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Calculation of dietary niche characteristics using stable isotopes has become a popular approach to understand the functional role of taxa across food webs. An underlying assumption of this approach is that stable isotopes accurately reflect the dietary breadth of a species over a temporal duration defined by tissue-specific isotopic turnover rates. In theory, dietary niche estimates derived from fast turnover rate tissues (e.g., blood plasma and liver) may augment stomach content-derived estimates more agreeably than slower turnover rate tissues (e.g., muscle or fin). We tested this hypothesis by comparing commonly used dietary niche estimates derived from stomach contents (nicheSCA: Levins’, Shannon–Wiener’s, and Smith’s), with those estimated using stable isotopes [nicheSIA: standard ellipse area (SEA), convex hull total area (TA), theta (θ), and ellipse eccentricity (E)] of liver and muscle tissue. Model species were three large-bodied sharks: white (Carcharodon carcharias), dusky (Carcharhinus obscurus), and scalloped hammerhead (Sphyrna lewini). Within-technique comparisons for nicheSCA and nicheSIA metrics (i.e., SEA vs. TA) were often correlated; however, we did not observe any statistically significant correlations between nicheSCA and liver/muscle tissue nicheSIA (i.e., Levins’ vs. SEA). We conclude that nicheSCA and nicheSIA do not provide comparable estimates of dietary niche, at least for the three predator species examined. This fundamental discrepancy highlights technique-specific limitations to estimating organismal dietary niche and identifies a need for the use of clearly defined niche metrics, i.e., the standardized use and reporting of the term isotopic niche as proposed by Newsome et al. (Front Ecol Environ 5:429–436, 2007). Finally, further investigation into the factors underpinning nicheSIA is required to better contextualize this popular ecological metric when compared to nicheSCA.

The Atlantic blue crab Callinectes sapidus (Decapoda, Portunidae) Rathbun, 1896 is native to the east coasts of North and South America and has recently expanded its distribution in the non-native range into the Gulf of Cadiz (SW Iberian Peninsula, Europe). Considering the impacts caused by this invasive species in numerous estuarine ecosystems and its generalist feeding behavior, this study aims to provide the first account of the Atlantic blue crab diet on the East Atlantic coast. We studied the species’ feeding habits using stomach content analyses to predict food web interactions and putative impacts. Samples were obtained in the Guadalquivir estuary (SW Spain, Europe), which was colonized in 2017. The main food items identified on their stomach were, fish (49.9%), mollusks (44.4%) and crabs (32.3%). They also consumed plant material (27.2%), and the sediment (32.3%) in their digestive tract was likely the result of secondary ingestion. The Atlantic blue crab exhibited the same omnivorous behavior as in the native area. There was no sexual variation in diet composition or feeding activity in general, but there was a seasonal variation in the diet composition of females. The decrease of the caramote prawn Penaeus kerathurus (Forskål 1775) observed in the Guadalquivir estuary since 2021 is likely not due to the Atlantic blue crab because they seldomly eat this prey. Overall, our study provides clear baseline information to expand the knowledge about the ecological roles of the Atlantic blue crab in non-native ecosystems.

Species range shifts have manifold consequences on the biota and functioning of recipient ecosystems. A thorough assessment at early dispersal stages may effectively contribute to foreseeing the outcomes implied. Here we capitalised on a combination of interdisciplinary tools to investigate the taxonomy, native ranges, invasion pathways, and impacts of four fish samples exotic to the Mediterranean. These are Zenopsis conchifer and Ephippion guttifer , two neonative species from the Atlantic, and Epinephelus striatus and “ Siganus clade C”, two new non-indigenous species (NIS) that are respectively critically endangered in the central-western Atlantic and potentially undescribed and native to the eastern Indo-Pacific. Faunal, molecular, and parasitological analyses suggested that neonatives still had an extra-Mediterranean origin, whereas NIS arrived through shipping. Dietary analyses showed that E. guttifer and E. striatus fed on multiple trophic levels and survived in the area of introduction scavenging on fisheries discards. This study contributes to solving taxonomic issues, shows how stomach content metabarcoding and parasite biotagging can aid monitoring species dispersal, and highlights the potential role of bycatch dumping in NIS spread. Finally, it emphasises how interdisciplinary approaches maximise evidence and proposes a workflow that may be applied to similar circumstances worldwide.

Antarctic krill ( Euphausia superba ) is a keystone species in the Antarctic marine ecosystem. The pelagic tunicate ( Salpa thompsoni ) interacts with krill in diverse aspects, which imposes krill distribution and even survival, especially under the context of changing climate. Understanding the diet of both species during autumn—a season with shifting food composition and availability—is critically understudied. Therefore, this study aims to investigate the diet composition and trophic ecological niches of krill and salps in the Bransfield Strait during autumn in 2022 using stomach content, fatty acid (FA), and stable isotope analyses. Stomach content analysis revealed that krill primarily consumed copepods, while salps primarily consumed diatoms. FA analysis confirmed significant differences between the FA markers of diatoms and copepods for each species, aligning with stomach content analyses. Stable isotope analysis also indicated low overlap of trophic niches. By consuming more copepods, krill may reduce direct dietary competition with the less prevalent salps during autumn. Although the feeding strategy (active or passive) exerted by krill is warranted to further investigate, the differentiated feeding pattern occupied by two species promotes their coexistence.

The first offshore wind farm in North America, Block Island Wind Farm (BIWF), was built on a pilot scale (five 6 MW turbines) approximately 5 km southeast of Block Island, Rhode Island. Potential effects of the BIWF on dietary habits were examined for fish collected in a trawl survey conducted monthly over 7 years (October 2012 to September 2019). Stomach content analysis was conducted on 3457 flounder, gadids, and black sea bass collected near BIWF and in 2 reference areas during baseline, construction, and operation time periods. Other trophic metrics such as fish condition, stomach fullness, and % empty stomachs were examined for an effect of wind farm operation. Temporal variation in trophic metrics was more common than spatial differences, with no consistent indication of an effect of wind farm operation across metrics or species. Prey accumulation curves indicated that diets were adequately characterized with sample sizes of approximately 40 stomachs for most time period by area combinations. Diet composition of hakes and flounder in all areas included a greater proportion of mysids and amphipods during the wind farm operation time period. Summer flounder and winter flounder condition was lower during the operation time period in all areas. Inclusion of mussels and associated epifauna (mysids) in fish diets following turbine installation indicate fish forage on the colonized turbines. Although substantial changes to fish diets were not evident at BIWF, cumulative trophic effects of larger wind farms should be examined as the offshore wind industry expands on the northeastern US coastline.

The food web structure of a coastal fish community (western Dutch Wadden Sea) was studied based on stomach content data from samples collected between 2010 and 2018. In total, 54 fish species were caught and 72 different prey items were identified. Fish species consumed from only a few up to >30 different prey species, suggesting the presence of both opportunistic and more specialized feeders. We found no significant differences between years or switches in food source with fish size. The trophic positions of the Wadden Sea fish community ranged from 2.0 to 4.7, with most trophic positions above 3.0. In the past, (near)-resident species were the most abundant guild in spring, and juvenile marine migrants in autumn. At present, all guilds are pre sent in similar but low abundances. The (near)-resident community consisted of about 20 species that fed primarily on amphipod crustaceans, brown shrimps and juvenile herring. There was only a slight overlap in diet with the group of juvenile marine migrants (5 species of juvenile flatfishes and clupeids), whose preferred prey were copepods, polychaetes and brown shrimps. About 15 species of marine seasonal visitors showed an overlap in diet with both the (near)-resident and the juvenile marine migrants, especially for brown shrimps and to a lesser extent herring and gobies. Our results illustrate (1) the pivotal position of a few key prey species (amphipod crustaceans, brown shrimps, juvenile herring and gobies) for the coastal Wadden Sea fishes and (2) that the substantial prey overlap in the diet of some predators cannot exclude intra- and inter-specific competition among these predators.

Due to their relationship with prey availability, elasmobranchs with an opportunistic diet can serve as indicators of changes in the marine community, thereby enhancing our understanding of the health of marine ecosystems. In this study, the Zearaja brevicaudata’s trophic ecology was studied, by analysing its stomach contents, aiming to provide evidence of long-term changes in diet resulting from shifts in the structure of the Argentine Patagonian marine community in response to fishing pressure. Changes were evaluated according to catch area, predator size, sex, maturity over time. Moreover, the relationship between Zearaja brevicaudata ’s dietary shifts and the fishing effort of Merluccius hubbsi was investigated. Our results suggest a gradual replacement of the main prey species with lower trophic level species. Although no correlation was found with fishing effort, the diet of Z. brevicaudata over time reflected changes similar to those associated to the fishing exploitation of M. hubbsi in the Patagonian system. In this context, the Z. brevicaudata’ s diet emerges as a valuable tool for monitoring and assessing changes in the marine community. By serving as an indicator, Z. brevicaudata’ s diet provides crucial insights into the conservation status of the marine ecosystem, enabling informed management and conservation efforts.

Large-bodied sharks can be critical for coupling disparate habitats and food webs, which is considered central for ecosystem stability. Understanding the role of sharks and their associated predator–prey relationships across spatial scales is also integral to the development of multi-species ecosystem models. A combined stomach content ( n  = 212) and multi-tissue stable isotope (fast [liver; n  = 101] vs slow turnover [muscle; n  = 108]) approach was used to investigate the feeding ecology of the copper shark ( Carcharhinus brachyurus ) in the temperate waters of Southern Australia. Sharks were sampled from fishery catches over 3 years, during the austral spring–summer seasons and across three distinct regions. Stomach content analysis identified the copper shark as a generalist predator that consumes a diverse prey base dominated by Sepia novaehollandiae, Sepioteuthis australis , and Sardinops sagax (36%, 21%, and 18% IRI). Regional differences in diet composition were evident, although no size- or sex-based variation was identified. Isotope mixing models and regional food web bi-plots also revealed that S. sagax was the most important prey species, but temporal variation in diet was observed that matched known movements. The copper shark was estimated to be a primary piscivore, feeding at trophic level 4.49. Data on the feeding behaviour of copper sharks will provide vital inputs into future ecosystem-based fishery models and guide conservation and management of this important marine predator in temperate Southern Australian coastal waters.

Assessing the ecosystem impact of a species requires information on its foraging ecology across life stages and full distribution. However, such data are often incomplete or entirely lacking for widely distributed species, such as pelagic sharks. We used stomach content and bulk stable isotope analysis in muscle and liver tissue to examine ontogenetic and sex-based variation in the foraging ecology of the salmon shark ( Lamna ditropis ) in the California Current Ecosystem (CCE), where individuals of all life stages overlap seasonally. Both methods showed that salmon sharks consume teleosts and cephalopods, but trophic niche width and dietary composition varied across sexes and age classes. Juvenile females exhibited the widest niche width among females in both tissues, which narrowed progressively as they matured. Conversely, male niche width decreased from the young-of-year to juvenile stages. Tissue-specific analyses revealed further differences. Liver tissue indicated a diet dominated by continental shelf species, reflective of recent nearshore foraging. Muscle tissue, representing long-term dietary trends, showed planktivorous salmonids and continental shelf species as primary prey for immature individuals, while adult females consumed largely deep-water squid and adult males showed a dietary composition of multiple prey groups, likely due to reliance of oceanic prey during seasonal migrations. These findings highlight the dietary diversity of salmon sharks and underscore the variability in foraging strategies and habitat use across age classes and sexes. Management strategies should consider these differences in foraging ecology, as they may render some specific life stages more vulnerable to environmental changes, anthropogenic impacts, and fluctuations in prey availability.

Effective marine ecosystem monitoring is critical for sustainable management. Monitoring seabird diets can convey important information on ecosystem health and seabird–fishery interactions. The diet of breeding black-browed albatross (Thalassarche melanophris) has previously been assessed using stomach content analysis (SCA) or stable isotope analysis (SIA), but not both methods together. Combining dietary sampling approaches reduces biases associated with using single methods. This study combines SCA and SIA to study the diet of black-browed albatross chicks, with a specific focus on fishery discard consumption, at two Falkland Islands colonies (New Island 51°43′S, 61°18′W and Steeple Jason Island 51°01′S, 61°13′W) during two consecutive breeding seasons (2019 and 2020). SCA provided high taxonomic resolution of short-term diet and priors for stable isotope mixing models, with multiple measures of dietary items (e.g. numeric frequency N%, frequency of occurrence FO%). By contrast, SIA of down feathers provided a single and more integrated dietary signal from throughout chick development. Although the two methods disagreed on the dominant prey group (SCA—crustacean; SIA—pelagic fish), the complementary information suggested a chick diet dominated by natural prey (SCA: 74%–93% [FO], 44%–98% [N]; SIA: minimum 87%–95% contribution). Nonetheless, SCA revealed that a high proportion of breeding adults do take discards. We detected consistent colony-specific diets in relation to prey species, but not in relation to higher discard use. Overall, discard consumption was highest in 2020, the year characterised by the poorest foraging conditions. Our results have implications for fisheries management and future dietary studies assessing discard use.