Marine finfish aquaculture and its appeal to bottlenose dolphins

This text was written for the blog of OceanCare, our partner organization and the main funder of this study.


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

Between October 2010 and April 2011, we conducted a research project in the Northern Evoikos Gulf, a semi-enclosed basin in central Greece, doing photo-identification and investigating factors affecting dolphin distribution throughout the Gulf. We then used statistical models to describe dolphin presence according to variables including bathymetry, sea surface temperature, chlorophyll, and various anthropogenic variables.

One of the most interesting findings concerned the interaction between dolphins and local fish farms. Our study showed that fish farms were the major factor determining dolphin distribution. The waters of the Northern Evoikos Gulf have been heavily overfished and it is quite possible that, today, fish farms are among the few places where dolphins can still find enough prey.

Luckily, dolphins do neither depredate nor damage the farms. Rather, they feed on fish attracted by a higher productivity around the cages, where a part of the food pellets given to farmed fish may drift away.

Although dolphin occurrence was generally higher near fish farms, some farms were found to have a much higher "appeal". Bottlenose dolphins actually spent most of their time close to a cluster of farms situated in the proximity of an appalling ferronickel smelting plant. This large plant, active year-round since 1969 and operating 24 hours a day, is regarded as the main producer of ferronickel in Europe.

By feeding in the immediate proximity of a heavy industry, dolphins are consistently exposed to polluted waters, contaminated prey and noise. All that is likely to result in long-term negative impacts on the animals. Several individuals photographed during this study actually showed tumours, body deformities and skin diseases, to an extent not found in other parts of the Mediterranean Sea where we have been studying bottlenose dolphins for decades.

The results of our study have been recently published in the peer-reviewed scientific journal Aquatic Conservation: Marine and Freshwater Ecosystems. By making this information available to the scientific and conservation community, we aim to contribute to a process that will hopefully lead to management action to protect the dolphins and the local ecosystem.

As we wrote in our published article:

"The consequences of feeding around fish farms situated in coastal waters exposed to industrial noise, smoke, runoff and large-scale disposal of metallurgic waste is a conservation concern, and more information is needed to assess long-term impact on the population dynamics of bottlenose dolphins. In addition, the close proximity of fish farms, industry, and the slag disposal area may pose threats that extend beyond ecology of the region, in the form of the quality of fish produced at fish farms that are in turn consumed by humans." 

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You may request a pdf copy of this paper to the first author, Silvia Bonizzoni.

Bonizzoni S., Furey N., Pirotta E., Valavanis V.D., Würsig B., Bearzi G. 2013. Fish farming and its appeal to common bottlenose dolphins: modelling habitat use in a Mediterranean embayment. Aquatic Conservation: Marine and Freshwater Ecosystems DOI: 10.1002/aqc.2401.