Marine finfish aquaculture and its "fishy" appeal to bottlenose dolphins

Bottlenose dolphins in the Mediterranean are increasingly attracted by marine finfish aquaculture facilities. Increased availability of prey near fish farms may help the dolphins survive in areas that have been intensively overfished. However, foraging near fish farms may expose the animals to pollution and anthropogenic noise.

Focusing on the analysis of visual survey and dolphin follow data collected in October 2010 and between March and April 2011, we investigated factors affecting the distribution of bottlenose dolphins throughout the Northern Evoikos Gulf, Greece. We used generalized additive modelling (GAMs) and generalized estimation equations (GEEs) to describe dolphin presence according to variables including sea surface temperature (SST) and Chlorophyll-a (Chl-a), distances to sources of anthropogenic influences (including fish farms, a ferronickel plant, and a slag disposal area). We looked at the effects of geographic, bathymetric, oceanographic and anthropogenic variables, also taking into account survey effort and sea state conditions. Visual survey data and dolphin follows were combined to obtain detailed information on habitat preferences by the animals.

Our study showed that fish farms were the major factor determining the spatial distribution of the animals. All other factors considered were considerably less important. Although dolphin occurrence was higher near finfish aquaculture facilities generally, not all fish farms had the same appeal. We therefore investigated and discussed the reasons behind the different attractiveness of individual facilities. Bottlenose dolphins turned out to spend most of their time close to a cluster of aquaculture facilities situated in the close proximity of a polluting industry — a ferronickel smelting plant situated in the town of Larymna. This large plant, active year-round since 1969 and operating 24 hours a day, is the main producer of ferronickel in Europe. The consequences of feeding around fish farms in waters exposed to industry noise, smoke, runoff and large scale disposal of metallurgic waste is a conservation concern.

State-of-the-art methods developed in our study can be used in other areas, to achieve a better understanding of the importance of marine finfish aquaculture for bottlenose dolphins and other species, as well as to investigate fine-scale interactions between dolphins and fisheries. Predictions of fish farm attractiveness can help allocate coastal areas designated for marine aquaculture, so that marine mammal occurrence is taken into account and any conflict is minimized.

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The manuscript's first author Silvia Bonizzoni is going to attend the 27th Annual Conference of the European Cetacean Society (Setúbal, Portugal, in April 2013) to present our study and disseminate its results among members of the scientific community. In addition, a manuscript has been formally submitted to a peer-reviewed science journal and is currently under review.

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