Multiyear social stability and social information use in reef sharks with diel fission-fusion dynamics

Abstract

Animals across vertebrate taxa form social communities and often exist as fission-fusion groups. Central place foragers (CPF) may form groups from which they will predictably disperse to forage, either individually or in smaller groups, before returning to fuse with the larger group. However, the function and stability of social associations in predatory fish acting as CPFs is unknown, as individuals do not need to return to a shelter yet show fidelity to core areas. Using dynamic social networks generated from acoustic tracking data, we document spatially structured sociality in CPF grey reef sharks at a Pacific Ocean atoll. We show that sharks form stable social groups over multiyear periods, with some dyadic associations consistent for up to 4 years. Groups primarily formed during the day, increasing in size throughout the morning before sharks dispersed from the reef at night. Our simulations suggest that multiple individuals sharing a central place and using social information while foraging (i.e. local enhancement) will outperform non-CPF social foragers. We show multiyear social stability in sharks and suggest that social foraging with information transfer could provide a generalizable mechanism for the emergence of sociality with group central place foraging.

Keywords: central place foraging; grey reef sharks; local enhancement; social network.

Figure 1.

Spatial and social assortment. (a) Palmyra Atoll US National Wildlife Refuge (red diamond) in the Central Pacific Ocean. (b) Space use measured as the 50% UD of sharks assigned to their respective communities, which were defined using community detection of movement networks in addition to residency behaviour (colours reflect communities in c). (c) Social networks and the distribution of weighted assortativity coefficients $(r_d^w)$ for 10,000 random networks (boxes) and observed networks (red circles) across 4 years of shark telemetry data. Each node in the network represents an individual shark, with clusters showing closely associated dyadic pairs. Networks were all significantly, positively assorted by community, represented as different coloured nodes. No assortment is illustrated by blue dashed line. (p < 0.05*, p < 0.01** and p < 0.001***). (Online version in colour.)

Figure 2.

Diel period predicts changes in group size within the two largest communities. (a) Number of acoustically tagged sharks detected at core receivers increase significantly throughout the day for individuals within the two largest communities (red and blue, figure 1). (b) Frame capture from an animal-borne camera of a grey reef shark engaging in close following behaviour. (c,d) Camera tag derived minimum group size changes throughout the day for two female grey reef sharks within community 2. (Online version in colour.)

Figure 3.

Strategy and survivorship modelling. Survival time of simulated sharks behaving either as central place foragers (CPF) or wanderers (do not use a central place) under different levels of food patch stability (a,b: 5% instability, c,d: 25% instability, e,f: 50% instability). Regardless of prey abundance (a,c,e: 100 prey patches, b,d,f: 200 prey patches), or the starting ratio between the two foraging strategies, under all conditions CPF individuals survive for longer durations, and only CPF successfully survive for the full duration of model time under more stable conditions. Once prey density is sufficiently stable and/or high, there is no variation in likelihood of survival of CPF. (Online version in colour.)