CONICET | Buscador de Institutos y Recursos Humanos

Co-occurrence of species with similar ecological requirements could bring individual advantages, but when resources are scarce, these species would segregate as a way to reduce competition and coexist. We studied co-occurrence, group dynamics and activity budgets, along with the seasonal and diel variation of two sympatric species, the common (Delphinus delphis) and dusky (Lagenorhynchus obscurus) dolphins, in Northern Patagonia, Argentina. Dolphin groups were recorded by nautical surveys between 2006 and 2018, and classified by the species (single- and multispecies groups), group size, age/sex composition, predominant activity, season and time block.Frequencies were arranged in contingency tables and analyzed by log-linear models. Activity bouts duration was also recorded by focal-group follows and compared by nonparametric tests. Single- and multispecies groups´ occurrence varied along the year, distinguishing a period of high co-occurrence in spring and summer, when both species and most of multispecies groups were present, and a period of low co-occurrence, when mostly common dolphins remained in the area. Common dolphins occur in groups of more than a hundred while dusky dolphins in groups of 50 animals at most. Feeding time budget was higher in dusky dolphin single- and multispecies groups. Common dolphins showed a seasonal and diurnal pattern in activity budget. During the period ofhigh co-occurrence, dusky dolphins´ group size showed a diurnal pattern, and both species increased their feeding time budgets, which indicate more favorable foragingconditions. During this period, they associate occasionally in multispecies groups, increasing feeding time budget and feeding bouts duration, suggesting that the association is beneficial.

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