Interrelations with Bottom-Trawl Fishery


The most common species off our Mediterranean shore, the common bottlenose dolphin, is frequently sighted in association with trawl fishing vessels. Some dolphins are seen in the vicinity of the vessel, without any clear interaction with it, others are observed to approach the vessel while the net is being hoisted on deck, and wait with the sea gulls until the waste is thrown back to the sea and still others are seen following the boat during the towing stage, sometimes continuously for hours .


The latter are positioned 300400 m behind the boat (over the mouth of the net), performing a series of 3-4 min dives broken by short surfacing intervals. This behavior is assumed to represent depredation on fish swimming at a fixed speed and course ahead of the mouth of the net, on parts of fish sticking out through the eyes of the net, or on fish that have already been trapped inside.

The latter practice is risky and at times fatal for the dolphins as they occasionally get caught in the net and drown. Some individuals must enjoy the venture enough to play around with the loose lazy-line, to the point of tight entanglement, another mode of incidental death by drowning .



Some of the trapped dolphins are thrown back to sea (with or without being reported), where they sink or eventually wash ashore. Dolphins that are handed over by fishing boat crew to IMMRAC's researchers are treated the same way as freshly beached carcasses. By-catch autopsies reveal that both sexes of all ages become trapped, except for very young calves . However, a closer look at the age distribution of overall mortality (beached and trapped), reveals an unnaturally high mortality rate among the 25 years age group. This may reflect contribution of unreported by-catch of recently weaned juveniles, no longer accompanied by their mother, that are not experienced enough in this foraging mode and therefore are at a higher risk of entanglement .

Some of the questions being studied are:

  1. Trawl-dolphin associations does the association indicate an additional adopted foraging strategy or is the population driven to use it due to a gradual decline in the abundance and availability of their normal prey (because of over-fishing and/or continued sea-bed habitat destruction by the trawlers)?
  2. Culture - Is this behavior common to the whole population, or is it a special skill, possibly inherited as tradition from mother to her young?
  3. Decrease by-catch - Is it possible to document predatory behavior around the net using under-water video recorders attached to the nets, in order to develop methods for preventing/reducing unintentional entrapment without compromising the catch, (for example, by selectively blocking the opening of the net with a very large mesh screen)?
  4. Modeling competition effort is made to develop quantitative models that will simulate the competition between the two principal predators, dolphins and fishermen, over shared and limited marine resources.