Abstract
Fish spawning aggregations (FSA) act as biological hotspots that concentrate food and nutrients across a broad trophic spectrum. In Pohnpei (Federated States of Micronesia), 20 female grey reef sharks (Carcharhinus amblyrhynchos) were acoustically tagged at two multi-species grouper (Epinephelidae) FSA to examine the likelihood that these mesopredators utilize FSA as a seasonal food source. Both FSA sites are within small-scale MPAs, thus providing a secondary opportunity to examine their conservation potential during these ephemeral events. Shark movement and residency was gauged against known spatial and temporal grouper reproductive patterns using an array of 15 and 50 acoustic receivers at Ant Atoll and Pohnpei (Island), respectively. Activity space was investigated using Kernel Density estimates of individual sharks, and residency indices (RI) were analyzed based on daily and monthly occurrence at the array. Three distinct residency patterns were identified: transient, semi-transient, or resident (Daily RI <0.40, >0.40 to 0.80, or >0.80, respectively). Generalized linear mixed models (GLMMs) were used to identify biological and environmental factors influencing shark activity space, including month, temperature, shark size, spawning month, and residency pattern. Findings revealed significant changes in average monthly residency indices and kernel densities during spawning months in support of an opportunistic foraging strategy around FSA. Monthly residency was higher during spawning months among semi-resident and transient sharks, while average monthly activity space was concentrated around FSA. Best-fit models for the GLMM indicated that activity spaces were most influenced by month and grouper spawning month. Seven of 20 sharks demonstrated inter-island movement and wide variations in individual movement and spatial requirements were shown. The concentration of sharks and groupers at unprotected FSA sites increases their vulnerability to fishing and supports the need for combined area and non-area management measures to effectively protect these species.
Highlights
Fish spawning aggregations (FSA) represent critical life history phases for a number of coral reef fishes [1] and serve as biological hotspots that provide food and nutrients to marine organisms across a wide trophic spectrum [2,3,4,5,6,7]
Model averaging of cumulative Akaike Information Criteria (AIC) weights supported the model selection, and parameters in the top candidate models for both 50% and 95% Kernel Density (KD) estimates were shown to influence activity spaces of grey reef sharks tagged at Pohnpei (Table 4)
Month was the most influential parameter for the core and extent of activity space, followed by grouper spawning period, with activity spaces contracting during these periods (Fig 4 and Table 4)
Summary
Fish spawning aggregations (FSA) represent critical life history phases for a number of coral reef fishes [1] and serve as biological hotspots that provide food and nutrients to marine organisms across a wide trophic spectrum [2,3,4,5,6,7]. But spectacular are the foraging events on FSA by subtropical and tropical reef-associated requiem sharks (Carcharhinidae), including lemon sharks (Negaprion brevirostris; Poey 1868) [24], grey reef sharks (Carcharhinus amblyrhynchos; Bleeker 1856) [14, 25], and blacktip reef sharks (Carcharhinus melanopterus; Quoy and Gaimard 1824) [26]. During these periods, sharks may become vulnerable to the same types of gear used for target aggregating fish Sharks may become vulnerable to the same types of gear used for target aggregating fish (Rhodes pers. observ.), protective measures that target spawning fishes may provide a temporary respite from fishing for sharks
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