Social predation allows groups of predators to search for, pursue, and capture prey with greater efficiency than using solitary hunting. During ~3,000 hours of underwater observations of nocturnal shark foraging activity, we report for the first time a heterospecific association between two shark species, with the nature of the interaction ranging from commensalism to kleptoparasitism and mutual-ism. These foraging associations with whitetip reef sharks offer grey reef sharks the opportunity to access different types of prey and increase their predation success. These findings add more evidence that heterospecific social associations may exist across animal groups including large marine predators.
Social foraging allows groups of predators to search for, pursue, and capture prey with greater efficiency than using solitary hunting. It can vary in complexity and take many forms ranging from cooperative hunting, to social information sharing and local enhancement (Lang and Farine 2017). Theoretical and empirical studies support the advantages of group foraging, although there will be trade‐offs between benefits, such as increased prey detection or capture success, and costs such as increased competition amongst group members (Gil et al. 2017).
Many species of groupers form transient fish spawning aggregations (FSAs) that are both spatially and temporally predictable, making them highly vulnerable to fishing. Consequently, many known aggregations have disappeared, making the collection of additional baseline data imperative to inform management actions and decisions that have the capacity to protect these important areas in the long term. Using acoustic telemetry and underwater observations, we document the spatio-temporal dynamics of the camouflage grouper Epinephelus polyphekadion at a FSA in Fakarava, French Polynesia. We show that grouper arrival at the aggregation site started 2 weeks before the full moon with a core area density that increased from 0.13 to 1.25 fish m−2 during the observation period. Following reproduction, almost all camouflage groupers left the FSA site within 48 h. Among 30 tagged groupers in 2011 and despite a relatively low receiver coverage in the pass, 30% returned to the FSA the following year confirming patterns of FSA site fidelity of the species found in previous studies at other locations. Our results confirm that the current management procedures protecting the FSA under the UNESCO biosphere reserve are critical in maintaining the functional role of the FSA by ensuring the persistence and sustainability of large and healthy populations of groupers and sharks.
The extent of the global human footprint [ 1 ] limits our understanding of what is natural in the marine environment. Remote, near-pristine areas provide some baseline expectations for biomass [ 2, 3 ] and suggest that predators dominate, producing an inverted biomass pyramid. The southern pass of Fakarava atoll—a biosphere reserve in French Polynesia—hosts an average of 600 reef sharks, two to three times the biomass per hectare documented for any other reef shark aggregations [ 4 ]. This huge biomass of predators makes the trophic pyramid inverted. Bioenergetics models indicate that the sharks require ∼90 tons of fish per year, whereas the total fish production in the pass is ∼17 tons per year. Energetic theory shows that such trophic structure is maintained through subsidies [ 5–9 ], and empirical evidence suggests that sharks must engage in wide-ranging foraging excursions to meet energy needs [ 9, 10 ]. We used underwater surveys and acoustic telemetry to assess shark residency in the pass and feeding behavior and used bioenergetics models to understand energy flow. Contrary to previous findings, our results highlight that sharks may overcome low local energy availability by feeding on fish spawning aggregations, which concentrate energy from other local trophic pyramids. Fish spawning aggregations are known to be targeted by sharks, but they were previously believed to play a minor role representing occasional opportunistic supplements. This research demonstrates that fish spawning aggregations can play a significant role in the maintenance of local inverted pyramids in pristine marine areas. Conservation of fish spawning aggregations can help conserve shark populations, especially if combined with shark fishing bans.