Who's Eating Whom
If sharks are threatened, it is not by the recreational fishing inspired by Jaws or by ecotourism. The simple fact is that people also like to eat sharks, a species whose life-history characteristics make it sensitive to overfishing and whose wide travels make fisheries management difficult. This appetite has resulted in the recent growth of fisheries for shark species worldwide. These fisheries have had a boom-and-bust history. Shark populations have decreased sharply after periods of intense fishing pressure.
Examples are the fisheries for the porbeagle shark (Lamna nasus) in the North Atlantic, the soupfin shark (Galeorhinus galeus) off California, the basking shark (Cetorhinus maximus) off Europe and Canada, and the spiny dogfish (Squalus acanthias) both in the North Sea and off British Columbia. For example, in 1961, Norwegian fishers began to target the porbeagle shark. The yearly landings of the fish- and squid-eating porbeagle shark in the Northeastern Atlantic rose to 8,060 tons in 1964, only to fall in the next three years to 207 tons. The landings have not exceeded 100 tons since the late 1970s.
Basking-shark fisheries have collapsed in both the Atlantic and Pacific oceans. A very localized fishery for the species arose off western Ireland in 1947 near Achill Island. Between 900 and 1,800 of these enormous, plankton-eating sharks were captured each year from 1950 to 1956. Over the following years the catch declined, shrinking to 119 by 1968. The fate of the soupfin shark, a relative of reef sharks, is another prime example. The need for high-grade oil by the military during World War II created a market for soupfin liver oil. The liver, impregnated with oil, can reach a third of the body mass of an adult shark. The price of oil rose from $50 per ton in 1937 to $2,000 per ton in 1941. The catch of soupfin rose from 270 tons per year in the early 1930s to a peak of 2,172 tons in 1941 and then dropped to 287 tons in 1994. (For a detailed review, read Camhi et al. 1998.)
Commercial fisheries for sharks have operated in the United States since the 1930s, but they were originally small and restricted to small areas. They did not begin to grow until the late 1980s. Massachusetts has the largest fishery for sharks in the United States (Figure 3). This fishery mainly targets a single species, the spiny dogfish , which (in addition to its other ranges) lives in large schools off New England in the spring and summer and then migrates to the waters off the southeastern United States during winter. It is served as "fish and chips" in restaurants in both the U.S. and Europe. The landings of this small but very abundant predatory species were still rising by the late 1990s.
Excluding dogfish from the catch, Florida has the largest shark fishery. This fishery targets many coastal species in the Gulf of Mexico. The landings of these reef sharks rose until the early 1990s, when they began to decrease. The growth in the commercial shark fisheries in the late 1980s for sharks in the Atlantic and Gulf of Mexico was partly due to a new public appreciation of the value of these species as food. However, a more important reason for the expansion of the shark fisheries was the demand in Asia for fins.
There are several reasons why sharks and rays are particularly vulnerable to intense fishing pressure. Most species are near the top of the food chain and not abundant. Sharks grow slowly, mature late in their lives, do not reproduce every year and have few young. They succeed in part because they are long-lived. As Merry Camhi of the National Audubon Society points out, "Unlike most bony fishes in which the survival of millions of eggs and larvae are often largely dependent on environmental variables, chondrichyans (sharks and rays) exhibit a much closer relationship between the number of young produced and the number of breeding adults." Kill a substantial proportion of adults, and the population cannot be expected to be replaced.
Take, for example, the sandbar and scalloped hammerhead shark, two species frequently caught in the Atlantic and Gulf of Mexico fisheries. The sandbar takes 13–16 years to reach maturity and then gives birth to 8–13 pups every other year. Female hammerheads can take up to 15 years to reach maturity and then give birth to 12–40 pups. The spiny dogfish has a similarly long growth period to reproductive maturity and produces 2–15 young every second year. The Atlantic cod, by contrast, reaches reproductive maturity in only 2–4 years, produces 2 million to 11 million eggs, and reproduces every year.
The impact of catching even a few of the really large predatory species of sharks—such as the bull (Carcharhinus leucas), tiger (Galeocerdo cuvieri) and white shark (Carcharodon carcharias)—can be even greater. When four large white sharks were caught on October 5, 1982, close to shore at the South Farallon Islands near San Francisco, the number of attacks on prey animals observed in local waters dropped by half during the next two years.
Why would the white shark be so vulnerable to fishing pressure? First, it is an apex predator, occupying the pinnacle of the food chain. Individuals of these species are never very common. The white shark feeds on seals and sea lions; they in turn feed on smaller prey such as fish and squid; these feed on even smaller planktonic animals; and these feed on planktonic plants. At each link in the chain some energy is lost, resulting in less animal mass.
The size of the local white shark population has been estimated in only two geographical areas, South Africa and South Australia. The centers of the range of estimation were 1,279 and 191.7, respectively. Naturalists at the Point Reyes Bird Observatory and I have identified nine to 14 white sharks over a 5-year period of observation each October and November at the South Farallon Islands. The same sharks often returned to the island year after year.
There are several reasons that white sharks are so rare. First, individuals of the species grow slowly. Male white sharks become sexually mature at a length of 4 meters, when they first develop rigid claspers or intromittent organs. A male of this size has 10 concentric growth rings on each cartilaginous element of the vertebral column, indicating an age of 10 years. Females mature at a larger size, between 4.5 and 5.0 meters long, and at an age of 14 years. It is probable that the white shark resembles most other sharks that do not give birth yearly, but every second or third year. Finally, the maximum size to a litter of pups is only 10 per female. In the context of this life history, it is not surprising that the capture of four white sharks at a single site would reduce the local population to half its former level. The fear of the negative impact that thrill-seeking fishers might have on the white shark population off California led during the first week of October 1993 to the passage of a bill in the California legislature protecting white sharks.