Strategic foraging: Understanding hawksbill (Eretmochelys imbricata) prey item energy values and distribution within a marine protected area

Abstract

Anthropogenic disturbances affect the health of coral reefs worldwide and may also impact hawksbill (Eretmochelys imbricata) foraging areas, potentially decreasing sponge numbers, while increasing macroalgae. Few studies have been conducted to understand energy content of hawksbill prey. We investigated observed (Geodia neptuni and Kallymenia limminghii) and potential (Xestospongia muta and Halimeda opuntia) hawksbill prey abundances and their energy contents in the Sandy Bay West End Marine Reserve, and related prey distribution to hawksbill distribution within the reserve. We analysed prey abundances by conducting in-water habitat transects followed by point count analyses. In-water hawksbill observations were recorded to provide total times turtles foraged on prey. We then measured energy content of prey types using microbomb calorimetry. Habitat assessments indicated sponges were most abundant in West Bay and West End, whereas macroalgae were most abundant in West End. Foraging observations indicated juvenile hawksbills spent more time foraging on G. neptuni (x̅ = 236.5 s) than K. limminghii (x̅ = 98.0 s) and no time foraging on either X. muta or H. opuntia. Energy content was higher for G. neptuni (4.09 kJ g⁻¹) and K. limminghii (12.88 kJ g⁻¹) than X. muta (2.48 kJ g⁻¹) and H. opuntia (1.27 kJ g⁻¹). Hawksbills were frequently observed feeding in West Bay where sponges were abundant and were also observed foraging on K. limminghii throughout this area. Fewer hawksbills were observed in West End and Sandy Bay than in West Bay, and these areas had fewer sponges compared with West Bay. Hawksbills benefit from foraging on the abundant observed sponge and macroalgae within their home ranges, allowing them to conserve energy and increase potential net energy gains from high energy prey.