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Chapter: The Diversity of Fishes: Biology, Evolution, and Ecology: Chondrichthyes: sharks, skates, rays, and chimaeras

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Shark conservation - Subclass Elasmobranchii

Shark populations globally have decreased 50–80% since the 1970s. Steepest declines occurred in the late 1980s, when international fishing accelerated, in no small part because of increased demand for fins (Rose 1996).

Shark conservation

 

 

Shark populations globally have decreased 50–80% since the 1970s. Steepest declines occurred in the late 1980s, when international fishing accelerated, in no small part because of increased demand for fins (Rose 1996). In the Gulf of Mexico and northwest Atlantic, both already heavily overfished, additional declines occurred between 1985 and 2000 (Baum et al. 2003; Baum & Myers 2004):  

 

Silky Sharks declined 90%, hammerheads were reduced another 89%, thresher sharks 80%, White Sharks 79% (>6000 captured!), Oceanic Whitetips 70–99%, Tiger Sharks 65%, the coastal species complex 61% (range 49–83%), and Blue Sharks 60%. Only mako sharks suffered losses of less than 50%. Although disagreement exists on the exact magnitude of the declines (e.g., Baum et al. 2005; Burgess et al. 2005), it is generally accepted that these numbers are likely representative of trends in other oceans, indicating that, “. . . overfishing is threatening large coastal and oceanic sharks . . . [and] several sharks may also now be at risk of large-scale extirpation” (Baum et al. 2003, p. 390).

 

Many of the life history features of sharks explain this vulnerability to exploitation. As apex predators, sharks seldom occur in the large numbers that characterize more resilient fish species (e.g., herrings and mackerels feed much closer to the base of the marine food chain). Importantly, sharks replace themselves slowly. Slow growth and maturation rates, long gestation periods, long intervals between reproductive bouts, and relatively small clutch sizes suggest that shark species have evolved in circumstances where juvenile mortality is typically low compared to the much more prolifi c bony fishes on which sharks feed. Under natural circumstances, sharks have few predators aside from other sharks, and most of these can be avoided by attaining large size. The unfortunate outcome of this particular life history strategy is that shark populations are not capable of overcoming the high mortality rates imposed by commercial exploitation.

 

Sharks are exploited for meat, skin, teeth (White Shark jaws can sell for as much as US$10,000; Heneman & Glazer 1996), organs for medicine (fallaciously), and fins for soup (reprehensibly). The history of commercial shark fishing is a history of collapsed fisheries. Examples include fisheries for Thresher Sharks in California, School Sharks in Australia, Spiny Dogfish in the North Atlantic, Porbeagles off Newfoundland, Basking Sharks off Ireland, Bull Sharks and sawfish in Lake Nicaragua, and Soupfin Sharks off the US Pacific coast.

 

Two examples typify the boom and bust pattern that characterizes commercial exploitation of shark populations. The Porbeagle (Lamna nasus) fishery of the western North Atlantic has been well documented. The fishery was wiped out in only 7 years. In 1961, uncontrolled exploitation began and about 3,500,000 pounds (1,575,000 kg) were caught. Catch peaked only 3 years later at 16 million pounds, then crashed. By 1968, only a few hundred sharks could be found. Twenty years later, populations had not returned to pre-exploitation levels (Campana et al. 2002). Before 1937, about 600,000 pounds or 6000 individual Soupfin Sharks, Galeorhinus zyopterus, were landed annually in California. With the development of a market for shark liver oil, this fishery expanded rapidly to 9,000,000 pounds (90,000 fish) in 1939, fell to 5,000,000 pounds in 1941, and by 1944 was back to 600,000 pounds, despite continued intensive effort. Importantly, catch rates fell from 60 sharks per set in 1939 to one shark per set in 1944, indicating a significant decline in the population. Thirty years later, populations had still not returned to pre-1939 levels (Moss 1984; Anderson 1990; Manire & Gruber 1991).

 

Most shark populations cannot withstand a fishing mortality even as low as 5% removal of the existing population each year (Pratt & Castro 1991). The estimated US maximum sustainable yield (MSY) of sharks has been estimated at 16,250 metric tons, but total mortality exceeds the MSY by nearly 6000 metric tons annually. Between 1986 and 1990, commercial shark landings doubled every year in the USA. All indicators suggest that North American shark populations are in decline and that management plans, including a moratorium on the capture of some species, are desperately needed.

 

The repeated scenario of large initial catches, rapid decline, and slow if any recovery highlight the need for careful management of all exploited shark stocks. Interestingly, sharks exhibit a remarkably close relationship between stock size and recruitment. Fisheries managers can predict future recruitment into populations based on existingreproductive stocks. This degree of predictability characterizes few other commercial species. The unfortunate fact is that shark fishing remains one of the least regulated commercial fishing activities. Management plans have been proposed but not implemented in many countries. The USA did not put a management plan into effect until 1993, and those regulations proved insuffi cient and had to be tightened further in 1997 (Poffenberger 1999). But even with implementation, shark conservation is complicated by shark biology. Local management is not suffi cient to protect shark populations because so many species undergo long distance movements through international waters. International efforts at conservation, which are historically difficult to negotiate, are crucial.

 

Against this backdrop of decline and mis- (or non-existent) management are efforts to conserve sharks, as well as economic ironies. The Shark Specialist Group of the IUCN assessed population status of about one-third of the world’s sharks, skates, and rays. The IUCN assigned high-risk status to 86 species: 14 were Critically Endangered, 26 were Endangered (including Barndoor and Common skates), and 46 were Vulnerable (including Great White and Whale sharks) (see www.redlist.org). Such attention often leads toprotective legislation at national and international levels. For example, in 2004, the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) listed Whale, Basking, and White sharks in Appendix II, regulating and restricting capture and trade in these species (see www.cites.org/eng/app/appendices.shtml).

 

Ironically, in many countries, including in the developing world where fishing provides an important livelihood for a large population segment, live sharks are worth considerably more than dead sharks. People are willing to spend money just to watch sharks, makingecotourismrelated shark viewing a profitable diving activity. Whale Shark watching is popular in the Maldives, Western Australia, Philippines, and Mexico, and in many locales divers spend large sums to watch reef sharks, manta rays, and stingrays. A shark can be captured once but can be viewed many times. Individual sharks in the Bahamas may be worth US$750,000 alive but only $40–50 dead (S. Gruber, pers. comm., in Daves & Nammack 1998). An economic analysis for the Republic of Maldives indicated an impressive 100- to 1000-fold difference in value (Anderson & Waheed 2001). Shark watching is an imminently sustainable commercial activity; shark fishing is not.


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