The IUCN/SSC Shark Specialist Group
Shark News 6: March 1996
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Evolution of sharks; hints for
successful management strategies
Erich K. Ritter, Green Marine, Miami, USA
Introduction
In the past 60 years, the demand for shark products has peaked and
waned, only to rise again. Very few shark fisheries existed in North
America and other countries until the 1930s, when the huge market
demand for liver oil caused rapid exploitation of shark stocks. The
arrival of synthesised products saved many shark stocks from collapse.
Shark populations were safe from exploitation until a new public
demand for shark fins and shark meat hit the market. For the past 25
years, shark stocks have been declining rapidly. They are heavily
overfished and many populations show alarming signs of collapse
(Bonfil 1994).
Although this overfishing must end to allow shark populations to
survive, regulations on fishing alone will not produce the necessary
or expected results. There is another major threat that must be
addressed-the destruction of coastal habitats. Loss of coastal habitat,
particularly the loss of nursery grounds, poses a significant threat to
shark stocks and marine biodiversity. A glimpse at life cycles and
evolutionary histories clearly indicates that stresses on coastal habitats
must be minimised to achieve adequate conservation of shark stocks.
Improved controls on directed shark fisheries (like this Taiwanese gillnet
fishery for shark fins) are essential, but must be combined with protection
of coastal
habitats and nursery grounds if stocks are to be protected.
Photo: J. Stevens.
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Evolution of sharks -importance of coastal
nursery grounds
Sharks are one of the most successful marine vertebrates in evolutionary
history. During more than 400 million years of evolution, sharks
encountered, with very few exceptions, one major predator - larger
sharks. This being one of the major selective forces, sharks developed
some remarkable features in their reproductive biology to avoid predators
and compensate for losses. Life history characteristics such as numerous pups,
fast growth rates, continuous female life cycles, or intra-uterine cannibalism
or oophagy aided offspring survival (Branstetter 1990). Arguably, one of the most
important strategies is the behavioural adaptation of giving birth in coastal
nursery grounds. Whatever the mechanism, all such strategies originated in coastal areas.
A look at the fossil record suggests that ancient sharks lived along
shores and shelf areas. For example, fossil records from Bear Gulch
Bay clearly suggest that as far back as the Upper Mississippian period,
approximately 320 million years ago, coastal areas were heavily
utilised by sharks and served as nursery grounds (Lund 1990). By
choosing protected coastal areas (bays, estuaries, and lagoons) to
give birth, adult sharks reduced the risk of predation on their offspring.
Shallow nursery grounds limit access to predators such as larger
sharks, offering a haven for juveniles.
Until recently these successful reproductive strategies gave sharks
a competitive advantage to remain at the top of most trophic webs.
Nevertheless, one of their oldest and most effective strategies, the
utilisation of coastal nursery grounds, is accelerating the demise of
many species. What has taken millions of years to evolve is now
threatened by human activities.
Destruction of coastal habitats
With more than half of the total world population living within
approximately 100 km of the ocean, there is inevitably destruction
and degradation of coastal habitat, rapidly altering coastal areas.
Sharks, however, do not have the mechanisms to adapt quickly to
rapid alteration in their environment. As typical K-selected species,
sharks rely on fairly stable environmental conditions to survive. With
the majority of shark species occupying the coastal zone during some
stage of their development, the rapidly declining quality of habitats
reverberates in the decline of stocks.
Habitat degradation affects entire ecosystems. Whole food webs
are disrupted. In highly protected areas where food limitation may be
a factor, even subtle declines in lower trophic levels can result in
severe alterations to top predator populations. In addition, a decline
in prey populations forces predators to expand their hunting activities.
In the case of juvenile lemon sharks Negaprion brevirostris, activity
space may be correlated with prey species composition (Ritter in
prep.). If a decline in preferred prey population leads to an increased
activity space, as suggested by a crisis model, juvenile populations
will increase their home range into less protected littoral waters,
competing with larger sharks. Such a scenario undermines the
advantage conferred by utilising protected nursery grounds.
Reproductive strategies and their implications for
management
Phylogenetic hypotheses are important tools to understand how
elements in nature developed. Fossil records combined with current
behaviour provide a window into evolutionary strategies. Natural
resource managers must take life history patterns into account when
instituting conservation methods and regulations (Ritter and Cardoch
in prep,). These patterns provide important hints as to the requirements
for sustaining a viable population. Attempts to manage shark fisheries
are few and efforts focus primarily on regulating total catch. However,
the reproductive strategies outlined above clearly indicate that efforts
to regulate catch alone are insufficient and beg consideration of
several recommendations.
Harvesting of sharks at the current rate could lead to extinction of
several species. Sharks did not evolve with the outside pressure of
human predation and do not reproduce quickly enough to compensate
for he losses caused by commercial fisheries. Present levels of
harvesting must end and management regulations must be reevaluated.
Current regulations on total catch ignore an important aspect of
shark biology - sharks take years to reach maturity and reproduce.
Therefore, management plans need to include restrictions
on takes of juvenile and subadult sharks. At a minimum, populations
at risk should have a total ban on fishing for juveniles and subadult
sharks to help stabilise their populations.
New-born lemon shark Negaprion brevirostris with placenta
still attached. This species gives
birth in nursery areas (here a shallow water
lagoon fringed by mangroves) where there are very
few marine predators. Photo: S. Gruber.
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Very few fishery services around the world are sensitive enough
to monitor shark stocks and populations. Of those that do, regulations
focus on stocks and dynamic fisheries, and not on entire life cycles.
In April 1993, the United States National Marine Fisheries Service
(NMFS) launched the Shark Fishery Management Plan of the Atlantic
Ocean in an attempt to include reproductive needs for monitoring
shark populations (NOAA 1993). The first of its kind, this large-scale
management plan focuses on shark fisheries in the Atlantic Ocean
and Gulf of Mexico. In addition to the expected stock regulations and
management of target species, the Shark Fishery Management Plan
recommends research into reproductive requirements and
identification of mating and nursery grounds. Furthermore, the plan
acknowledges the need to include habitat initiatives for effective
stock management. Although attempts have been made to include
those recommendations in actual management decisions (e.g. Carrier
1995 and 1996, M. Bailey 1995 pers. comm.), NMFS continues to
invest the majority of its efforts indefining and enhancing commercial
values and goals. Without consideration of species-specific habitat
requirements sustainable yields will drop due to insufficient knowledge
and protection of the essential neonate and juvenile populations.
Many shark species are highly migratory and move through
different national boundaries and different exclusive economic zones.
Therefore, international cooperation is needed to ensure that protective
measures taken in one country are not undone by migration into
another country lacking similar practices. An international effort
should be launched to ensure protection for all shark species at all
stages of their life cycle.
Conclusions
Shark reproduction strategies provide valuable information for successful
stock management. In an age of limited funding
opportunities, protective measures must be employed at points
ensuring maximum effectiveness. Shark life history patterns tell us
where those points are. They begin with protection of coastal habitats
focusing primarily on mating and nursery areas. It is critical to enact
regulations that are synchronous with life cycles and that protect all
ontogenetic stages. Without them, efforts to protect adult stocks will
be insufficient. Negligence will result in a loss greater than just shark
depletion and a gamble too high to take - the loss of entire marine
ecosystems and biodiversity.
References
Bonfil, R. 1994. Overview of world elasmobranch fisheries. FAO
Fish. Tech. Paper 341, FAO, Rome. 119pp.
Branstetter, S. 1990. Early life-history implications of selected
carcharhinoid and lamnoid sharks of the Northwest Atlantic. In:
H.L. PrattJr., S.H. Gruber and T. Taniuchi (eds). Elasmobranchs
as living resources: advances in biology, ecology, systematics,
and the status of the fisheries. NOAA Tech. Rep. NMFS 90.
pp. 17-28. US Department of Commerce.
Carrier, J. 1995. Identification of mating and nursery grounds for
nurse sharks Ginglymostoma cirratum in the Florida Keys and
initial habitat management planning. Abstract. Am. Fish. Soc.
Meeting. August 1995. Tampa, Florida.
Carrier, J. 1996. Identification and closure of nurse shark breeding
grounds. Shark News 6:9.
Lund, R. 1990. Chondrichthyan life history styles as revealed by the
320 million years old Mississippian of Montana. Env. Biol. Fish.
27:1-19.
NOAA. 1993. Fishery management plan for sharks of the Atlantic
Ocean. NOAA, NMFS, US Department of Commerce.
Ritter, E. In preparation. Modelling of prey-predator interactions in a
semi-enclosed, low energy, Bahamian lagoon.
Ritter, E., and Cardoch, L. In preparation. Evolutionary history and reproductive strategies insharks: advice for resource management.
Erich K. Ritter,
Green Marine, P.O. Box 331793, Miami, FL 33233, USA.
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