The information reviewed here indicates an extensive worldwide interaction between marine mammals and fisheries. Most notably, in almost all the statistical areas where studies have been conducted, large numbers of certain small cetaceans or pinniped species, especially the coastally distributed species, are affected by coastal gillnet fisheries. For several marine mammal species, including the baiji, vaquita, Mediterranean monk seal, and Hector's dolphin, operational interactions with fisheries have been proposed as threats to species survival. Populations of certain primarily coastal species such as the harbour porpoise, finless porpoise, and hump-backed and bottlenose dolphins, may also be threatened locally.
In fact, most species of marine mammal, with the exception of some of the rarer oceanic beaked whales for example, have been recorded at some time or other caught in some type of fishing gear. It should be remembered in these cases that only a very small proportion of catches are ever actually recorded, so that the reporting of just a few individuals in a specific fishery may be indicative of a larger interaction. Only when a detailed study of a fishery is made can this be assessed. Furthermore there are large areas of the world where it seems likely there may well be interactions between marine mammals and fisheries, but for which there is, as yet, no information, and no idea of any impact that such fisheries may be having.
Assessing the actual impact of a fishery on marine mammal populations has only been achieved in a very few cases. The general problem is that for most cetacean species at least, it is very difficult to assess population size, or to assess the consequences of an uncertain and unpredictable catch rate. This does not imply, however, that there is no problem where the impact of a fishery cannot be assessed. On the contrary, where large scale catches are observed it seems reasonable to take a precautionary approach rather than wait for detailed assessment of a population decline, as marine mammal populations have been repeatedly shown to be highly vulnerable to uncontrolled additional mortalities. This approach has been adopted by the United Nations in its resolution (44/225) on large-scale pelagic drift net fishing and its impact on the living marine resources of the world's oceans and seas.
Solutions to the problem of marine mammal entanglement have been sought in several parts of the world with a variety of techniques, and were discussed at the IWC workshop on mortality of cetaceans in passive fishing nets and traps at La Jolla in October 1990. No general solution to the problem has been found, but in one or two cases some reduction in the numbers of marine mammals taken in gillnets has been noted by, for example, rigging driftnets to fish some few metres below the surface. In other areas, however, such subsurface sets have not been successful. It would appear that, short of banning the use of gillnets entirely, solutions to the problem will have to be found on a fishery by fishery basis.
Many of the studies on marine mammal entanglement in fishing gear which have been reviewed above have been stimulated by concerns for the status of certain marine mammal populations. In contrast to this situation, competition between marine mammals and fisheries has been comparatively little studied in the past few years. Growing concerns about the possible consequences of competition between marine mammals and fisheries would suggest that this will become an increasingly important issue in fisheries management and conservation in future.
