Mixed-species dolphin groups in a submarine canyon-associated coastal ecosystem under anthropogenic pressure

Interspecific associations among dolphins provide key insights into social structure and ecological interactions within coastal marine ecosystems. This study presents the first systematic analysis of mixed-species groups (MSG) between the common dolphin ( Delphinus delphis ) and the striped dolphin ( Stenella coeruleoalba ) in the Bay of Algeciras-Gibraltar, a small-scale ecosystem influenced by a submarine canyon and subject to intense human activity. A total of 2410 sightings were recorded over 593 days between March 2017 and March 2020. MSG were observed in all seasons, with peak frequencies in summer and winter. Age-class composition showed a predominance of adults in D. delphis and juveniles in S. coeruleoalba , with increased presence of calves and neonates during summer. These associations may reflect coordinated foraging strategies or social cohesion in response to anthropogenic disturbance, particularly in an area lacking natural predators. Although no spatial or temporal overlap was observed with bottlenose dolphins ( Tursiops truncatus ), a recurrent association was documented between a solitary female T. truncatus and groups of D. delphis , although this case does not represent a functional MSG. A synthesis of dietary studies from nearby areas allowed the identification of possible prey species potentially shared by both species. The results highlight the ecological and conservation relevance of MSG in this region and support the proposed establishment of a micro-sanctuary for D. delphis , given that the Mediterranean subpopulation has been listed as “Endangered” since 2003. • Mixed-species groups of D. delphis ( Dd ) and S. coeruleoalba ( Sc ) occur in the BA-G. • Dd and Sc differ in group structure and age-class composition across seasons. • All age classes of Dd and Sc were present year-round in the study area. • Dd consistently outnumbered Sc in all mixed-species group sightings. • Associations between Dd and a solitary Tursiops truncatus ( Tt) female were also recorded.