Interactions with fishery
Fisheries can affect cetaceans both directly and indirectly. Effects on the animals may include:
- direct bycatch;
- injuries or death by fishermen perceiving the animals as competitors;
- prey depletion or changes in food prey composition/distribution caused by overfishing;
- habitat loss and/or degradation (e.g., from bottom trawling);
- short- to long-term modifications in cetacean behaviour leading to emigration, dispersion or reduced reproductive rates as a consequence of direct or indirect interactions with fisheries.
There is a long history of interactions between cetaceans and fisheries in the Mediterranean Sea. Cetaceans were always attracted to fishing gears; they attempt to remove bait and catches during commercial and recreational fisheries. Cetaceans also feed at mariculture (fish farm) enclosures. With some exception, current interactions mainly involve coastal, small-scale artisanal fishery. Gears likely to have the most interaction with cetaceans are trammel and bottom gillnets, driftnets, trawls, longlines and purse seines.
Interactions between cetaceans and coastal fisheries may negatively affect the fisheries through:
- Abrasion and wounds to fish caused during capture attempts or while “playing” with fish during fishing operation, even when they are satiated. Cetaceans may take portions of fish or the entire fish, rendering them nonmarketable.
- Catches’ reduction disturbing fishing operation. Cetaceans can be causing fish schools to disperse and escape from the net. In the case of fish farms, dolphins may attack and harass fish through the pen walls, thus stressing, scarring and wounding the fish and resulting in lower product quality through reduced value or reduced fish weight.
- Gears’ damaging (gear may not fish as efficiently and a loss of catch may result). Additional costs include time for repairing fishing gears and expenses for new material.
- A real or perceived ecological competition with cetaceans, based on the conviction that depredation – particularly by dolphins – reduces the amount of fish available to fisheries
Information on the economic effects of dolphin interactions with Mediterranean fisheries is qualitative and inadequately documented.
Detailed quantitative information on the spatial, seasonal, and operational features of small-scale coastal trammel and gillnet fisheries in the region is missed.
Identification of hotspots where overlap occurs (i.e. high dolphin densities matched with high levels of fishing activity) should be followed by rigorous site-specific studies to characterize and quantify the costs of dolphin depredation.
Mediterranean dolphins are often thought to compete with fishermen reducing fishery yields, but no robust scientific investigation support this hypothesis.
On the other hand, it has been demonstrated how competitive effects are more likely to affect dolphins than humans, having the total biomass removed by fishery exceeded that removed by dolphins.
Interactions in the study area
In the study area, cetacean-fishery interactions involved a variety of gears.
Specimens of Risso’s dolphin and sperm whale were found entangled in surface longline systems. The entangled Risso’s dolphin was alive, swimming with difficulty without diving, and escorted by the other members of the group; the line and the hooks were attached to the dorsal fin and wrapped around the body up to the caudal fin. The sperm whale was found entangled in an abandoned longline, the animal was in an advanced state of decomposition and it was not possible to recover the body.
Several specimens of striped dolphin and sperm whale were victims of the interaction with driftnets (spadara and ferrettara).
The animals have been found offshore, adrift, completely enveloped by the nets; in some cases, the animals have stranded, showing evident signs (mutilation of the tail, scars) of interaction with the driftnets.
Interactions with fishery were recorded also for the trawlers, also known as “paranza“.
The involved species were bottlenose dolphin, striped dolphin, and fine whale.
Cetaceans were observed following the operating trawlers and making long dives, likely taking advantage of the movement created by the net on the seafloor and/or the fish escaping from the net.
The interviewed fishermen affirmed that for the bottlenose dolphin species the predation happens also inside the net, in particular when the net is recovered slowly; in this time the aperture of the net is greater and allows the dolphins to prey on it in turn.
The historical interaction involves trammels and bottom gillnets, and consists of the depredation, by the bottlenose dolphin species, of the nets. Trammels and gill nets represent the small-scale artisanal fishery, most often family-owned. Typically traditional, involve relatively small fishing boats, making short fishing trips, close to shore, mostly for local consumption.
The feeding behaviour of dolphins on the nets it is hard to record. Defining interaction when dolphins are present around a fishing net, or assuming that activities visible at the surface are representative of activities beneath the surface could lead to a misinterpretation of the real dolphin behaviour.
The presumably dolphin-caused net damage may result not only from entanglement with bottom debris, natural substrate or inadequate handling/maintenance of the fishing gear, but also from interaction with other predators like logger head sea turtles (Caretta caretta), Mediterranean moray Muraena helena, and other species as European otter (Lutra lutra) and tope shark (Galeorhinus galeus). The morphological damage category ‘Bite’, with its characteristic shape and size, is sometime attributable to the action of other predators cuttlefish (Sepia spp.), common octopus (Octopus vulgaris), European conger (Conger conger).