Species of Anisakis typically infect the stomach of cetaceans worldwide, often causing ulcerative lesions that may compromise the host's health. These nematodes also cause anisakiasis or allergic reactions in humans. To assess the risks of this emerging zoonosis, data on long-term changes in Anisakis infections in cetaceans are necessary. Here, we compare the prevalence and severity of ulcerative lesions caused by Anisakis spp. in five cetacean species stranded along the north-west Spanish coast in 2017–2018 with published data from 1991–1996. Open ulcers were found in 32/43 short-beaked common dolphins, Delphinus delphis; 3/5 striped dolphins, Stenella coeruleoalba; 1/7 bottlenose dolphins, Tursiops truncatus; and 1/3 harbour porpoises, Phocoena phocoena meridionalis; a single individual of long-finned pilot whale, Globicephala melas, was found uninfected. In common dolphins, the mean abundance of open ulcers per host was 1.1 (95% confidence interval: 0.8–1.3), with a maximum diameter (mean ± standard deviation) of 25.4 ± 16.9 mm. Stomachs with scars or extensive fibrosis putatively associated with Anisakis were detected in 14 and five animals, respectively. A molecular analysis based on the mitochondrial cytochrome c oxidase II gene using 18 worms from three cetacean species revealed single or mixed infections of Anisakis simplex sensu stricto and Anisakis pegreffii. Compared with the period 1991–1996, we found a strong increase of prevalence, abundance and extension of ulcerative lesions in most cetacean species. Anisakis populations could have increased in the study area over the last decades, although we cannot rule out that a higher environmental stress has also boosted the pathological effects of these parasites.

Diet studies on the endangered and genetically distinct Iberian population of the southern harbour porpoise Phocoena phocoena are scarce. The present study provides updated information on the feeding ecology of this cetacean along the Galician coast (NW Spain) over the last 3 decades (1990-2018). The stomach contents of 72 stranded harbour porpoises were analysed to determine diet composition, to study which factors affect dietary variability and to estimate overlap between harbour porpoise diet and the target species of the fisheries in the study area, one of the most important fishing regions in Europe. Results showed that harbour porpoises are mainly piscivorous. We identified 33 prey taxa, but only 4 were important in the diet: fish of the genus Trisopterus, blue whiting Micromesistius poutassou, Atlantic horse mackerel Trachurus trachurus and European hake Merluccius merluccius. Interannual and ontogenetic variability in the diet were statistically significant, although differences in diet between sexes or between seasons were not detected. The diet of harbour porpoises from the Galician coast shows a partial overlap with fisheries catches in the area in terms of commercial fish species (≈61%) and size classes (≈45%), confirming the potential vulnerability of the Iberian population to interactions with fishing activities (i.e. bycatch in fishing gear and/or reduced prey availability).

The common dolphin (Delphinus delphis) is one of the most abundant small cetaceans along the Galician coast and a definitive host for the nematode parasite Anisakis, which is transmitted to cetaceans through the food chain. This study aimed to analyse which factors, including dolphin diet, affect the parasitic load. Samples of stomach contents from stranded dolphins along the Galician coast (2004–2024) were examined. The number of parasites was counted, and the contribution of different prey species to the diet was analysed based on hard remains. Generalised Additive Models (GAMs) were used to assess the relationships between parasitic load (number of Anisakis in the stomach) and various putative explanatory variables (e.g., year, month, size, sex, latitude, body condition, cause of death and diet of the dolphins). Results showed an increase in parasitic load over the years and a seasonal pattern, with numbers peaking in the first months of the year. A significant positive relationship was found between dolphin length and Anisakis load. In addition, dolphins that died from bycatch had the lowest parasitic loads. The numbers of Atlantic mackerel (Scomber scombrus) and blue whiting (Micromesistius poutassou) in the stomach had a significant effect on parasite load: parasite abundance decreased as the numbers of these prey species in the stomach increased. This result confirms the influence of diet on Anisakis load, although it does not reveal which species contribute the most to the parasite load. The study offers insights into how diet and other ecological factors influence the parasitic load in D. delphis.

As climate change continues to disrupt marine ecosystems, reliable body condition indices (BCIs) for sentinel megafauna species such as dolphins will become increasingly important tools for long-term ecosystem monitoring. We developed novel BCIs from routinely collected morphometric measurements and applied them to a 22-year dataset (1998-2019) of 597 stranded common dolphins (Delphinus delphis) from Northwest Spain. BCIs were calculated as residuals from Generalized Additive Models relating body length to girth and blubber thickness measurements, thereby controlling for body size effects. The indices were first validated against visually assessed nutritional status and then used to investigate long-term trends and potential environmental and trophic drivers of body condition. The girth-based BCI outperformed the blubber thickness-based index in predicting nutritional status. The indices differed in their sensitivity to confounding variables. Both indices revealed an increase in male body condition until 2009-2010 followed by a sustained decline through 2019, whereas no long-term trend was detected in females. By integrating data on climate variability (North Atlantic Oscillation), oceanographic conditions (upwelling intensity, sea surface temperature, chlorophyll-a), prey availability, and prey quality, we found evidence that changes in body condition was associated with climate- and oceanography-driven changes in prey communities and reductions in the quality of prey available to dolphins. Our findings suggest that body condition in common dolphins integrates signals across multiple components of marine ecosystems and can serve as a sensitive indicator of ecosystem change. Given growing reports of declining body condition in marine predators worldwide, the approach presented here provides a practical and transferable framework for long-term ecosystem monitoring using existing stranding network datasets.