Diet shift and site-fidelity of oceanic whitetip sharks Carcharhinus longimanus along the Great Bahama Bank

Abstract

Identifying the driving forces behind oceanic pelagic shark movements is key to a bet- ter understanding of their life history. Some oceanic pelagic shark species have been shown to ag- gregate in specific regions to mate and/or exploit abundant food resources. The oceanic whitetip shark Carcharhinus longimanus, a subtropical, ectothermic, oceanic pelagic shark that has experi- enced severe population declines, aggregates seasonally around Cat Island (CI) in The Bahamas. Large pelagic teleosts (e.g. billfish, tunas, and dolphinfish) are abundant in this region and oceanic whitetips are anecdotally reported to feed heavily on recreationally caught teleosts. However, it was unknown whether feeding habits at CI substantially differ from longer-term feeding habits. We used tag-recapture to assess site-fidelity of adult oceanic whitetips to CI and stable isotope analysis (SIA) of 2 different tissues (blood plasma and white muscle) to compare short- and long-term feeding patterns. The relatively high recapture rate (20.3%) confirmed that individual whitetips ex- hibit site-fidelity to CI. The aggregation consisted of adult individuals; females were more common, more than half were gravid, and no physical or behavioral evidence of mating or parturition was ob- served at CI. SIA-based Bayesian mixing model estimates of short-term (near CI) diets showed more large pelagic teleosts (72%) than in long-term diets (47%), showing a spatiotemporal differ- ence in oceanic whitetip feeding habits. This suggests that availability of large teleost prey is a possible mechanism underpinning site-fidelity and aggregation of whitetips at CI. These results provide insight into the function of one of the last known aggregations of this once-abundant top predator.