The study of stranded animals is a valuable aid to monitoring marine mammals globally. However, the utility of strandings data depends on their quality and representativeness, which is affected by various biological, physical, social and economic factors. An analysis of how stranding networks work could help understand limitations in the data collected and facilitate correcting for or even eliminating them. In 2021, the International Council for the Exploration of the Sea’s Working Group on Marine Mammal Ecology carried out an expert consultation using a questionnaire to provide insight into the contribution of European stranding networks as a monitoring tool in European countries with Northeast Atlantic and adjacent coasts (hence also including some networks operating along the Mediterranean coast). A key aim was to identify ways to improve data on mortality of marine mammals due to fishery bycatch. The present paper is the first of a two-part series based on the responses to the questionnaire by 45 organisations from 19 countries, and focuses on characterising the activities and capacities of the stranding networks surveyed, identifying differences within and between countries, highlighting strengths and weaknesses, and providing recommendations to enhance the value and credibility of the information collected. The second paper will focus on the information specifically related to mortality due to fishery bycatch. Stranding networks provide extensive spatio-temporal coverage of European coastlines, but their activities may be constrained by limited resources as well as limitations imposed by the stranding process. There is a need for better coordination and standardisation of the collection and analysis of data and samples and increased spatial coverage to fill gaps. To improve data quality, in particular to support assessment of impacts of threats such as bycatch, more necropsies and associated sample analysis are needed. It would also be advantageous to collect more information from less fresh animals, record search effort, and give greater attention to pinnipeds and non-marine mammal taxa. We also highlight the need to make information available and the potential value of a common database. Streamlining the reporting of results at the European level and providing systematic funding to stranding networks in accordance with their needs are necessary steps to optimise their role as a tool for the long-term monitoring of marine mammals and other marine megafauna in Europe.

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Between the eighteenth and twentieth centuries, many small cetaceans (particularly dolphins) were killed in large numbers off the coasts of Europe and North Africa, due to perceived competition with fisheries. Dolphins were long considered as pests, responsible for reduced fish catches and damage to fishing gear. This conflict was particularly acute between the second half of the nineteenth century and the first half of the twentieth century, when governments from various countries encouraged and subsidised the extermination of small cetaceans. While the precise number of killings and the effects these had on the past and current status of cetacean populations are largely unknown, historical records and osteological collections can help us appreciate their magnitude. Here we first summarise information available regarding conflict between fishers and small cetaceans in various countries around the Mediterranean and Northeast Atlantic. Then, based on an extensive analysis of historical literature that included landing and bounty reports, and on a review of osteological collections, we provide new and extensive information on conflicts in France and Algeria. For France and Algeria, we (1) provide information regarding the context and chronology of killings and culling campaigns, (2) describe the methods used to deter and kill cetaceans, (3) identify the species affected, and (4) attempt to quantify the number of individuals killed. Our results suggest that tens of thousands of small cetaceans were killed in France and Algeria, primarily between the 1880s and the 1930s. Total mortality of small cetaceans due to human conflict in the Mediterranean Sea and Northeast Atlantic is certainly much higher, as that includes the killings in all other countries where similar conflicts existed. Such a high mortality likely had an important negative impact on dolphin population trajectories, particularly in the Mediterranean Sea. In the second half of the twentieth century, intentional killings largely stopped, but small cetaceans became exposed to a variety of other anthropogenic threats. Reconstructing past mortality can help 121us understand current population trends, viability and potential for recovery, as well as help us to set meaningful baselines for conservation. <!----> <!----> <!----> <!---->

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.