Environmental variables influencing occurrence and distribution of Delphinus delphis in the eastern Aegean Sea (eastern Mediterranean Sea)

1. Cetaceans are considered bioindicators of the health state of marine ecosystemsowing to their wide distribution across the different aquatic ecosystems in theworld and their significant top-down control role in the food chain, despite theirlow biomass. At the same time, effective management of wild cetaceanpopulations severely affected by human pressure requires extensive knowledgeon species distribution, habitat use, and associated threats. In this context,defining the factors that directly influence the local occurrence and distribution ofcetaceans is one of the underlying challenges and is essential for theirconservation and long-term survival.2.Delphinus delphissightings data, collected between 2017 and 2021 during284 standardized vessel-based surveys, were used to set up a presence–absencedistribution model in the eastern Aegean Sea, eastern Mediterranean Sea.Binomial generalized additive models with logit as link function were run usingthe R package mgcv (restricted maximum likelihood method) and differentbiogeochemical explanatory variables collected from different sources.3. Longitude, latitude, salinity, chlorophylla, dissolved ammonium, and dissolvedphosphate were selected as non-collinear predictive variables. Through a modelvalidation based on a 10-fold cross-validation approach and a random datasplitting procedure of 70%/30% (train/test dataset), a model formula has beenselected with an explained deviance of 38.10%, an Akaike information criterionvalue of 1,661.3, and an area under curve of 0.91.4. The study confirms that long-term time series of satellite-derived data are usefulto assess the occurrence and the spatial distribution ofD. delphis, suggesting theneed for a better understanding of the influence of these environmental factorsespecially in the framework of climate changes. 5. Outcomes highlight the need to test further variables and further methods inorder to provide increasingly reliable results in view of the conservation measuresthat must be adopted to stop or reduce the degree of pressure to which thesespecies are subjected.