Preliminary insights of a mixed-species shark aggregation: a case study of two carcharhinids from the Mediterranean Sea

Social and non-social animals can aggregate at a specific site for various reasons such as reproduction, feeding, or other synchronized patterns of movements. While shark aggregations are well documented, mixed-species aggregations are less studied and therefore poorly understood. To overcome this, a combination of acoustic telemetry and social network analysis was used to investigate population structure and behavior based on the temporal overlap of 22 individuals of 57 tagged sharks of two species, the sandbar (Carcharhinus plumbeus) and the dusky (Carcharhinus obscurus) sharks, that form a winter mixed-species aggregation in front of an Israeli coastal power plant. The results suggested that if both species co-occurred and share the study site, their finer-scale associations revealed temporal partitioning between species and species assortment in sandbar sharks. The multi-species network was also structured by sex. The difference between species may indicate separate strategies and temporal niche partitioning at the aggregation site. The particularly warmer temperatures (~ 5–10 °C warmer) caused by the electric power plant suggest that female dusky sharks follow the thermal niche–fecundity hypothesis by selecting warmer waters to optimize gestation, while male sandbar sharks socialize at the site. This study represents the first attempt to examine the fine-scale structure of a mixed-species aggregation of sharks and provides new insights into the shark’s social structures through tolerance of each other and social-niche partitioning in this mixed-species aggregation.