Dolphins not to be blamed

A new paper by Tethys authors has recently been published in Aquatic Conservation: Marine and Freshwater Ecosystems

Biomass removal by dolphins and fisheries in a Mediterranean Sea coastal area: do dolphins have an ecological impact on fisheries?

The study evaluates the biomass removed by of dolphins and the fishing fleet on a coastal ecosystem in the Ionian Sea. They found that the total biomass consumed annually by local dolphin populations – 15 short-beaked common dolphins and 42 common bottlenose dolphins – was 15.5 and 89.8 tonnes, respectively. The total biomass removed by the local fishing fleet (307 fishing boats) was 3469.2 tonnes, i.e. about 33 times greater than that removed by dolphins.

In other words, dolphins remove 2.9% of the total biomass and fisheries 97.1%.

The message is clear: dolphins in Mediterranean coastal waters are too often blamed for competing with fisheries and deemed responsible for reduced catches, but reality can be different when measured.

SB

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Figure (click to enlarge): Total biomass removed by dolphins and fisheries, as calculated by Bearzi et al. 2010; dolphins remove 2.9% of the total biomass, fisheries 97.1%.

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Bearzi G., Agazzi S., Gonzalvo J., Bonizzoni S., Costa M., Petroselli A. 2010. Biomass removal by dolphins and fisheries in a Mediterranean Sea coastal area: do dolphins have an ecological impact on fisheries? Aquatic Conservation. DOI: 10.1002/aqc.1123

ABSTRACT

1) Dolphins are often claimed to compete with fisheries, including through removal of substantial biomass. To calculate the biomass removed by fisheries and the degree of resource overlap with dolphins in a coastal area of Greece, estimates of dolphin abundance based on photographic capture–recapture were combined with an assessment of fishing effort and catch.

2) The estimated total biomass consumed annually by local dolphin populations – 15 short-beaked common dolphins and 42 common bottlenose dolphins – was 15.5 and 89.8 tonnes, respectively. The total biomass removed by the local fishing fleet (307 fishing boats) was 3469.2 tonnes, i.e. about 33 times greater than that removed by dolphins.

3) Dolphins removed 2.9% of the total biomass, fisheries 97.1%. Nine purse seiners (representing only 3% of the active fishing fleet) were responsible for 31.9% of biomass removal. Similarity of biomass composition between dolphins and fisheries was expressed by a Pianka index of 0.46 for common dolphins and 0.66 for bottlenose dolphins.

4) Overlap differed according to fishing gear. Common dolphin overlap was higher with purse seiners (0.82), and lower with beach seiners (0.31), bottom trawlers (0.11) and trammel boats (0.06). There was virtually no overlap with longliners (0.02). Bottlenose dolphin overlap was higher with trammel boats (0.89) and bottom trawlers (0.75), and lower with longliners (0.38), purse seiners (0.24) and beach seiners (0.18). There was minimal overlap (0.12) between the two dolphin species.

5) This study suggests that ecological interactions between dolphins and fisheries in this coastal area have minor effects on fisheries. Conversely, prey depletion resulting from overfishing can negatively affect dolphins. Fisheries management measures consistent with national and EU legislation are proposed to ensure sustainability and to protect marine biodiversity.

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A post by Carolyn Kraft