Inter-regional variation in feeding patterns of skipjack tuna (Katsuwonus pelamis) inferred from stomach content, stable isotope and fatty acid analyses

Assessment of metal and organic pollutants in combination with stable isotope analysis in tunas from the Gulf of Cadiz (east Atlantic)

Bioaccumulation patterns of heavy metals (Pb, Cd, Cr, Ni, Fe and Cu) and organic (priority and emerging) pollutants, in combination with stable isotope analysis (SIA), were assessed in muscle and liver of three tuna species from the Gulf of Cadiz (Atlantic bluefin tuna, Thunnus thynnus; Atlantic bonito, Sarda sarda, and skipjack tuna, Katsuwonus pelamis). SIA and contaminant (heavy metal and organic) profiles separately discriminated between species. There was no significant overlap between the trophic niches estimated from isotopic data, suggesting that there are diet differences which may determine differential bioaccumulation patterns. The levels of heavy metals and persistent organic pollutants in muscle of all the individuals analyzed were below the allowable limits established by the current legislation. Concentrations of most contaminants were higher in liver than in muscle, underlining the powerful detoxifying capacity of the liver in tunas. In addition to diet, other factors such as size and age (exposure time to environmental chemicals) explain differences in pollutant accumulation patterns in tissues between species, each with varying degrees of involvement depending on the pollutant class. Our results show that combining contaminant profile data with trophic features based on SIA may help understand pollutant bioaccumulation patterns in upper levels of marine food webs.

DNA metabarcoding to assess prey overlap between tuna and seabirds in the Eastern tropical Atlantic: Implications for an ecosystem-based management

Overfishing has been drastically changing food webs in marine ecosystems, and it is pivotal to quantify these changes at the ecosystem level. This is especially important for ecosystems with a high diversity of top predators such as the Eastern Atlantic marine region. In this work we used high-throughput sequencing methods to describe the diet of the two most abundant tuna species, the Skipjack tuna (Katsuwonus pelamis) and the Yellowfin tuna (Thunnus albacares), highly targeted by fisheries off west Africa. We also explored prey diversity overlap between these tuna species and the seabird species breeding in Cabo Verde that are most likely to share prey preferences and suffer from bycatch, the Brown booby (Sula leucogaster) and Cape Verde shearwater (Calonectris edwardsii). Overall, the diet of both tuna species was more diverse than that of seabirds. Skipjack tuna diet was dominated by prey from lower trophic levels, such as krill, anchovies, and siphonophores, while the Yellowfin tuna diet was mainly based on epipelagic fish such as flying and halfbeak fishes. Some of the most abundant prey families detected in the Yellowfin tuna diet were shared with both seabird species, resulting in a high prey diversity overlap between this tuna species and seabirds These results have implications for the management of tuna fisheries in the Eastern Tropical Atlantic, because a large decrease of both tuna species might have cascading effects on both primary and secondary consumer levels, and the decrease of these underwater predators may have implications on the viability of tropical seabird populations.

Trophic niche partitioning of five sympatric shark species in the tropical eastern Pacific Ocean revealed by multi-tissue fatty acid analysis

Fatty acid (FA) analysis of consumer tissues has recently shown utility in drawing further inferences about trophic niche dynamics of marine predators such as sharks. In this study, we examined liver, plasma, and muscle FAs in five coexisting pelagic sharks (blue (Prionace glauca), silky (Carcharhinus falciformis), bigeye thresher (Alopias superciliosus), pelagic thresher (Alopias pelagicus), and smooth hammerhead (Sphyrna zygaena)) inhabiting the tropical eastern Pacific Ocean. Results showed complex inter- and intra-individual and tissue variation among the five shark species. Based on multivariate analysis of the muscle FAs, P. glauca and C. falciformis have the largest FA niche widths, indicating diverse feeding habits or habitat isolation, whereas A. pelagicus and S. zygaena occupied a narrower niche width, reflecting increased trophic specialization. High percentages of muscle FA niche overlap indicated strong resource competition between S.zygaena and C. falciformis and a degree of dietary isolation by P. glauca. Interpretations of feeding ecology differed based on the analysis of plasma FAs, which could be attributed to higher dietary FA turnover rates. The liver was deemed unsuitable to examine FA niche metrics based on high and unexplained intra-specific variance in liver FAs as well as the unique lipid metabolism in chondrichthyans. Overall, our multi-tissue approach revealed the magnitude of potential competitive interactions among coexisting tropical shark species. It also expanded our understanding of inter-tissue variability and best practices when using FA analysis to estimate trophic niche metrics of sharks.

Trophic Dynamics and Feeding Ecology of Skipjack Tuna (Katsuwonus pelamis) off Eastern and Western Taiwan

The skipjack tuna (Katsuwonus pelamis) is a mesopredator fish species with seasonal abundance in waters off Taiwan. Regional ecological and life-history information has been historically lacking for this species. In recent years, stable isotope analysis (SIA) of carbon and nitrogen has been used to assess predator feeding ecology and broader ecosystem trophic dynamics. This study evaluated comparative skipjack feeding ecology in distinct regions off Taiwan, combining traditional stomach content analysis with SIA of individuals off western (n = 43; 2020) and eastern (n = 347; 2012–2014 and n = 167; 2020) Taiwan. The stomach content analysis showed the most important prey to be ponyfish (Photopectoralis bindus) in western Taiwan and epipelagic squids (Myopsina spp.) and carangids (Decapterus macrosoma;) in eastern Taiwan from 2012 to 2014 and epipelagic carangids (Decapterus spp.) and flying fishes (Cheilopogon spp.) in eastern Taiwan in 2020, suggesting that the skipjack tuna is a generalist predator across regions. In contrast, time-integrated diet estimates from Bayesian mixing models indicated the importance of cephalopods and crustaceans as prey, potentially demonstrating more mesopelagic feeding in less productive waters during skipjack migrations outside the study regions. Skipjack off western Taiwan had a slightly higher estimated trophic position than in the waters off eastern Taiwan, potentially driven by the varying nutrient-driven pelagic food web structures. Skipjack SI values increased with body size off eastern Taiwan but not in western waters, suggesting that opportunistic predation can still result in different predator–prey size dynamics between regions.

Species-specific heavy metal concentrations of tuna species: the case of Thunnus alalunga and Katsuwonus pelamis in the Western Mediterranean

Albacore Thunnus alalunga and skipjack tuna Katsuwonus pelamis are highly migratory species that are usually caught together in the Western Mediterranean. These species are top predators that are highly affected by the biomagnification process through the trophic chain. Bioaccumulation pattern of the main metal pollutants (mercury, Hg; lead, Pb; and cadmium, Cd) were analyzed in muscle tissues of 52 individuals (26 T. alalunga and 26 K. pelamis) of these highly consumed species in order to address two objectives: (1) compare the species-specific bioaccumulation between these large-pelagic species, and (2) assess the healthy properties of such valuable resources based on the trace metal limits established by the European Commission Regulation (ECR). Both generalized linear mixed models and redundancy analysis indicated a differential bioaccumulation between these two tuna species. While T. alalunga accumulates higher concentrations of Hg (0.1996 ± 0.0602 mg·kg^-1 weight wet-ww), K. pelamis accumulates higher concentrations of Cd (0.0076 ± 0.0049 mg·kg^-1 ww) and Pb (0.0031 ± 0.0017 mg·kg^-1 ww). Size and trophic ecology support the differences detected in the bioaccumulation pattern. Heavy metal concentrations were below the tolerable limits considered by ECR (1, 0.1, and 0.3 mg·kg^-1 ww for Hg, Cd, and Pb, respectively).

Diet and feeding behaviour of albacore Thunnus alalunga in the western Mediterranean Sea during the spawning period

Mature albacore tuna (Thunnus alalunga) are expected to have high energy requirements at the time of breeding. However, there are no descriptions of the diet of albacore in the Mediterranean Sea that can help us to understand if such requirements can be obtained from feeding during reproduction. In this study, we analysed the stomach contents of reproductively active albacore captured from 2010 to 2015 in the oligotrophic waters of the western Mediterranean Sea, one of their main spawning grounds. Estimates of stomach fullness revealed intense feeding activity, and prey composition indicated important consumption of mesopelagic fish, including barracudinas, myctophids and small pelagic crustaceans. Plastic debris occurred in 25%-53% of the stomachs sampled across all years. Prey composition was not different between males and females. However, females fed at higher rates and had higher hepatosomatic index values than males, suggesting that increased feeding could contribute to meet their higher energy demand associated with offspring production. We observed a diet shift from small crustaceans to fish prey along fish size. During the spawning period, albacore showed a specialist feeding behaviour by preying on aggregations of vertically migrating myctophids and small crustaceans, probably when they were near the surface. This study provides information and biological data to support ecosystem modelling and to increase the understanding of albacore ecology.

Characterization and comparison of the digestive physiology of two scombrids, Katsuwonus pelamis and Sarda sarda, in the Gulf of Cádiz

Fish and other marine animals have a unique and intimate interaction with their surrounding environment. Diet type or trophic level play significant roles in modulating species digestive physiology. However, little is known about how the trophic niche influences digestive activity and gut microbiota in scombrids species. The aim of the present study was to analyse and describe the digestive physiology of the skipjack tuna (Katsuwonus pelamis) and the Atlantic bonito (Sarda sarda) as bioindicator of the feeding ecology and trophic niche for both species in the Gulf of Cádiz (Spain). For that, fish proximate composition, pH of digestive organs and digestive enzymes activity levels were analysed in 10 individuals of each species to gain insight into the digestive physiological adaptations of the two species of scombrids. In addition, intestinal microbiota composition was determined for the skipjack tuna. The integration of the results suggested a clear trophic niche segregation between both species. Stomach pH was associated in both species with infrequent feeding events. Body proximate composition and digestive lipases activity patterns pointed to a high predominance of lipids in the Atlantic bonito diet, suggesting oily fish as main prey. On the other hand, results supported the skipjack tuna as a highly opportunistic feeder with a more varied diet, including fish but also crustaceans as preys. The gut microbial community in the latter species is dominated by Firmicutes and Tenericutes at phylum level, and by Mycoplasma, Blautia and Dorea at genus level. The present study contributes to advance the knowledge on the feeding behaviour and physiology diversity in fish species as a result of adaptation to a particular habitat.

Isotope-based inferences of skipjack tuna feeding ecology and movement in the southwestern Atlantic Ocean

Skipjack tuna (Katsuwonus pelamis) sustain a large-scale fishery in the southwest Atlantic Ocean (SWA), but information about its foraging ecology in this region is still limited. Here we use carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analysis of muscle from individuals collected in 2017-2018 (n = 383) to quantify diet composition and characterize movement patterns. We found a relatively small degree of variation in δ¹³C (range: -18.9 to -16.5‰) in comparison to δ¹⁵N values (6.7-14.7‰). At higher latitudes in the southern area (30-34°S), individuals had higher mean (±SD) δ¹⁵N values (12.2 ± 1.3‰) in comparison to those collected in the northern area (9.7 ± 1.5‰) between 20-26°S. At the northern area, isotope mixing models with informative priors showed that lanternfish (median: 50%) and krill (31%) were the primary foods. In the southern area, lanternfish (53%), krill (23%) and small pelagic fish (23%) were the primary food sources. Spatial shifts in diet composition were related to warming events that likely resulted in low abundance of sardines in the northern area. The latitudinal pattern in skipjack and krill δ¹⁵N values mirrored that of regional zooplankton isoscapes, suggesting residency at the timescale of isotopic turnover for muscle (~2-4 months), and that geographical variation in the baseline isotopic composition can be exploited to characterize seasonal movements of skipjack and other top marine consumers in this region.

Predicting Skipjack Tuna Fishing Grounds in the Western and Central Pacific Ocean Based on High-Spatial-Temporal-Resolution Satellite Data

Skipjack tuna are the most abundant commercial species in Taiwan’s pelagic purse seine fisheries. However, the rapidly changing marine environment increases the challenge of locating target fish in the vast ocean. The aim of this study was to identify the potential fishing grounds of skipjack tuna in the Western and Central Pacific Ocean (WCPO). The fishing grounds of skipjack tuna were simulated using the habitat suitability index (HSI) on the basis of global fishing activities and remote sensing data from 2012 to 2015. The selected environmental factors included sea surface temperature and front, sea surface height, sea surface salinity, mixed layer depth, chlorophyll a concentration, and finite-size Lyapunov exponents. The final input factors were selected according to their percentage contribution to the total efforts. Overall, 68.3% of global datasets and 35.7% of Taiwanese logbooks’ fishing spots were recorded within 5 km of suitable habitat in the daily field. Moreover, 94.9% and 79.6% of global and Taiwan data, respectively, were identified within 50 km of suitable habitat. Our results showed that the model performed well in fitting daily forecast and actual fishing position data. Further, results from this study could benefit habitat monitoring and contribute to managing sustainable fisheries for skipjack tuna by providing wide spatial coverage information on habitat variation.