There are no comprehensive studies on strandings of marine megafauna off the coast of the southern Portuguese mainland (Algarve). This study investigates spatio-temporal patterns, size and sex differences, and causes of death of stranded cetaceans (n=1207) and sea turtles (n=694) from 1978-2023. The most common stranded cetaceans were common dolphins (Delphinus delphis), striped dolphins (Stenella coeruleoalba), minke whales (Balaenoptera acutorostrata), bottlenose dolphins (Tursiops truncatus), and harbour porpoises (Phocoena phocoena). The most frequently stranded sea turtles were loggerheads (Caretta caretta) and leatherbacks (Dermochelys coriacea). Since the establishment of a local stranding network in 2010, the number of strandings has increased significantly. This study found significantly more minke whales and loggerhead sea turtles stranding in spring and summer, while leatherback strandings peaked in summer and fall, all correlating with migration patterns. Strandings occurred along the whole Algarve, with hotspots in areas most likely due to presence of species, detectability, and oceanic topography. All commonly stranded species had significantly high mortality due to bycatch, trauma, and undetermined causes. To mitigate these high levels of bycatch, effective fisheries management and mitigation measures are essential. Further research is needed to explore undetermined causes of death and improve conservation efforts for these ecologically important species.
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