Odontocete cetaceans foraging behind trawlers, worldwide

摘要

Several populations of odontocete cetaceans, including at least 19 species, have modified their behavior and adapted to foraging in association with trawlers. We review information on odontocete interactions with different types of trawlers across 13 Food and Agriculture Organization fishing areas around the world. We also review knowledge gaps, the effects on odontocete ecology, distribution, behavior and social organization, the main mitigation options, and some management avenues that could help reduce incidental mortality. Trawlers involved in the interactions varied greatly in gear and target species, implying odontocetes have developed behavioral specializations to forage under a variety of conditions. Specialized behavior included venturing into a moving trawl net to feed on the organisms trapped in the net, feeding on fish stirred up by the net, extracting fish from the outer mesh, feeding on catch lost during hauling, and scavenging on discarded catch. Foraging behind trawlers facilitates access to prey, and in some instances may compensate for scarcity of natural prey within areas exposed to intensive fishing or environmental degradation. This opportunistic foraging strategy, however, exposes the animals to potential harm and mortality in trawl gear. The combined effect of facilitated foraging and bycatch on the status and trends of odontocete populations is unknown. The economic damage caused by odontocetes, e.g. in terms of loss of marketable catch and gear damage, remains largely conjectural. Attempts to reduce depredation and/or bycatch in trawl gear have included acoustic deterrents and exclusion devices installed in nets, although neither technique has proven to be consistently effective.