Mortality rates for two shark species occupying a shared coastal environment

Abstract

Mortality plays a crucial role in the dynamics of all shark populations. However, our understanding of mortality and the factors that affect survival in wild populations remains limited, particularly for adult sharks. Long-term acoustic monitoring data were used to estimate mortality rates for juvenile pigeye (Carcharhinus amboinensis) and adult spottail (Carcharhinus sorrah) sharks within the same coastal area. Estimates of mortality were low for both species (C. amboinensis: natural = 0.00–0.05 y−1, fishing = 0.07–0.11 y−1, total = 0.11–0.13 y−1; C. sorrah: natural = 0.05 y−1, fishing = 0.03 y−1, total = 0.08 y−1). There were no significant differences in mortality rates between species (natural: p = 0.972; fishing: p = 0.965; total: p = 0.935), despite the difference in species life stages. All monitored individuals were similar in body size, however, suggesting that size may play a greater role than age in the mortality rates for some shark species. High survivorship indicates that this coastal area provides a low mortality environment for both juvenile C. amboinensis and adult C. sorrah. Low mortality for these species is likely a reflection of low predation risk and sufficient prey resources. This, coupled with marine protected areas, may provide juvenile C. amboinensis and adult C. sorrah with similar opportunities and levels of protection.