Trophic niche partitioning between the white shark (Carcharodon carcharias) and the shortfin mako (Isurus oxyrinchus) in the central Mediterranean Sea

Context Large predatory sharks such as the white shark (WS) and shortfin mako (SMK) have been historically depleted to Critically Endangered levels in the Mediterranean Sea. Despite their low abundance, the Tunisian Plateau seemingly plays a crucial ecological role as a potential nursery and feeding ground, supporting the early life stages of these species. Aims Here we investigated the trophic ecology of WS and SMK, which co-exist in the Tunisian Plateau, focusing on juveniles and young-of-the-year (YOY). Methods We conducted stable isotope analysis of carbon (δ13C) and nitrogen (δ15N) from muscle samples to assess the trophic niche breadth and overlap between the two species. We estimated the possible prey contribution with Bayesian mixing models under two prey-grouping schemes, namely, functional prey categories (cephalopods, small pelagics, large pelagics, demersal fishes, dolphins) and habitat-based categories (coastal-pelagic, oceanic-pelagic, coastal-demersal, bathyal-demersal). Key results White sharks had significantly higher δ15N values than did shortfin makos, but no differences in age classes or sexes were detected, and no inter- or intraspecific variation in δ13C values were observed. Corrected standard ellipse areas were similar, with only ~12% core-area overlap between species, providing evidence for niche partitioning. The mixing model results were consistent across prey grouping schemes; WS seemingly display a generalist diet both in functional preys and foraging habitat, whereas SMK rely more on small pelagic fishes largely derived from coastal-pelagic habitats. Conclusions Trophic segregation between WS and SMK supports their co-existence on the Tunisian Plateau. Such differential resource use is likely to minimize interspecific competition and promote stable sympatry in this productive area. Implications Our results constitute the first isotopic and mixing-model-based dietary assessment of early life stage WS and SMK in the central Mediterranean. Their sightings may reflect both a higher population abundance in the region and intense fishing pressure. Given their trophic roles and the potential ecological consequences of their decline, incorporating trophic information with complementary methods (e.g. telemetry) could be useful to track feeding-ground utilization. Such integrated approach could inform timing and placement of mitigation measures (e.g. gear modifications) tailored to each species’ trophic habits, helping sustain their survival in the Mediterranean Sea.