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Threats

Type Of Threats

In recent years, there has been a general increase in the directed taking of small cetaceans in lesser developed countries. These new and expanding fisheries require careful, but tactful monitoring. Many fisheries are for cetaceans in general rather than for specific types, so managing them on the basis of individual species is difficult. In Japanese waters, there is extensive directed take, mainly for human consumption. Some fisheries are for certain species of small cetacean, while others are generic. In South America, some of the resident species are hunted for crab bait, and many of those species are already vulnerable to incidental take. Sri Lanka has many generic small cetacean fisheries which might threaten some local populations.

Incidental takes most often occur when specimens get caught in large nets set to catch fish. Unable to escape or reach the surface, they drown and perish. Certain types of fishing gear, such as set nets, are particularly dangerous to small cetaceans. This problem may represent a greater threat to cetaceans than directed takes. For several populations, including the Baiji of the Yangtze River, the hump-backed and bottlenose dolphins off eastern South Africa, and the striped dolphins of the Mediterranean Sea, the mortality rate due to incidental taking is greater than the maximum amount that the population can sustain without significant decline. The effects of this type of threat on the dusky dolphins of the eastern South Pacific and the northern right whale dolphins of the central North Pacific is also a source of concern.
A major concern is large-scale monofilament pelagic driftnetting. It causes overexploitation of the fish population, and marine mammals and seabirds often get entangled in them. Also, discarded nets, sometimes called "ghost nets," remain active for several years, during which they continue to entrap cetaceans. In a resolution made on 22 December 1989, the United Nations General Assembly agreed to end pelagic driftnetting by the end of June, 1992 unless and until procedures to reduce the problems of this type of fishery could be implemented. Some countries took steps to forbid any use of pelagic driftnets within 360 kilometers of their coasts.                                  

 The encroachment of human civilization on natural habitats also represents a serious threat to the survival of some stocks.Costal development may destroy fish nursuries, eliminating the food supply of the local cetaceans. Coastal mangrove swamps, found in tropica regions, are a very important habitat for some species of cetecean, especially the hump-backed dolphins of South Asia and West Africa. These areas are both fish nursuries and shelters for adult fish. Continued urban development and industrialization threatens these regions. The damming of rivers on such systems as the Ganges and the Amazon has a nasty effect on riverine populations. In addition to the ecological effect resulting from the change in the flow of water, riverine populations may be permanently separated. Individual stocks may not be viable in the long term and seasonal migrations may be disrupted.

Pollutants enter the food chain near its bottom and build up in top predators, such as cetaceans. In addition, cetaceans may injest the wastes and debris from discarded fishing gear. Mineral exploration and exploitation are also a concern, but unless cetaceans are caught in survey explosions or oil spills, the danger is probably minimal. The effects of such toxins on cetaceans are not entirely clear. The death of cetaceans due to pollution is difficult to recognize, especially when it occurs in large numbers. In other mammals polychlorinated biphenyls (PCBs) adversely affect reproduction, and the same may hold true for cetaceans.

Ironically, an important method of improving public awareness about cetaceans and promoting conservation measures may also be a serious threat to these creatures. In addition to normal ship traffic causing death due to collisions and propeller injuries, the increase in visitors that come for whale watching expeditions may disturb these animals. Studies show that if the visitors are careful and treat the resident cetaceans with propriety, the animals will have no difficulty acclimating. Commercial whale watching outfits are careful to ensure that visitors are tactful, but private individuals may not understand all of the necessary provisions.