Cetacean bycatch (or cetacean by-catch) is the accidental capture of non-target cetaceans such as dolphins, porpoises, and whales by commercial fisheries.Alverson et al. (1994) 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.Demaster et al. (2001) This trend is likely to continue due to the pressures of human population growth on the environment, the growing popularity of seafood, and the commercialization of deep-sea fishing as operations expand into waters which were previously protected. These commercial fishing vessels come into both direct and indirect contact with cetaceans. An example of direct contact is the physical contact of cetaceans with fishing nets. Indirect contact occurs through marine food chains, which are destabilized by the commercial fishermen severely reducing local fish populations that cetaceans rely on for food. In commercial fishing practices, cetaceans are captured as bycatch but then retained because of their value as food or bait.Read et al. (2006)
Bycatch trends
Generally, cetacean bycatch is increasing. Most of the world's cetacean bycatch occurs in gillnet fisheries. The mean annual bycatch in the U.S. alone from 1990 to 1999 was 6,215 marine mammals, with dolphins and porpoises being the primary cetaceans caught in gillnets. Based on global bycatch observation of U.S. fisheries, an estimated 653,365 marine mammals, comprising 307,753 cetaceans and 345,611 pinnipeds were caught from 1990 to 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.Silvani et al. (1999) They are fatal for small toothed whales (Odontocetes) and sperm whales, as well as other marine vertebrates such as sharks, sea birds and sea turtles. Many fisheries routinely use driftnets exceeding the EU size limit of 2.5 km/boat.Miragliuolo et al. (2002) This illegal drift-netting is a major issue, especially in important feeding and breeding grounds for cetaceans.
The tuna industry has achieved successes in reversing cetacean bycatch trends. International recognition of the problem of cetacean bycatch in tuna fishing led to the Agreement on the International Dolphin Conservation Program in 1999 and overall there has been a dramatic reduction in death rates. In particular, dolphin bycatch in tuna fishing in the East Tropical Pacific has dropped from 500,000 per year in 1970 to 100,000 per year in 1990, to 3,000 per year in 1999, and to 1,000 per year in 2006.
A shark culling program in Queensland, which has killed roughly 50,000 sharks since 1962, has also killed thousands of dolphins as bycatch.https://www.news.com.au/technology/science/animals/aussie-shark-population-is-staggering-decline/news-story/49e910c828b6e2b735d1c68e6b2c956e Aussie shark population in staggering decline. Rhian Deutrom. December 14, 2018. Retrieved December 26, 2018. "Shark control" programs in both Queensland and New South Wales use shark nets and drum lines, which entangle and kill dolphins.https://web.archive.org/web/20181002102324/https://www.marineconservation.org.au/pages/shark-culling.html "Shark Culling". marineconservation.org.au. Archived from the original on 2018-10-02. Retrieved December 26, 2018. Queensland's "shark control" program has killed more than 1,000 dolphins in recent years,http://www.afd.org.au/news-articles/queenslands-shark-control-program-has-snagged-84000-animals Action for Dolphins. Queensland’s Shark Control Program Has Snagged 84,000 Animals. Thom Mitchell. November 20, 2015. Retrieved December 26, 2018. and at least 32 dolphins have been killed in Queensland since 2014.https://hsi.org.au/blog/separating-fact-from-fiction-in-qlds-shark-culling-program Separating fact from fiction in QLD’s shark culling program. Nicola Beynon. October 19, 2018. Retrieved December 26, 2018 A shark culling program in KwaZulu-Natal, South Africa has killed at least 2,310 dolphins.http://www.sharkangels.org/index.php/media/news/157-shark-nets "Shark Nets". sharkangels.org. Retrieved December 26, 2018.
Cetaceans at risk Bycatch is recognized as a primary threat to all cetaceans. The following cetaceans are at high risk for entanglement in gillnets:
Atlantic humpback dolphins The Atlantic humpback dolphin (Sousa teuszii) is endemic to West Africa. Several stocks have been identified with numbers ranging from tens to a few hundred.Reeves et al. (2005) Abundance estimates are lacking. Gaps in the species range and hence distribution are evident. Bycatch is only documented in a few West African countries. Surveys and evaluations need to be conducted to determine the presence/absence of humpback dolphins in their historical range. Conservation measures need to be implemented to save this species. Because many people live off the sea, it is not feasible to have complete gillnet closures. Some areas may be designated as off-limits to gillnet fisheries. Eco-tourism may be implemented successfully because of high species diversity.
Baleen whales
Baleen whales (Mysticeti) are often taken in gill-nets and in fisheries that use vertical lines to mark traps and pots. Large cetaceans such as humpback and right whales may carry off gear after entanglement. This explains the large scars borne by whales along the U.S. Atlantic coast.Knowlton (2003) Analyses show that 50-70% of Gulf of Maine humpback whales (Megaptera novaeangliae) and North Atlantic right whale (Eubalaena glacialis) have been entangled at least once in their lifetime. The North Atlantic right whale is one of the most endangered large cetaceans; only 300-350 individuals remain.WWF (2004) Minke whales (Balaenoptera acutorostrata) are also at risk.
Burmeister's porpoises The Burmeister's porpoise (Phocoena spinipinnis) is one of three cetaceans that are most often bycaught in Peru and Chile. Several thousand porpoises are caught each year in Peru alone. Bycatch is a frequent occurrence for this species because of the inability to detect them in the water. Surveys have shown that bycatch remains a concern in that area today, and it is unknown whether the population is declining. Reliable data, conservation measures, and public awareness are all lacking. These porpoises are cryptic, making surveying a challenge. It is also difficult to estimate the extent of the bycatch, because the sale of porpoise meat is no longer available at markets.
Commerson's dolphins
The expanding trawl fisheries devastated the Commerson's dolphin (Cephalorhynchus commersonii) populations in Patagonia. Trawl fisheries greatly expanded for twenty years until they crashed in 1997. Pelagic squid fisheries took over, which use pelagic trawls that are harmful to both dusky, short-beaked common dolphins and Commerson's dolphins. There are approximately 21,000 Commerson's dolphins remaining today. Two stocks have been identified in the population, but genetic information and bycatch levels are unknown. With anchovy fisheries expanding, it is imperative to assess the Commerson's dolphin population before these fisheries grow. The seasonal operation of in-shore gillnet fisheries are known to involve bycatch of cetaceans. Presently, there are no known estimates of gillnet bycatch. The bycatch problem in Argentina is political in nature. Improvements in fishing technology, public awareness, and a large-scale survey of the impact of bycatch on Commerson's dolphin populations are essential.
La Plata dolphins The La Plata or Franciscana dolphin (Pontoporia blainvillei) is the most threatened small cetacean in the southwest Atlantic Ocean due to bycatch. They are only found in the coastal waters of Argentina, Brazil, and Uruguay. This species has been divided into four ranges (FMU's: Franciscana Management Units) for management and conservation purposes. These populations are genetically different. Mortality rates are 1.6% for FMU 4 and 3.3% for FMU 3, but it is unknown whether these estimates are accurate. Aerial surveys have proven inconclusive so far as to the population numbers of franciscanas. To rectify this situation, more surveys are needed as well as political commitment, awareness campaigns, and bycatch mitigation techniques.
Harbour porpoises There is substantial incidental catches in fishing operations.Hammond et al. (2002) Often, the harbour porpoise (Phocoena phocoena) is killed by incidental bycatch (10, 11, 12). Gillnets pose a serious threat to the harbour porpoise as they are extremely susceptible to entanglement.Jefferson & Curry (1994) A study by Caswell et al.Caswell et al. (1998) in the western North Atlantic combined the mean annual rate of increase of the harbour porpoise with the uncertainty of incidental mortality and population size. It was found that the incidental mortality exceeds critical values, thus showing bycatch is a significant threat to the harbour porpoise. Harbour porpoises become entangled in nets due to their inability to detect the nets before collision.Kastelein et al. (1999) In 2001, 80 harbour porpoises were killed in salmon gillnet fisheries in British Columbia, Canada.
Hector's and Maui's dolphins
In New Zealand, these dolphins have a high rate of entanglement. Hector's dolphin (Cephalorhynchus hectori) is endemic to the coastal waters of New Zealand and there are about 7,400 in abundance. A small population of Hector's dolphins is isolated on the west coast of the island and have been declared a subspecies called Maui's dolphin. Maui's dolphins (Cephalorhynchus hectori maui) are often caught in set nets and pair trawlers resulting in less than 100 left in the wild. For protection, a section of the dolphin's range on the west coast has been closed to gillnet fisheries.
Indo-Pacific humpback and bottlenose dolphins Drift and bottom-set gillnets are the biggest conservation threat to these dolphins in the Indian Ocean. There have only been assessments in some areas, such as Zanzibar. Hunting, until 1996, reduced the population and contributed to its decline. Now hunting has been replaced with eco-tourism. It was estimated in 2001 that there are 161 bottlenose dolphins (Tursiops aduncus) and 71 Indo-Pacific humpback dolphin (Sousa chinensis) remaining based on photo-identification mark-recapture techniques. A study revealed over 160 incidences of bycatch since 2000. Approximately 30% of bycatch is in drift and bottom-set gillnets. Mortality is about 8% and 5.6% for bottlenose and humpback dolphins, respectively . The mitigation of bycatch is imperative for these species and for eco-tourism.
Irrawaddy dolphins
Based on a survey in 2001, fewer than 70 Irrawaddy dolphins (Orcaella brevirostris) remain in the upper region of the Malampaya Sound in the Philippines and only 69 individuals in the Mekong River. They have been severely impacted by lift nets and crab gear, thus critically endangered. It is estimated that mortality from bycatch may be greater than 4.5% in Malampaya Sound and 5.8% in the Mekong River. The population is declining dramatically. Current bycatch levels are unsustainable, and bycatch reduction measures, as well as long-term systematic monitoring, are urgently required. The elimination of gillnets from areas of high use is needed, and economic incentives need to be provided to the local people.
Spinner and Fraser's dolphins