Dolphins restructure social system after reduction of commercial fisheries

Abstract

Although human activities are known to affect the social behaviour of group-living animals, the resilience of animals' social structure to disturbance is poorly understood. In the 1990s, bottlenose dolphins, Tursiops aduncus , in Moreton Bay, Australia, formed two distinct social communities (‘trawler’ and ‘nontrawler dolphins’) based on foraging interactions (or lack thereof) with commercial prawn trawlers. Members of the two communities almost never associated, despite overlapping home ranges. Since then, changes to fisheries legislation have substantially reduced trawling in Moreton Bay. We used association analyses and social network metrics to compare patterns of sociality among bottlenose dolphins across two periods: 1997–1999 (during trawling) and 2008–2010 (post trawling). Over this decade, their social network became less differentiated and more compact (average geodesic distance between individuals decreased), with significantly more and stronger associations between individuals (mean and maximum half-weight indices increased). The previously described partitioning into two communities has disappeared, with former ‘trawler’ and ‘nontrawler dolphins’ now dispersed over the entire social network and associating with each other. This restructuring suggests that although fisheries can influence the social behaviour of bottlenose dolphins, their social structure represents a complex adaptive system that is resilient to disturbance.