Moreton Bay is a small patch of ocean bounded by Queensland, Australia, on the west and on the east by Moreton Island and North Stradbroke Island. The bay is home, by various estimates, to between six hundred and eight hundred Indo-Pacific bottlenose dolphins (Tursiops aduncus). A study conducted in the late 1990s found that the dolphins of Moreton Bay generally clustered into two social groups that were defined by the ways in which they interacted with commercial prawn trawlers. The "trawler" community was often seen following the trawlers, chowing down on discarded by-catch, while the "nontrawler" community consisted of the dolphins that foraged in other ways. (Scientists sure are creative with their designations, aren't they?)
Despite the fact that the two groups of dolphins coexisted in the same physical space, their different foraging methods meant that the two groups rarely interacted. "Almost all associations," the researchers say occurred "within rather than between these communities." A decade later, the researchers returned, led by graduate student Ina C. Ansmann of the University of Queensland. Everything had changed.
The change was distinctively human in origin. In 1997, the State of Queensland implemented the Moreton Bay Marine Park Zoning Plan, followed in 1999 by the Trawl Management Plan. The Marine Park Zoning Plan identified portions of the bay that were protected from activities such as trawling, and the Trawl Management Plan limited the number of fishing vessels permitted to operate in the non-protected portions of the bay, as well as the number of days and nights on which fishing could occur. Together, the two plans reduced in fishing-related activity in Moreton Bay by fifty percent.
In the decade following the implementation of these laws, the two dolphin communities melted into one. Dolphins form groups in what researchers call fission-fusion societies, which are quite similar to those of primates such as chimpanzees and humans. Characteristic of fission-fusion societies are patterns of association containing many weak ties and a handful of stronger ones. Groups-within-groups often form (think of a set of best friends among a larger group of close friends), usually as a function of sex or reproductive status.
What could explain such a marked shift in population structure? One possibility is that the reduction in commercial trawling led to an increase in the availability of fish in the bay (just as Hurricane Katrina allowed fish stocks to increase in the gulf, thanks to a grounded fishing fleet). As the availability of food increases, competition for that food naturally decreases. Wars are not often fought during times of plenty. Recovering fish stocks may have allowed dolphins to socialize in a non-competitive way, since the "benefits of sociality outweighed the reduced cost of competition for food."
Ansmann points out that the reduction of trawling in Moreton Bay is likely beneficial for the conservation of bottlenose dolphins, not just because of reduced human invasion into their space, and not just because of the increase in food, but because larger communities make for more genetically stable and diverse populations. Larger communities may also result in increased opportunities to learn new behaviors due to social learning. As dolphins will certainly be faced with new problems to which they must adapt as the climate changes at global and local levels, flexible problem-solving will become increasingly important for their survival.
Ina C. Ansmann, Guido J. Parra, B. Louise Chilvers, & Janet M. Lanyon (2012). Dolphins restructure social system after reduction of commercial fisheries Animal Behaviour, 84, 575-581 DOI: 10.1016/j.anbehav.2012.06.009
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Images: Indo-pacific bottlenose dolphin via Flickr/CW Ye. Diagrams adapted from Ansmann et al. (2012).