Cetacean at Risk

Cetacean face an uncertain future. Environmental contamination of oceans, seas, and rivers is a concern. Pesticides, heavy metals, plastics, and other industrial and agricultural pollutants which do not disintegrate rapidly in the environment reduce dolphin and Whales populations, and cause tissue buildup of high levels of contaminants. Injuries or deaths due to collisions with boats, especially their propellers, are also common.

Various fishing methods, most notably purse seine fishing for tuna and the use of drift and gill nets, unintentionally kill many dolphins. Accidental by-catch in gill nets and incidental captures in antipredator nets that protect marine fish farms are common and pose a risk for mainly local dolphin populations and called byCatch.

Cetacean bycatch is the incidental capture of non-target cetacean species by fisheries[1]. Species which are seriously affected by this include dolphins, porpoises, and whales. Bycatch can be caused by entanglement in fishing nets and lines, or direct capture by hooks or in trawl nets.

Cetacean bycatch is increasing in intensity and frequency. This is a trend that is likely to continue because of increasing human population growth and demand for marine food sources, as well as industrialization of fisheries which are expanding into new areas. These fisheries come into direct and indirect contact with cetaceans. An example of direct contact is the physical contact of cetaceans with fishing nets. Indirect contact is through marine trophic pathways where fisheries are severely reducing fish stocks that cetaceans rely on for food. In some fisheries, cetaceans are captured as bycatch but then retained because of their value as food or bait. In this fashion, cetaceans can become a target of fisheries.

Bycatch is increasing
Most of the world’s cetacean bycatch occurs in gillnet fisheries.[3] The mean annual bycatch in the U.S. alone from 1990–1999 was 6,215 marine mammals, with dolphins and porpoises being the primary cetaceans caught in gillnets.[3] A study by Read et al.[3] estimated global bycatch through observation of U.S. fisheries and came to the conclusion that an annual estimate of 653,365 marine mammals, comprising 307,753 cetaceans and 345,611 pinnipeds were caught from 1990–1994.

While gillnets are a principal concern, other types of nets also pose a problem: trawl nets, purse seines, beach seines, longline gear, and driftnets. Driftnets are known for high rates of bycatch and they affect all cetaceans and other marine species.[4] They are fatal for small toothed whales (Odontocetes) and sperm whales, as well as other marine mammals and fish such as sharks, sea birds and sea turtles. Many fisheries routinely use driftnets exceeding the EU size limit of 2.5 km/boat.[5] This illegal drift-netting is a major issue, especially in important feeding and breeding grounds for cetaceans.

Cetaceans have become the bane of many long-line fishermen in areas where certain whale populations have made a significant comeback in recent years. Whales often follow the boats, devouring the catch and leaving little more for them than fish heads when it is hauled in.

References
1. Alverson, DL, Freeburg, MH, Murawski, SA and JG Pope (1994). A global assessment of fisheries bycatch and discards. Fisheries Technical Paper. Food and Agriculture Organization, Rome. 339 pp.
2. Animal Freedom (2008) Cetacean Bycatch
3. Barlow, J & Cameron, GA (2003). Field experiments show that acoustic pingers reduce marine mammal bycatch in the Californian drift gill net fishery. Marine Mammal Science. 19: 265-283.
4. Demaster, DJ, Fowler, CW, Perry, SL, and ME Richlen (2001). Predation and competition: the impact of fisheries on marine mammal populations over the next one hundred years. Journal of Mammology. 82: 641-651.
5. Jefferson, TA and Curry, BE (1994). A global review of porpoise (Cetacea: Phocoenidae) mortality in gill nets. Biological Conservation. 67(2): 167-183.
6. Miragliuolo, A, Mussi, B, and G Bearzi (2002). Observations of driftnetting off the island of Ischia, Italy, with indirect evidence of dolphin bycatch. European Cetacean Research. 4pp.
7. Read, AJ, Drinker, P, and S Northridge (2006). Bycatch of marine mammals in the U.S. and Global Fisheries. Conservation Biology. 20(1): 163-169.