The IUCN/SSC Shark Specialist Group
Shark News 6: March 1996
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Identification and closure of
nurse shark breeding grounds
Jeffrey C. Carrier, Albion College, USA
World-wide efforts to deal with dwindling shark stocks have resulted
in numerous initiatives to protect local populations as well as legislation
to protect specific species, most notably white sharks. South Africa,
California and Australia boast of regulations protecting these animals,
and the shark fisheries management plan to regulate fisheries along
the east and Gulf coasts of the United States (NMFS 1993) represent
successful attempts at addressing depletion of local populations of
sharks, generally from overfishing. The inability of local populations
to rapidly recover from overfishing can be attributed to reproductive
strategies which are incompatible with an intensive fishery. Most
studies suggest that sharks grow slowly, mature late in life, and
produce comparatively few offspring, biological characteristics which
may in fact be an epitaph for commercially valuable species.
All of the legislative endeavours to date have been
directed toward reducing loss of animals from local
populations. Few efforts have been directed toward
increasing rates of replenishment of populations by
identifying and managing breeding and nursery grounds.
This has been hampered by the inherent difficulty of
observing shark mating in the wild and actually identifying
breeding grounds and assessing the importance of habitat
to mating activities. Our study of nurse shark
Ginglymostoma cirratum mating in the Florida Keys is
unique in many aspects, and it has begun to reveal the
critical role of habitat to successful mating in this species.
This note will describe recent efforts which have led to
closure ofthe breeding grounds during the mating season.
E.W. Gudger (1912) first described mating activities in
the islands of the Dry Tortugas National Park in the western
Florida Keys. Our observations of these activities began in
1977 and the mating behaviours have been systematically
studied by Wes Pratt and I each year for the last five vears
(see Carrier et al. 1994 for a recent review). All of these studies and
observations have shown that nurse sharks utilise the same specific
area each year and, in fact, our tagging studies suggest that the same
animals, at least the males, faithfully return to this site each year. This
is not particularly surprising since very little migration has been
shown for this species (Carrier 1985, Carrier and Luer 1990).
Mating activities generally occur in the shallow waters of the study
site. The study
suggests
that shallow waters may afford some protection for females
attempting to
escape from
pursuing males.
Photo: Copyright Jeffrey C. Carrier.
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During the course of our studies, we have been able to approach
mating pairs and groups to videotape and photograph their mating
behaviours (see Pratt and Carrier 1995). During our studies, when it
appeared on occasion that our presence disrupted the mating event
we moved away to observe activities from a distance. While making
these observations, we often witnessed boats and swimmers entering
the area, unaware of the presence of pairs or groups of sharks, and
observed that a mating event could be interrupted and terminated by
such intrusions.
With the knowledge that the area has had historical importance
as a mating grounds for this species, and with the observation that
human presence interrupts the mating activities, we approached the
National Park Service with a proposal to close the area to human
activities during the months of May through August, the time frame
we have identified as the breeding season. We proposed to place
buoys adjacent to the site, an area measuring approximately 400 m
by 100 m. The site is also adjacent to a mangrove island which is
a rookery for several species of terns and frigate birds and which
is already protected from human activities by the same system of
buoys.
Following an extensive review process by Park Service biologists,
including site visitations, the proposal was approved. Permanent
underwater moorings ('mantas') are to be placed around the site, and
buoys will be attached to the moorings in May and removed in
August. In this fashion activities will be limited only during the time
when they are potentially disruptive.
We believe that this protection is important for two reasons
beyond its obvious value to nurse shark mating. The area closure
extends recognition of breeding grounds, with the appropriate
protective measures, to an aquatic realm in a fashion similar to the
protection historically reserved for terrestrial species. Secondly, it
creates an important precedent for the protection of habitats recognised
as critical for successful breeding of sharks. Though the nurse shark
is of limited commercial value, appearing mostly as incidental
bycatch, we believe our studies of mating in this species may serve
as a model for describing mating in other species and for protecting
near shore areas which are identified as critical for their mating. Since
we have also observed, collected, and tagged new-born animals and
juveniles in this area, we believe the protection will eventually also
recognise the value of this area as a nursery and protection can be
extended to protect juveniles in the same fashion as the adults will
now be protected.
References
Carrier, J.C. 1985. Nurse sharks of Big Pine Key, Florida (USA): an
investigation of growth and movements, and a comparison of
several types of external tags. In: C. Gabrie and V. Harmeli (eds.).
Proc. 5th Intemat. Coral Reef Congr., Tahiti, vol 6., pp. 655-660.
Antenne-Museum-EPHE, Moorea, French Polynesia.
Carrier, J.C., and Luer, C.A. 1990. Growth rates in the nurse shark
Ginglymostoma cirratum. Copeia 1990(3): 676-682.
Carrier, J.C., Pratt, H.L., and Martin, L.K. 1994. Group reproductive
behaviours in free-living nurse sharks, Ginglymostoma cirratum.
Copeia 1994(3): 646-656.
Gudger, E.W. 1912. Summary of work done on the fishes of Tortugas.
Carnegie Inst. Wash. Yearbook 11: 148-150.
National Marine Fisheries Service. 1993. Fishery Management Plan
for the Sharks of the Atlantic Ocean. US Department of Commerce,
February 25, 1993.
Pratt, H.L. and Carrier, J.C. 1995. Wild mating of the nurse sharks.
National Geographic 187(5): 44-53.
Jeffrey C. Carrier, PhD
Department of Biology, Albion College, Albion, MI 49224, USA
Email: jcarrier@alpha.albion.edu
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