January 31, 2007
Shark Biology Contributes to Population Decline and Fishery Collapses
A shark's life history is much closer to that of a whale or dolphin than to a cod or herring.
Sharks are known as "K-selected" species, which means that they have a life history strategy featuring slow growth, delayed maturation, long gestation, and the production of few young.
Many shark species grow only a few centimetres per year, reaching maturity at perhaps seven or eight years of age. Females are often pregnant for a year or more and typically have only a handful of pups. These pups are "ready to go" at birth, able to catch their own food and, if they can avoid predators, fend for themselves from day one.
This strategy, while spectacularly successful for the past 400 million years, has come up against an even more "successful" species—humanity. A combination of modern fishing technologies, global markets, rapid airfreight and sheer greed have allowed us to decimate shark populations in the past few decades. A life history strategy that served the sharks well for hundreds of millions of years has now become the instrument of their own destruction.
Ignoring shark biology has catastrophic consequences
Take, for example, the case of the spiny dogfish (Squalus acanthias), a small shark that ranges across both the Atlantic and the Pacific. In the 1980s and early 1990s, the US fishing industry was encouraged to focus on this highly commercial species in order to allow time for over-fished populations of groundfish to recover. On the surface, this may have seemed a good idea, but it took no account of the life history of sharks in general and dogfish in particular.
Atlantic dogfish are thought to live for at least 30-40 years while, in the Pacific, their lifespan is now thought to reach 70 years. The female dogfish does not mature until her mid "teens" to early twenties. After mating, she is pregnant for 18-24 months and gives birth to an average litter of 6 live pups, some of which will not survive to maturity.
It is then another year at least before she becomes pregnant again. Combine this with the fact that mature females school together and that they are significantly bigger than males, and you end up with a scenario where fishers specifically target schools of mature females, many of which are at some stage of pregnancy, thus removing not one but two generations.
Collapse of fishery-targeted shark populations
Not surprisingly, the plan to use the dogfish as an economic bridge while other species recovered was a spectacular failure, resulting in the decimation of dogfish populations in the western Atlantic. Within a handful of years, scientists were warning that populations would take decades to recover—if they recovered at all. They are now being proposed for special international protection.
And it is not only the USA that has overestimated the ability of shark species to recover. Almost every known fishery that has specifically targeted sharks has collapsed within a few years.
- During the 1960s the Norwegians and Danes began fishing for porbeagle sharks (Lamna nasus) in the northwest Atlantic; between 1961 and 1964 their catch rose from 1,800 metric tonnes to 9,300mt and then declined to less than 200mt.
- The common skate (Dipturus batis) in the Irish sea is now considered to be commercially extinct as a result of short-term overexploitation.
- A historical harpoon fishery for the basking shark (Cetorhinus maximus) off the west coast of Ireland was briefly revived in the 1940s but the catch quickly peaked and declined by the end of the 1950s.
- A fishery for bluntnose sixgill sharks (Hexanchus griseus) began in the Maldives in 1980, peaked in 1982–84 and collapsed by 1996. Other fisheries for this species, in Australia, New Zealand, France and Brazil are all reported to have declined.
- The collapse of the soupfin shark (Galeorhinus galeus) fishery in the US Pacific follows a typical pattern. The fishery expanded enormously in 1938, with the discovery that liver oil was rich in vitamin A. The catch peaked at 4,000mt in 1940, crashed in 1942 and by 1944 was down to only 300mt. Only about 40mt are now caught annually.
- Catches of porbeagle sharks in the north-eastern Atlantic peaked in 1947 then declined; catches temporarily rose again during the 1960s as the fishery spread to the northwestern Atlantic, but then declined to a low level in the mid 1980s.
- US Pacific angel shark (Squatina california) catches peaked in 1985–86 at 560mt but decreased quickly to 120mt three years later. A ban in 1994 is likely to have averted a total population collapse.
- In the early 1980s a fishery for sevengill sharks (Notorhynchus cepedianus) in San Francisco Bay, USA, crashed within a few years.
- Many more shark fisheries are likely to have declined drastically, but have never been formally documented.
If we are to conserve the world's remaining shark populations, management schemes, fishery regulations and trade regimes must take into account the extraordinary life history of this fascinating group of species.