Luis Cardona

The Mediterranean Cetacean Migration Corridor is one of the largest marine protected areas in the Mediterranean Sea. Nevertheless, little is known about the abundance and distribution of cetaceans and sea turtles in the area. A combination of aerial and boat surveys conducted in 2023 revealed the presence of seven cetaceans and two sea turtle species therein. The community was dominated numerically by two epipelagic foraging species, the striped dolphin (Stenella coeruleoalba) and the loggerhead turtle (Caretta caretta). However, based on population estimates, the majority of the community biomass was contributed by sperm whales (Physeter macrocephalus) and fin whales (Balaenoptera physalus). The population numbers of Cuvier’s beaked whales (Ziphius cavirostris) and Risso’s dolphins (Grampus griseus) were in between. When migrating fin whales were excluded from the analysis, deep divers with a high trophic position (sperm whales, Cuvier’s beaked whales and Risso’s dolphins) made a much larger contribution to the overall community biomass than epipelagic predators with a lower trophic position (striped dolphins and loggerhead turtles). Bottlenose dolphins (Tursiops truncatus), long-fined pilot whales (Globicephala melas) and leatherback turtles (Dermochelys coriacea) were observed during the surveys, but were too scarce to attempt any population estimate. Random forest models and generalized additive models identified the concentration of chlorophyll-a and the eastward current velocity as the major drivers of the distribution of epipelagic species. Conversely, the distribution of deep divers was best explained by a combination of bathymetric (standard deviation of the slope) and hydrographic (eddy kinetic energy, sea surface height and eastward or northward sea water velocity) variables. Finally, the distribution of fin whales was poorly predicted by the environmental variables considered. This evidence indicates that dynamic spatial closures might be needed to reduce the impact of fishing and maritime traffic on epipelagic predators, whereas static closures might suffice for deep divers.

We use data collected by onboard observers to assess the bycatch and immediate mortality of sea turtles in the Canary Current Large Marine Ecosystem (CCLME) and adjoining areas from 2016 to 2021. First, a dataset including 698 sets by drifting longliners, 6300 tows by trawlers and 1029 sets by purse seiners was used to calculate the catch per unit of effort for each species and fleet. Second, effort data compiled by the International Commission for the Conservation of Atlantic Tunas (ICCAT) and Global Fishing Watch were used to escalate bycatch to the whole fleet. Third, we used post‐release mortality rates from the literature to estimate the total number of turtles killed annually and provide a baseline to assess their potential demographic impact. Accordingly, the international fleet of longliners was estimated to cause the annual mortality of 6800 loggerhead turtles Caretta caretta (95% CI: 5825–7628), 740 olive ridley turtles Lepidochelys olivacea (95% CI: 560–926) and smaller numbers of hawksbill Eretmochelys imbricate and green turtles Chelonia mydas . Nothing is known about the post release mortality rate of leatherback turtles, but bycatch was estimated as 3529 leatherback turtles (95% CI: 2463–4889). Regarding bottom trawling, the whole fleet was estimated to case annually the death of 151 green turtles (95% CI: 67–191), 95 loggerhead turtles (95% CI:33–108) and 42 olive ridley turtles (95% CI: 23–87). Finally, purse seiners were estimated to cause negligible mortality to loggerhead and leatherback turtles, the only species bycaught by that fleet. Our study provides comprehensive and precise baseline data on sea turtle bycatch within the three main industrial fishing activities, crucial for understanding the demographic impacts on sea turtle populations. Moreover, it underscores the CCLME as a significant hotspot for marine megafauna bycatch.

Global change is rapidly reshaping species' habitat suitability ranges, hence leading to significant shifts in the distribution of marine life. Contrasting distributional responses among species can alter the spatial overlap between predators and prey, potentially disrupting trophic interactions and affecting food web dynamics. Here, we evaluate long‐term changes in the spatial overlap of habitat suitability ranges for trophically related species, including crustaceans, fish, penguins, and pinnipeds across 12 Large Marine Ecosystems from the Southern Hemisphere, merged into three primary regions: South America, Southern Africa, Australia and New Zealand. To this aim, we first use Boosted Regression Trees (BRTs) to hindcast and project species‐specific changes in suitable habitat from 1850 to 2100 under two future climate scenarios: SSP1‐2.6 (low climate forcing) and SSP5‐8.5 (high climate forcing). We then analyze changes in species habitat suitability and potential predator–prey spatial overlaps. Findings reveal that marine species generally exhibit changes in their suitable habitats, with pronounced shifts towards higher latitudes under the SSP5‐8.5 scenario. However, contrasting trends emerge among predators across functional groups and regions of South America, Southern Africa, Australia and New Zealand. These variations highlight the need for species and regional‐specific management responses. We also project contrasting spatial mismatches between predators and prey: predators experiencing declines in suitable habitat tend to exhibit greater overlap with their prey in future scenarios, whereas those with expanding suitable habitat show reduced spatial overlap with their prey. This study provides valuable insights that can inform spatial management strategies in response to climate change and illustrate how climate change may weaken species' ability to adapt to climate‐driven environmental changes due to trophic disruptions.

Early juvenile loggerhead turtles (Caretta caretta) rely on gelatinous zooplankton, whereas individuals larger than 40 cm curved carapace length are adapted to crush hard-shelled invertebrates. Nevertheless, fish were reported to be the staple food of loggerhead turtles in the western Mediterranean 30 years ago. Here, the temporal consistency of such a fish-based diet of loggerhead turtles is assessed through gut content analysis and stable isotope analysis of samples from the Mediterranean coast of Spain spanning three decades. The gut contents of 134 juvenile loggerhead turtles (curved carapace length range: 27–71 cm) from three different periods (1991, 1999–2008 and 2010–2017) were analyzed, as well as a subsample of the same turtles (n = 10 in each period) for both bulk and compound-specific stable isotope ratios (CSIA-AA). Gut content analysis revealed a decline in the frequency of occurrence and numerical abundance of fish and an increasing contribution of gastropods and bivalves throughout time, although pelagic tunicates were always the most frequently observed prey. The δ¹⁵N_bulk of turtle bone also dropped throughout the study period, but the values of the stable isotope ratio of N in phenylalanine (δ¹⁵N_Phe) indicated that 52.5% of that variability was due to a baseline shift over time. Accordingly, the trophic position estimated from CSIA-AA did not follow the decreasing pattern of δ¹⁵N_bulk, but fluctuated throughout time. The overall evidence indicates that fish consumption by loggerhead turtles in the study region declined through time, but the trophic position of loggerhead turtles did not change simultaneously. This is probably because low trophic prey such pelagic tunicates and filter-feeding bivalves and suspension-feeding gastropods were the bulk of the diet during the whole study period and fish played a minor role, even when their frequency of occurrence peaked. Past levels of high fish consumption might be due to high levels of fishery discards, currently declining because of the recent reduction of the fishing fleet.

The response to climate change in highly dimorphic species can be hindered by differences between sexes in habitat preferences and movement patterns. The Antarctic fur seal, Arctocephalus gazella, is the most abundant pinniped in the Southern Hemisphere, and one of the main consumers of Antarctic krill, Euphausia superba, in the Southern Ocean. However, the populations breeding in the Atlantic Southern Ocean are decreasing, partly due to global warming. Male and female Antarctic fur seals differ greatly in body size and foraging ecology, and little is known about their sex‐specific responses to climate change. We used satellite tracking data and Earth System Models to predict changes in habitat suitability for male and female Antarctic fur seals from the Western Antarctic Peninsula under different climate change scenarios. Under the most extreme scenario (SSP5‐8.5; global average temperature +4.4°C projected by 2100), suitable habitat patches will shift southward during the non‐breeding season, leading to a minor overall habitat loss. The impact will be more pronounced for females than for males. The reduction of winter foraging grounds might decrease the survival of post‐weaned females, reducing recruitment and jeopardizing population viability. During the breeding season, when males fast on land, suitable foraging grounds for females off the South Shetland Islands will remain largely unmodified, and new ones will emerge in the Bellingshausen Sea. As Antarctic fur seals are income breeders, the foraging grounds of females should be reasonably close to the breeding colony. As a result, the new suitable foraging grounds will be useful for females only if nearby beaches currently covered by sea ice emerge by the end of the century. Furthermore, the colonization of these new, ice‐free breeding locations might be limited by strong female philopatry. These results should be considered when managing the fisheries of Antarctic krill in the Southern Ocean.

Understanding consumer trophic status and long‐term dietary changes can yield information about impacts of altered habitats on their ecology. In Hawai'i, coastal ecosystems have been significantly modified by the introduction of invasive seaweeds and mangroves, high nutrient load and overfishing, but so far, much is still to be understood about how these changes have affected the green turtle (Chelonia mydas). This study analyzed stable carbon, nitrogen, and sulfur isotope ratios in the bone tissue of modern and museum specimens of green turtles collected from 1901 to 2020 in Oahu and the North‐western Hawaiian Islands to understand how their isotopic niche has changed through time, a crucial step towards restoring the ecological role of a formerly decimated species. The standard ellipse size and the total area of the convex hull of the isotopic niche of green turtles in three periods (1901–1951, 1992–2008, and 2018–2020) were calculated. The stable isotope values of ancient green turtles (1901–1951) suggest that they relied heavily on macroalgae even before the introduction of exotic species and eutrophication promoted the development of algal pastures. However, a few ancient green turtles relied heavily on seagrasses, and others complemented their macroalgae‐based diets with significant amounts of animal matter. Such diet specialists were missing from the sample of current green sea turtles, suggesting that these foraging strategies are less common or perhaps even absent in the current population. The results suggest that green turtles have converged on the use of the most abundant resource, red macroalgae, likely because of the homogenization and simplification of coastal habitats and food webs due to anthropogenic influences. Restoring the population size of herbivorous fishes and a more diverse ecosystem structure may be necessary to recover the array of trophic strategies formerly present in the Hawaiian green turtle population.

The western Mediterranean Sea has traditionally served as a primary foraging ground for juvenile loggerhead turtles from three distinct regional management units: the North‐western Atlantic, the Mediterranean and Cape Verde. Nesting activities were sporadic. Recently, nesting activity on the beaches of eastern Spain has increased, due to warmer sand and water temperatures during the summer months. The study has integrated stable isotope analysis of carbon (C), nitrogen (N) and sulphur (S) with satellite telemetry to identify the foraging grounds of these new colonizers nesting on Spanish beaches. Results indicate that the majority of these adult females forage oceanically in the Algerian Basin, with a few exhibiting distinct stable isotope signals, tentatively associated with foraging in shallow coastal areas in the central Mediterranean Sea. The dominance of oceanic foragers in this new population is noteworthy, given the Algerian Basin's oligotrophic nature. This contrasts with the prevalence of neritic foraging in adult females nesting in the central and eastern Mediterranean Sea. The use of the Algerian Basin as a foraging ground for adult loggerhead turtles exposes them to bycatch from drifting longlines. The authorities should refrain from increasing bluefin quotas imposed on longliners in the Algerian Basin unless the fishery avoids using squid as bait and agrees to deploy hooks deeper than 25 m to reduce loggerhead turtle bycatch.