Longevity and Causes of Death
Longevity
Little is known about the growth and age of elasmobranchs.
Many of the conventional methods for aging animals, such as
examining teeth, will not work with elasmobranchs.
Sharks grow slowly compared to bony fishes, possibly due to
sharks'slow digestive time and feeding rates. There is
considerable variation in age and growth rates between species
and even between populations of the same species.
Aging studies
Growth rings are periodically deposited on the vertebrae
of some sharks. Vertebrae can be stained and examined for these
growth rings. Growth rings may stop developing in older sharks.
Examining the vertebrae of captive-born sharks after their
death enables researchers to compare the number of growth rings
with the shark's known age
In some areas, tagged sharks are providing information
about growth rates. Once a shark is caught, it is measured,
tagged, and released. The shark is measured again when it is
recaptured. Researchers correlate the measurements with the
number of years since recapture and calculate a yearly growth
rate.
Predators
Depending on the species, sharks and batoids have several
predators, including other sharks, elephant seals, and killer
whales.
Human interaction
Sharks are vulnerable to overfishing. Because sharks are
slow-growing and a single female produces only a few hundred pups
or less in a lifetime, depleted populations may take years to
recover.
Recreational and commercial shark harvesting has
increased in the past several years due to an increased
demand for sharks and shark products. Each year,
thousands of sharks are taken unintentionally in nets set
out to catch other types of fish. One particularly
wasteful type of shark fishing is known as fanning.
Only the fins of the shark, which will be sold for use in
sharkfin soup, are removed. The rest of the shark is
simply discarded.
Fisheries management programs are necessary for a sensible
shark harvest.
The National Marine Fisheries Service (NMFS) develops
management plans for sharks, setting catch quotas for
target shark species. On April 26, 1993, NMFS
implemented a plan to manage U.S. shark fisheries of the
Atlantic Ocean, Gulf of Mexico, and Caribbean Sea. The
plan includes the following features:
- Annual commercial quotas, which are divided
into half-yearly quotas.
- Provisions for
closing a fishery for a species group when the
semi-annual quota is met.
- Catch limits for recreational anglers.
- Permit requirements for commercial vessels
that catch sharks.
- A requirement that vessels land fins in
proportion to carcasses (effectively prohibiting
the practice of shark finning).
- A requirement that when sharks are not
kept, they are released in a manner that ensures
the probability that they will survive.
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