is a species with many prey-predator relationships in the southeastern GC food webs that may be considered a critical trophic link. This information is crucial for an ecosystem-based fisheries management in the Gulf of California.