The U.S. government estimates that as many as 100,000 marine mammals are killed or injured by the U.S. commercial fishing industry each year. These deaths and injuries result from boat collisions, entanglements in fishing gear, and entrapments as nontarget species (often called bycatch). Many more marine mammals are killed, injured, and harassed by foreign fleets, most infamously by the tuna fleets in the Eastern Tropical Pacific (ETP), despite large-scale efforts by environmental and animal protection groups to change fishing practices in the area. Because spotted and spinner dolphins associate closely with yellowfin tuna in the ETP, the nets used to capture the tuna encircle, traumatize, and may entangle and kill these dolphins.
Drift-netting, a practice that has yet to be completely outlawed internationally, is a deadly threat. Made of fine nylon mesh, drift nets can be miles long and many feet deep. The nets are left adrift for days and even weeks before fishing boats return to check their catch. These nets catch everything in their paths, including mammals, birds, turtles, and sharks. Many of these victims are considered worthless bycatch and are discarded. Drift nets are the indiscriminate stripminers of the seas.
Fisheries pose another danger, for they may compete with marine mammals for the same fish and in the process reduce fish populations that marine mammals depend on for food. Fisheries in many regions have cut their fleet sizes because they have overfished populations, and some have had to close because fish stocks have become commercially extinct, too small for continued commercial exploitation.
The loss of these fish populations has a negative effect on the marine ecosystem. For example, the decimation of the Steller sea lion population in the north Pacific is attributed to the collapse of fish stocks the sea lions have depended on, stocks that were overfished by certain Alaskan fisheries. Marine mammals cannot survive without a plentiful food supply, so overfishing by fisheries presents a real danger to the health of the marine ecosystem, especially when marine environments are also beset by pollution and other forms of habitat degradation.
The seal and sea lion populations on the west coast of the United States have responded positively to the protections they are afforded under the Marine Mammal Protection Act. Their relatively healthy numbers are perceived by the fishing industry, both commercial and recreational, as a growing threat to their business. Commercial fishermen claim that seals and sea lions steal fish from nets and lines and also that they prey on endangered stocks of salmon, thus driving these fish populations toward extinction.
Recreational fishermen claim seals and sea lions are threatening their livelihoods by following charter vessels out to sea and taking fish from sportfishermen's lines. While it is true that some seals and sea lions have taken advantage of foraging opportunities provided by commercial and recreational fishing boats and dams and locks at the entrance to fish spawning rivers, there are non-lethal solutions to these interaction problems. In addition, the real cause of declining fish stocks is not growing seal and sea lion populations but the growing human population. Too many people are moving to the coast and contributing to unsustainable habitat degradation and exploitation of resources.
