# Parasite (Anisakis spp.) Load and Its Relationship with Diet in Common Dolphins (Delphinus delphis) Along the Coast of Galicia (NW Spain)

## TL;DR

This study examines how diet and other factors influence the number of Anisakis parasites in common dolphins along the Galician coast, finding that diet and seasonal patterns play a role.

## Contribution

The study identifies diet and ecological factors as key influences on Anisakis parasite load in common dolphins, offering insights for marine ecosystem and public health assessments.

## Key findings

- Parasite load in common dolphins has increased over the years and peaks in early months.
- Dolphins with more Atlantic mackerel and blue whiting in their diet had lower Anisakis loads.
- Larger dolphins and those that died from bycatch had lower parasite loads.

## Abstract

The common dolphin (Delphinus delphis) is one of the main cetacean species along the Galician coast and acts as the final host for the nematode parasite Anisakis. These parasites are acquired through the consumption of infected fish and cephalopods. Understanding the relationship between these parasites and D. delphis can help us assess the status of Anisakis populations in the marine ecosystem and, since this parasite can be transmitted to humans, contribute to improving public health safety by providing insights into potential risks. This study aimed to investigate which factors, including diet, influence the number of parasites in the stomachs of D. delphis. Results revealed that the number of parasites has increased over the years and is higher in the first months of the year. Larger dolphins had more parasites, while dolphins that died due to incidental capture in fishing gear had fewer parasites. Dolphins with a higher number of Atlantic mackerel and blue whiting in their stomachs showed a lower parasitic load. This confirms that the diet might be an important factor in determining Anisakis load in D. delphis even if it does not clearly identify which prey species contribute most to the Anisakis load in dolphins. This study offers insights into how diet and other ecological factors influence the parasitic load in D. delphis.

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.