The IUCN/SSC Shark Specialist Group
Shark News 5: October 1995
|
Critically Endangered Large Coastal Sharks, a Case Study: the
Sandbar Shark, Carcharhinus plumbeus (Nardo, 1827)
Jack Musick, School of Marine Science,
Virginia Institute of Marine
Science,
College of William and Mary, Gloucester Point, VA 23062
Taxonomy
The sandbar shark, Carcharhinus plumbeus (Nardo, 1827), is a medium
to large-sized requiem shark (Carcharhinidae) with a relatively short
rounded snout, distinctive high triangular dorsal fin placed over or
before the pectoral insertions, dermal ridge between the dorsal fins,
large serrated triangular upper teeth, and narrow awl-shaped lower
teeth. In life this species is light grey above and pristine white below,
often with a brassy hue and white stripe along the flank. Originally
described from the Adriatic Sea, it was subsequently described under
various other names from other localities. Some names in recent use
include: Carcharhinus millberti, Eulamia milberti, Galeolamna stevensi
and Carcharhinus japonicus.
Figure 1. Sandbar shark being landed on long-line vessel.
|
Distribution and ecology
The sandbar shark is a coastal species typical in many aspects of its
biology of many other common coastal sharks. The species has been
recorded from the western Indian Ocean, south-east Asia, Japan,
Australia, and Hawaii. Its occurrence in the Eastern Pacific is debatable.
The sandbar shark occurs in the eastern Atlantic and Mediterranean and
is the most abundant large coastal shark in the western north Atlantic
and eastern Gulf of Mexico. Tagging and genetic studies suggest that
sandbar sharks from Cape Cod, Massachusetts, to the northern Yucatan
peninsula in Mexico are comprised of a unit stock separate from the
population reported from Trinidad to Brazil.
In the western North Atlantic, the sandbar shark exhibits strong
seasonal movements. Adult female sandbar sharks migrate north into
the middle Atlantic Bight in May and early June, when sea water
temperatures approach 19°C, and use estuarine waters such as
Chesapeake Bay and Delaware Bay as pupping grounds. Immediately
after pupping these large females move offshore to 20-50 m depth.
Neonates and juveniles aged 1-4 years utilise estuarine habitats during
the summer. Larger juveniles use shallow coastal habitats (<20 m).
Although the juvenile population in the middle Atlantic Bight exhibits
approximately a 1:1 ratio of females to males, the adults are represented
almost solely by females (very occasionally adult males are taken at
greater than 100 m depth at the edge of the continental shelf). Adult
males appear to inhabit the southern part of the range and are common
off Florida and in the Gulf of Mexico. Sandbar sharks migrate south
below Cape Hatteras, North Carolina, and further in September or
October when seawater temperatures fall to 18-20°C. Some large
juveniles and adults may migrate as far as southern Florida, Cuba and
Mexico, while small juveniles and other larger juveniles and some
adults may winter in warm waters at the edge of the Gulf Stream off the
Carolinas. Off South Africa similar seasonal migrations into high
latitudes in spring and lower latitudes in fall appear to occur. Island
populations such as in Hawaii appear to be seasonally resident.
Sandbar sharks are euryphagous predators feeding on a wide
variety of smaller demersal teleosts and elasmobranchs, as well as on
cephalopods, and various crustaceans.
The sandbar shark is viviparous with a yolk sac placenta.
Gestation has been estimated at 9-12 months in the western North
Atlantic, 11-12 months off South Africa and the East China Sea,
and 10-12 months off Taiwan. Females apparently have young onlyevery other year, with about 50% of mature females being pregnant off
Taiwan. Conversely, only 17-27% of mature females captured off
Florida were pregnant. However, most of the mature females examined
in the mid-Atlantic Bight of the US in summer are pregnant or recently
have born young. Therefore, the pregnancy rate in the western North
Atlantic is probably near 50%, but it is difficult to obtain a synoptic
sample of the entire population of mature females because of their
wide geographic distribution and seasonal movements. Litter size is
variable and depends in part on the size of the mother. In the western
Atlantic, where female sandbar sharks mature at about 179-183 cm
TL, litter size averages 8.4-9.3 (range = 1-14). Whereas in Hawaii
where females sandbar sharks mature at 150 cm TL, mean litter size
is only 5.5 (range = 1-8). Within a given geographic area litter size
is only very weakly correlated with the size of the mother. In general,
size at maturity, maximum size and litter size decrease from the
western Atlantic to the western Indian Ocean, to Taiwan and Australia,
to the east China Sea, and to Hawaii. Size at birth varies slightly by
region, but does not follow the same geographic pattern. New born
pups range from 56-75 cm TL with pups averaging 60-65 cm TL in
most areas. Maximum reported size is 234 cm TL for females and 226
cm TL for males.
Sandbar sharks are slow-growing K-selected species. Although
growth and age at maturity may be accelerated under captive conditions,
wild populations grow very slowly and mature at a relatively late age.
In the western Atlantic the von Bertalanffy growth coefficient, K, has
been estimated to be very low (0.039 to 0.089) in validated studies using
annuli on vertebral centra. Maturity in these studies was estimated at
13-16 years. However, in another study based on growth rates calculated
from tag/recapture data, growth was considerably slower and age at
maturity was estimated to be 29 years. Considerable debate has
arisen concerning the discrepancy between the two methods,
including the small tag/recapture sample size and the possible effects of tagging on growth rates. Regardless, sandbar sharks grow slowly and mature late.
Conservation status
Sandbar sharks are significant components of coastal shark fisheries
world-wide. In the western Atlantic this species contributes up to 60%
of the catch and 80% of the landings in the directed long-line fishery.
In addition, the sandbar shark is second only to the blue shark (a pelagic
species) in the US Atlantic recreational shark fishery.
During the last twenty years, recreational and commercial fisheries
for sharks along the US Atlantic coast and in the Gulf of Mexico have
expanded at rapid rates. Recreational catch has been estimated at
2.5 million sharks ( ca. 35,000 mt) annually; 20-40% of this is killed.
Driven by increased marketability, the commercial fishery has rapidly
expanded since 1985, with landings exceeding 7,100 mt in 1989.
Increased exploitation of sharks prompted the development of a US
Fisheries Management Plan (FMP), implemented in 1993 for the shark
resources of the Atlantic and Gulf coasts. In addition, several states
(Virginia, North Carolina, Texas and Florida) have enacted laws to
regulate shark fishing in their respective regions (14% of commercial
and 64% of recreational catches occur in state controlled waters).
Regardless, a Scientific Review Panel of Experts concluded in April
1994 (Anon., 1994) that the stocks of large coastal sharks were depleted
to much lower relative levels than realised in the FMP, and that stock
recovery would take decades rather than two years as stated in the plan.
Consequently, the Panel recommended that the total allowable catch
(TAC) of sharks not be increased in 1995 as recommended in the
management plan, but that the TAC remain constant. Some members
of the Panel suggested that the TAC be reduced instead of being held
constant or that the fishery be closed.
Figure 2. Catch per unit of effort of sandbar sharks expressed as sharks per 100 hooks
for the years 1974-1993. Data collected during scientific surveys conducted
by the Virginia Institute of Marine Science in coastal waters of the mid-Atlantic
Bight of the USA. (Some years pooled to equalise sampling effort.)
|
The annual rate of replacement (r) used in the FMP model, 26% per
year, is much higher than that calculated to be biologically possible for
both fast-growing and slow-growing carcharhinids using accepted
demographic models. Recent modelling in our laboratory suggests that
for sandbar sharks the annual population increase rate can vary from
2.5% to 11.9% with an age at maturity of 15 years. If a more conservative
age of first maturity of 29 years is used, then the maximum annual
population increase rate would be 5.2%. These low rates of intrinsic
increase are probably close to the real situation and reflect the K-selected
life history parameters typical of virtually all sharks. The
unrealistic r value used in the FMP was calculated using a surplus
production fishery model based on a time series of commercial catch
data. Such models may be useful for fast-growing, short-lived teleosts,
but are inappropriate for slow-growing, long-lived fishes such as
sandbar sharks. Most sharks, and large coastal species in particular,
have life history characteristics that make them particularly vulnerable
to overfishing. In the western Atlantic sandbar shark stocks have been
reduced by 85-90% in just ten years because of over-exploitation. In
addition, the age structure of the population has been shifted dramatically
toward younger age classes. Adult females are very uncommon. Under
the IUCN criteria for listing threatened species, the western North
Atlantic population of sandbar sharks would be classified as "Critically
Endangered" (IUCN, 1994).
This species continues to support a substantial fishery after such a
severe population decline only because of the very large size of the
original stock. Under the current FMP, the target fishery mortality
(F = 0.25) can only lead to a continued population decline (Sminkey
and Musick, in press). The western North Atlantic shark fishery is a
multi-species fishery. Many species less common than the sandbar
shark have undergone similar population declines, and at least one, the
dusky shark Carcharhinus obscurus, has undergone an even greater
population decline. This species matures at a larger size and later age
than the sandbar shark and may reproduce only every three years.
The naive assumption of some resource managers that marine fish
populations are not vulnerable to extinction because they are 'open',
with large geographic ranges and unlimited immigration, is unfounded
(Huntsman, 1994). Coastal stocks of even large migratory species such
as sandbar sharks have discrete geographic boundaries. Over-fishing
can rapidly deplete K-selected species. It may be true that fisheries will
collapse of their own accord when stocks become so reduced that they
are no longer profitable to pursue, but the notion that fisheries will
become economically extinct before extinction of target species is not
true. In a mixed-species fishery, where all species are subjected to the
same fishery mortality rate, less-abundant species could be driven to
extinction while numerically dominant species still continued to support
the fishery. Thus, Manire and Gruber's (1990) concern that many shark
species might be vulnerable to extinction appears to be well founded.
Even if the fishery were completely closed, stock recovery of the
sandbar shark and other large coastal species in the western North
Atlantic would take several decades. The collapse of large coastal shark
stocks in the western North Atlantic provides strong support for Congdon
et al.'s (1993) contention: "The concept of sustainable harvest of
already-reduced populations of long-lived organisms appears to be an
oxymoron".
Bibliography
Anonymous. 1994. Report of the shark evaluation workshop, March 14-18, 1994. NOAA, NMFS, Southeast Fishery Center, Miami:
47pp.
Congdon, J.P., A.E. Dunham and R.C. Van Loben Sels. 1993. Delayed
sexual maturity and demographics of Blanding's turtles ( Emydoidea
blandingii). Implications for conservation and management of
long-lived organisms. Conservation Biology 7(4): 826-833.
Huntsman, G.R. 1994. Endangered marine finfish: Neglected resources
or beasts of fiction. Fisheries 19(7): 8-15.
IUCN. 1994. IUCN red list categories. IUCN Species Survival
Commission, Gland, Switzerland: 13 pp.
Manire, C. and S. Gruber. 1990. Many sharks may be headed toward
extinction. Conservation Biology 4(1): 10-11.
Sminkey, T.R. and J.A. Musick. In Press. Demographic analysis of
sandbar sharks in the western North Atlantic. Fishery Bulletin.
Editor's note: The above is an abbreviated version (excluding most
references) of the material supplied by the author for the Shark Action
Plan. The IUCN threatened species assessment is provisional until
agreed by the Shark Specialist Group.
|
|
|
|
|
