Chondrichthyes (cartilaginous fish,
including sharks, rays and chaemeras) >
Conservation and management of cartilaginous fish
Do cartilaginous fishes need protection? We
think so, but in southern Africa most people might think otherwise.
There seem to be plenty of sharks, rays, and chimaeras about, and to
many people these `inedibles' are trash fish, `man-eaters' and
objects of hatred, that could stand for culling or even
extermination. However, cartilaginous fishes, because of their
essentially mammalian life-history style, are endangered by
ever-increasing human exploitation. The threefold increase in
fisheries for cartilaginous fishes since World War II highlights the
biological, environmental and management difficulties with these
fishes that need careful consideration to insure their long-range
survival. Cartilaginous fishes are typically slow-growing,
long-lived, mature late in life, and have a low reproductive rate.
In addition the females of most species grow to a larger size than
males. These life-history characteristics make cartilaginous fishes
easy to overexploit, and make sustained fisheries for them nearly
impossible with the nonselective practices of most fisheries
operations. Numerous case histories for such fisheries indicate that
initial exploitation usually results in rapid declines in catch
rates and in some instances a complete collapse of the fishery.
Cartilaginous fishes are limited by their mode
of reproduction, which produces low numbers of offspring and allows
them little flexibility in compensating for increased exploitation.
This is very different from bony fishes that sustain massive
fisheries, such as cod and hake, which produce millions of eggs per
female fish and have enormous reproductive potential to replenish
the next generation despite heavy exploitation of adults.
Exploitation of cartilaginous fish populations diminishes the
replenishment of young by direct predation on the young and adults;
fewer young are available to become adults, fewer young are produced
by the diminished adults, and numbers rapidly decline. Some species
such as the
bluntnose spiny dogfish (Squalus megalops) seem a vast,
inexhaustible resource, but this is deceptive because, with heavy
exploitation, their low reproductive rates cannot sustain their
numbers as individuals are removed, and fisheries accordingly will
crash.
It is currently not known how to `fine-tune' a
fishery for cartilaginous fishes, to produce sustainable yields over
an indefinite period. Methods of doing so might be based on those
used to regulate
mammal populations with low fecundity rather than
bony fishes, including selective fishing of individuals that might
be surplus to the reproductive population, as well as strict
seasonal, geographic, and catch limits on the fishery. The
difficulties with management of cartilaginous fishes arise from
severe limits to our knowledge of the biology of these fishes, the
unselective nature of most fisheries for cartilaginous fishes, and
the fact that fisheries for cartilaginous fishes worldwide are
largely unnoticed, unstudied, unregulated, and are essentially out
of control. Many fisheries take cartilaginous fishes as a bycatch,
and are driven by primary catches of more fecund bony fishes; thus
declining catch rates for cartilaginous fishes need not regulate or
even effect a given fishery as bony fishes can still be caught in
commercially viable quantities.
It is instructive to estimate the numbers of
individual sharks, rays, and chimaeras taken in the world
cartilaginous fish catch. Using the 627,000 tonne catch of 1986, and
assuming that each individual weighed as much as a human being (68
kg), some 9.2 million cartilaginous fishes were killed in 1986,
compared to an estimated 30 humans killed annually by sharks. The
yearly `human attack' fatality rate on cartilaginous fishes was thus
some 322,000 times higher than the shark attack fatality rate.
However, the actual numbers of cartilaginous fishes taken are
probably much higher, because the average size of cartilaginous
fishes is smaller than for human beings. Using an average weight of,
say, 10 kg, the number of cartilaginous fishes caught worldwide in
1986 would be approximately 63 million, about twice the human
population of southern Africa. This sets shark paranoia and hysteria
into another perspective; from a shark's viewpoint, humanity is a
deadly danger, while sharks pose a miniscule problem to the vast
armed legions of humankind.
World catches of cartilaginous fishes are not
keeping pace with those of more fecund bony fishes, and seem to be
leveling off in the present decade. In the next few decades we
suspect that world cartilaginous fish catches will begin to decline
as increasing fisheries pressure, driven by ever-burgeoning
humanity, exhausts most accessible populations.
A particularly worrisome, huge and shadowy
fishery that affects shark populations worldwide is the oriental
sharkfin trade. The value of shark fins have soared in recent years.
The sharkfin fishery fits in nicely as a bycatch operation of
fisheries for bony fishes; and sharks that otherwise had low value
and might compete with more valuable bony fishes for hold space in a
fishing vessel can be turned into a high-value, low-volume bonus for
commercial fishermen. Thus the rigging of pelagic longline vessels
are festooned with hundreds of drying sharkfins, and pelagic shark
populations are being decimated. The sharkfin trade is powered by a
tremendous market in the populaces of China and Taiwan and in
Chinese communities worldwide. Virtually nobody seems to notice the
effects of this trade or cares, although much hue and cry was given
in the late 1960's to the plight of spinner dolphins, which were
being killed by tuna purse-seiners in the eastern south Pacific.
`Popularity' may be destroying at least one
species of shark. The `success' of the JAWS films touched an
atavistic streak in the minds of humankind, and in the aftermath of
these science-fictional films macho-men swarmed out to pit their
virility against the
great white shark. Here and elsewhere, off
California, New York, and Australia, shark after shark was caught,
by anglers, divers, and commercial fishermen, with the perpetrators
congratulating themselves in conquering these horrible man-eating
monsters and ridding humankind of a supposed scourge. Entrepreneurs
sprang up, photographing live white sharks, leading expeditions of
the nature-loving rich to huddle in shark cages off Dangerous Reef
in South Australia, exhibiting the frozen or preserved carcasses of
white sharks, and selling their mercury-laden meat, teeth, and jaws.
Other sharks were not spared either, as shark fishing became
extremely popular. Large public aquaria captured white sharks alive
for display, which so far have uniformly died after a few days or
weeks. The value of large white shark jaws soared to fantastic
heights, and erratic and lucrative spot fisheries for jaws arose in
southern Africa and elsewhere. Dangerous Reef, where most of the
spectacular cinemagraphic and still-photography of white sharks
occurred, was recently rid of most of the small group of white
sharks that regularly attended boats as big-game anglers and
commercial fishermen moved in after the cinema people,
diver-photographers and shark-cage entrepreneurs.
All this pressure on the great white shark may
be creating problems for its survival equivalent to those afforded
elephants and
rhinoceros by the persistent ivory and rhino horn
trades. As white sharks feed on seals and other large marine
organisms that in turn feed on commercially important bony fishes,
the effects of removing white sharks may be detrimental. We suggest
that proposals for the protection of the white shark and banning of
the white shark jaw trade be considered and placed into operation
both locally and internationally before this shark is exterminated.
We note that, JAWS and much adverse publicity notwithstanding, many
local anglers have sympathy for white sharks and oppose their
capture, because of their predation on
Cape fur seals.
The prospect for cartilaginous fishes in the
next century is grim, as the human population soars and these fishes
are increasingly hammered by fisheries. The difficulties in managing
and conserving cartilaginous fishes are enormous, and exacerbated by
the negative `man-eating' JAWS image afforded sharks, which hampers
conservation and encourages anti-shark measures, unregulated
exploitation, and wanton destruction of cartilaginous fishes. The
ecological impact of the removal of cartilaginous fishes, top
predators in the sea, is unclear, but we suspect that it will be
far-reaching and ultimately detrimental to marine ecosystems and
ultimately humankind.
We call for the public to acquaint itself with
the local cartilaginous fishes, and to realize that they are of
little harm and are integral to healthy marine environments. We hope
that at least some people will take heed of their plight, and will
acquire a mature, respectful, appreciative attitude towards a group
of animals honed to predatory perfection by over 450 million years
of evolution. Many land predators were quite recently considered
vermin, to be shot on sight, but are now conserved and protected.
Conservationists and the general public in the West have helped to
conserve those species of whales that were threatened by the
excesses of international whaling. Such conservation efforts towards
sharks and other cartilaginous fishes will be extremely difficult
but increasingly necessary in the immediate future. It is an act of
blind self-indulgence to morbidly embrace shark-attack paranoia or
actively exploit such fear while sharks and other cartilaginous
fishes are quietly being overfished and destroyed.
We propose that cartilaginous fishes should be viewed as a necessary
prerequisite to maintaining a sound marine ecosystem, rather than as
unnecessary vermin that should be exterminated. It has become clear
in recent years that in order to maintain many of the large game
reserves the predators need to be protected rather that eliminated.
Unfortunately, people have been slow to recognize that this same
principle applies to the marine environment. Often we have heard
that by killing a shark one would save a seal or sea otter, yet this
is a very one-sided and narrow-minded approach. Not only are sharks
a positive stabilizing factor in the maintenance of healthy marine
ecosystems, but they are interesting and rewarding animals to
observe. More detailed studies of cartilaginous fishes will reveal a
better and hence more complete understanding of marine communities
by examining the chief predators. If one is truly concerned about
maintaining the marine ecosystem, then we need to consider the
impact that is being imposed on it by the removal of these top
predators.
We hope that much of what is written in this
website gives YOU the reader some insight into the diversity of
cartilaginous fishes and the intricate and delicate relationships
between these fishes and their environment. For these are extremely
complex animals whose biology we are only beginning to understand.
Hopefully, the information we have presented will cause you, as an
environmentally concerned individual, to stop and think of these
creatures not as monsters or pests, but as an integral and essential
part of the WHOLE MARINE ENVIRONMENT.
Text by Leonard J.V. Compagno, David A. Ebert
and Malcolm J. Smale
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