The IUCN/SSC Shark Specialist Group
Shark News 7: June 1996
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Population genetics: an accessory to tagging studies
Ed Heist, Center for Biosystematics and Biodiversity, Texas A&M University
Many species of sharks span vast geographic ranges but may be
divided into isolated breeding populations. If conservation efforts are
to succeed, these individual populations need to be identified so that
we can determine whether locally-depleted regions can be replenished
from other areas. One way to do this is to examine movements of
individual sharks through traditional tag and recapture. Another,
more recent, technique involves analysis of population genetics using
polymorphic characters. The purpose of this article is to discuss the
relationship between traditional tagging and genetic studies, and to
demonstrate how both kinds of research can add to our understanding
of shark populations.
While tagging studies detect movements of individuals within a
single lifetime, genetic studies track movements of genes over many
generations. When two populations become isolated reproductively,
gene frequencies change over generations through random genetic
drift. Ultimately isolated populations become fixed for alternate
alleles; for many generations however only frequency differences will
exist. Reproduction between individuals from different regions results
in gene flow, homogenising gene frequencies between regions. By
examining frequencies of polymorphic genes from different geographic
regions we can estimate the amount of gene flow that has taken place
over past generations.
Both techniques, tagging and population genetics, have strengths
and weaknesses. Genetic homogeneity between regions is not proof
that separate fishery stocks do not exist: populations may not have been
isolated long enough for differences in allele frequencies to develop; or
an exchange of only a few individuals per generation is sufficient to
maintain the same alleles in different populations. Tagging studies may
thus reveal significant stock structure in the absence of any genetic
structure. On the other hand, genetic studies may reveal details about
reproductive life-histories of organisms not detectable by tagging
studies. Some marine organisms, sea turtles and whales for example,
have separate breeding populations that overlap during part of the year.
Studies in these organisms have revealed significant population structure
despite the fact that individuals from separate breeding units intermingle.
Many species of sharks deliver their pups in coastal nursery areas distant
from the locations where adults are usually found. Genetic studies may
tell us whether female sharks return to the same nursery area from which
they came, and therefore whether local depletion of juvenile sharks in
a coastal nursery area will have a long-term effect on future generations
of pups.
Population genetic studies have been published on sandbar
sharks in the US and on the gummy shark, spot-tail shark, and
Australian blacktip shark in Australia. Other projects under way
include studies on the great white and whale shark. Recently, John
Graves, Jack Musick and I published a paper on population
genetics in the shortfin mako (Heist et al. 1996). We examined
mitochondrial DNA genotype frequencies in makos from the North
Atlantic, South Atlantic, North Pacific and South Pacific. We found
there was a highly significant difference in genotype frequency
between the North Atlantic and other samples, suggesting little or no
exchange of makos between the North Atlantic and other oceans. This
finding is also consistent with the tagging data of Casey et al. (1992)
that shows long-distance movement of tagged makos within the North
Atlantic, but no recaptures south of the equator or in other oceans.
Both population genetics and tagging can thus provide important
information for the conservation of shark populations.
References
Casey, J.G., and Kohler, N.E. 1992. Tagging studies on the shortfin
mako shark ( Isurus oxyrinchus) in the western North Atlantic.
Aust. J. Mar. Freshwater Res. 43: 45-60.
Heist, E.J., Musick, J.A., and Graves, J.E. 1996. Genetic population
structure of the shortfin mako ( Isurus oxyrinchus) inferred from
restriction fragment length polymorphism analysis of mitochondrial
DNA. Can. J. Fish. Aquat. Sci. 53: 583-588.
Ed Heist, Center for Biosystematics and Biodiversity, Texas A&M
University, USA. Email: e-heist@tamu.edu
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