Chondrichthyes (cartilaginous fish,
including sharks, rays and chaemeras) >
Size, age and growth
Most cartilaginous fishes are smaller than
people. About 50% of living sharks reach a maximum total length of between 15
cm and 1 m long, and 82% reach two m or less, while 18% percent of
the species reach over 2 m total length. The average maximum length for living
sharks is about 1.5 m total length. Some 4% of the species of sharks are
gigantic, 4 to 12 or more meters long. These include the largest
fishes, the
Whale and
Basking sharks, which overlap the larger
cetaceans in size; the basking shark may reach 12 m and the whale
shark over 15 m long.
Most rays are less than 1 m long at maturity,
and probably less than 10% of living rays exceed a width of 3 m or
length of 4 m. The largest living rays include the
sawfishes
(Pristidae) 7.3 to 9.8 m long and mantas (family Mobulidae) 6
to 7 m wide.
In contrast, some sharks and rays are dwarves,
mature at a length of 10 to 20 cm. The smallest sharks are
dogfish
(family Squalidae) and
finback catsharks (Family Proscylliidae)
about 15 cm. long at maturity; some
electric rays are mature at only
10 cm long. Chimaeras are small to moderately large at maturity, and
reach a maximum size of 0.5 to about 2 m long.
Most cartilaginous fishes are slow growing and
long-lived, with some species requiring up to 20 years or more to
reach adulthood and with a maximum age of 75 years. Much emphasis
has been placed on determining age and growth of cartilaginous
fishes in recent years due to the increase in fisheries exploitation
of many species and its importance in managing such fisheries.
Age of cartilaginous fishes is usually
estimated by counting bands across the vertebral centra or rings on
the dorsal fin spines when present. These bands are usually enhanced
by staining techniques, sectioning, or by radiographing (x-raying)
them. Bands are then counted in much the same way tree rings are
counted to estimate the ages of trees. However, verification of the
periodicity of band deposition remains unknown for most species.
Bands are assumed to be annual in many species, but this has been
verified for only a few species, including the Lemon shark
(Negaprion brevirostris), Leopard shark (Triakis semifaciata), and
the Thornback skate (Raja clavata). Verification of the period of
band deposition is usually done by injecting live fishes with
tetracycline, an antibiotic which places an identifiable chemical
`mark' on vertebrae or spines where new hard tissue is being
deposited. Fishes are tagged and returned to the sea and when the
fishes are recaptured after a year or more at liberty their
vertebrae or spines can be examined for the tetracycline mark. The
number of bands deposited after the mark was induced can then
correlated with the time the fish was at liberty. Another technique
is to compare the banding on the edge of vertebral centra or the
dorsal spine base from fish caught sequentially during a year, which
may enable one to estimate how long it takes to deposit one band
from its increase in width.
Males of the
Spotted spiny dogfish (Squalus
acanthias) may require up to 14 years to mature, while females may
take at least 20 years to mature. The
Great white shark may take
between 10 and 15 years to mature and has been estimated to reach a
maximum age of at least 23 years based on a six meter female caught
off Gans Bay in 1987. Other species for which age at maturity have
been estimated include;
Spinner shark (Carcharhinus brevipinna) 6 to
7 years for males, 7 to 8 years for females;
Blacktip shark
(Carcharhinus limbatus), 4 to 5 years for males, 7 to 8 years for
females;
Zambezi shark (Carcharhinus leucas), males 14 to 15 years
for males, over 18 years for females;
Silky shark (Carcharhinus
falciformis), males 6 to 7 years, females 7 to 9 years;
Tiger shark
(Galeocerdo cuvier), 7 to 10 years for both sexes;
Scalloped
hammerhead (Sphyrna lewini), males 10 years, females up to 15 years;
Thresher shark (Alopias vulpinus), 3 to 7 years for both
sexes; Blue shark (Prionace glauca), 5 to 6 years for both sexes;
Shortfin mako shark (Isurus oxyrinchus), a minimum of 7 to 8 years
for both sexes.
Text by Leonard J.V. Compagno, David A. Ebert
and Malcolm J. Smale
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