Prey size differentiation as a mechanism of trophic segregation in sympatric otariids in the Peruvian Humboldt Current System

Trophic segregation facilitates the coexistence of sympatric predators by reducing resource competition. In the Humboldt Current System, the South American sea lion (Otaria byronia, SASL) and the South American fur seal (Arctocephalus australis, SAFS) overlap spatially and exploit similar prey resources. Yet, the role of prey size as a segregation mechanism remains poorly understood. We analyzed sagittal otoliths from five shared fish species, converting otolith length to total length, and compared prey sizes consumed by both pinnipeds under warm and cold sea surface temperature conditions. Significant interspecific differences in prey size were detected under warm conditions for three prey species, suggesting enhanced size-based partitioning when ocean temperatures increase. Under cold conditions, prey sizes consumed by SASL and SAFS were largely similar, except for Normanichthys crockeri. Intraspecific analyses showed greater prey size variability in SAFS than SASL, particularly for Merluccius gayi, and Normanichthys crockeri. Both predators also consumed prey below the minimum landing size established for Peruvian fisheries, indicating partial overlap between pinniped diets and fishery resources. These results provide novel evidence that size-based dietary differentiation is an axis of trophic segregation between sympatric otariids, particularly during warm conditions, and highlight the importance of integrating predator–fishery interactions into ecosystem-based management.