Philopatry, Natal Homing and Localised Stock Depletion in Sharks
Robert E. Hueter,
Center for Shark Research, Mote Marine Laboratory, USA
No one knows for sure what dictates the precise patterns of shark
distribution in time and space, and our understanding of the
motives and mechanisms underlying shark migratory patterns is
crude at best. This essay is an attempt to tie together three
scientific concepts - philopatry, natal homing and localised stock
depletion - to promote a hypothesis about shark distribution and
migration, and to issue a challenge to shark researchers and
students around the world.
Philopatry
Philopatry is a term from animal behaviour and ecology derived from
the Greek for 'home-loving'. In his 1963 book Animal Species and
Evolution, Ernst Mayr defined philopatry as the drive or tendency of
an individual to return to, or stay in, its home area, birthplace, or
another adopted locality. The term is commonly used to describe the
migratory habits of microorganisms, invertebrates, and many
vertebrates including mammals and especially birds. It is less common
in the fish literature and is almost non-existent in the elasmobranch
literature. Shark biologists talk of migratory routes, home ranges,
activity spaces, and sometimes territories, but these are all
expressions of a simpler temporal-spatial pattern: that of an animal
such as a migratory shark choosing to go to, or stay in, a specific
geographic location. This is philopatry.
Natal Homing
The next term, natal homing, is perhaps the extreme form of
philopatry in which an animal migrates back to its specific
birthplace, usually to reproduce. The term was applied in 1967 by
the great sea turtle biologist Archie Carr to the migratory habits of
adult female sea turtles, which Carr believed return to nest at their
natal rookeries. Although long-term tag returns of sea turtles have
yet to confirm Carr-s natal homing hypothesis (not surprising
given their 30-year maturity time and the only recent development
of tools like PIT tags), genetic evidence supporting
his hypothesis is accumulating. In fish, natal
homing is well-known to occur in salmon, in
which the primary mechanism for this behaviour
is olfactory imprinting, based on the work of
Hasler and his students dating back to the 1960s.
Localised Stock Depletion
The last term, localised stock depletion, is a
fisheries concept that refers to the depletion of a
species in a highly restricted part of its geographic
range. Species density is "hole-punched" in a
specific locality, typically through either localised
intensive fishing or degradation of habitat.
Certainly the effects of habitat changes on shark
distribution are understandable in this regard. But
it is less clear how migratory sharks could be
easily fished out in a specific place when
abundance and tagging data indicate their
conspecifics are in good supply nearby, apparently
passing by suitable, unclaimed habitat.
And yet, examples of this can be found. Data from recreational
shark tournaments in Florida in the 1970s and 1980s indicate localised
depletion through concentrated overfishing, as shark abundance and
size in the recreational fishery dropped dramatically in one Florida
coastal site after another - but not all at the same time (Hueter 1991).
This started well before the region's commercial shark fishery expanded
in the mid-1980s. Was this an indication that shark populations sorted
themselves out on a much finer scale than was realised, such that stock
structure is dictated to a great extent by philopatry?
Question
Adult blacknose shark Carcharhinus acronotus on a sport fisherman's
line prior to release in Tampa Bay, Florida. Photo: N. Summers.
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If salmon and sea turtles do it, why not sharks? Are individual sharks
in non-insular environments philopatric for specific places in their
ranges, such as feeding areas, mating areas, and in the case of adult
females, specific nurseries? Or do migrating sharks simply 'aim at a
moving target', a set of environmental conditions that does not
always have the same earth coordinates (Cury 1994)?
The latter
process, which leads to individual dispersal, has been generally
assumed to apply to coastal sharks in regions where suitable habitat
is widespread.
Evidence
The evidence needed to answer this question includes long-term
tagging and tracking data, catch and abundance studies looking
for localised depletions, and population genetic data. The Florida
case suggests evidence of philopatry from catch data, and others
can be found, such as those described in the March 1996 issue of
Shark News (Walker 1996). Genetic data have been slow in
coming, as the field has gone from allozymes to mitochondrial
DNA and now microsatellite DNA to find the right probe for shark
population differences.
Tagging and tracking data will ultimately provide the most
direct evidence of philopatry. To examine this and other questions
about the life history of sharks, the Center for Shark Research of
the Mote Marine Laboratory has operated a shark-tagging program
over the past seven years. CSR biologists have tagged more than
5,700 small sharks of 16 species, and results from over 200
recaptures suggest philopatric tendencies in some coastal sharks
of the Florida Gulf of Mexico.
Blacknose Sharks
Among the species studied, we have tagged juvenile and adult
blacknose sharks Carcharhinus acronotus, primarily in Tampa Bay
along the central Gulf coast. Male and female blacknose sharks come
into the lower Bay in late spring for mating and feeding, and they
vacate the area entirely by late summer. So far, we have received 14
long-term recaptures of these sharks, and all came back in almost
exactly one-year, two-year, three-year, or four-year cycles. Eight of
these annual-cycle recaptures were found within 0-5 miles of the
tagging site, two were found nine miles away, and only four were
recaptured more than 10 miles from the tagging site. Longest time at
liberty so far has been 1,452 days (4.0 years), and this shark was
recaptured at exactly the same place where it was tagged four years
earlier.
Although we don't know precisely where these sharks go in the
winter, we do know they are well outside the Tampa Bay area,
perhaps hundreds of miles away. Could they simply be lurking
offshore, following a set of environmental conditions that will
bring them back in the summer? Possibly, but why are they so
close to the tagging site exactly one, two, three, or four years later?
Are these individual sharks philopatric for these specific areas and
returning on an annual basis?
Blacktip and Other Sharks
For juvenile blacktip sharks Carcharhinus limbatus, the data are more
numerous but less clear, and yet a similar pattern is emerging. When
we examine all of our blacktip recaptures and disregard the season
when the sharks were tagged (although the vast majority of these were
tagged in the spring and summer months of May, June, or July),
temporal migratory patterns are mixed, except for one: the annual
cycle. Even though some sharks were recaptured nearly 300 miles
away during mid-years, at the end of each complete year at liberty
they were usually right back at the tagging sites. In this case it is very
unlikely that these sharks are simply lurking offshore during the
winter, because our winter blacktip recaptures have all been at
relatively distant locations from the tagging sites, about 100-300
miles away.
Data from these and from other species are accumulating.
Similar patterns are appearing for the Atlantic sharpnose shark
Rhizoprionodon terraenovae and the less well-travelled
bonnethead Sphyrna tiburo. The jury is still out on juvenile bull
sharks Carcharhinus leucas and other species because there
have been relatively few recaptures thus far. Meanwhile, we
are continually scrutinising our new recapture data. Could it be
that our data are biased because, for some reason, only fishermen
near the tagging sites recognise our tags and turn in the data?
Probably not, because we have received off-cycle returns from
distant locations, and we have also had success with recaptures of
these same tags from throughout the Gulf, including in Mexican
waters.
Hypothesis
Although these data are preliminary, a trend is emerging. I believe it
is time to 'raise the bar' on this hypothesis and more formally state that
which many shark biologists have thought about, talked about, and
written about, to wit:
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That many, if not most, shark species are philopatric for their
natal nursery areas and other critical parts of their ranges, such as
winter feeding grounds. This philopatry, furthermore, makes them
even more susceptible to regional overfishing and habitat
destruction.
The requirements for philopatric behaviour and natal homing in
sharks would include:
- Defined nursery areas - sharks certainly have these;
- Migratory routes and patterns - also well documented;
- Neural and sensory equipment required for a homing
mechanism.
After studying the senses and brains of sharks for nearly 25 years,
it is clear to me that they are more than qualified for the job. Their
large brains certainly surpass those of salmon and sea turtles, and their
senses, including electroreception, are among the most well-developed
in the animal kingdom.
Conclusions, and a challenge
I consider the issue of philopatry and natal homing in sharks to be
the most important issue in shark biology today, and I challenge
all shark researchers to test this hypothesis rigorously in their
respective research areas. Nearly every type of shark research can
play a role in this, for the ramifications of philopatry, if true for
most shark species, would be profound. It certainly would affect
our views of shark evolution and genetics, and it would shape new
perspectives on the physiology and ecology of shark species. It
would fundamentally affect studies of shark population dynamics,
and perhaps most importantly, it would drastically change
conventional views of shark fisheries science for the management
and conservation of shark populations.
References
Carr, A. 1967. So Excellent a Fishe: A Natural History of Sea Turtles.
Scribner, New York, NY. 280 pp.
Cury, P. 1994. Obstinate nature: an ecology of individuals. Thoughts
on reproductive behavior and biodiversity. Can. J. Fish. Aquat.
Sci. 51: 1664-1673.
Hasler, A.D. and A.T. Scholz. 1983. Olfactory Imprinting and Homing
in Salmon. Springer-Verlag, New York, NY. 134 pp.
Hueter, R.E. 1991. Survey of the Florida recreational shark fishery
utilizing shark tournament and selected longline data. Mote
Marine Laboratory Tech. Rept. 232A: 94 pp.
Mayr, E. 1963. Animal Species and Evolution. Belknap Press of
Harvard University Press, Cambridge, Mass. 797 pp.
Walker, T. 1996. Localised stock depletion in sharks: does it occur for
sharks? Shark News6: 1-2.
Robert E. Hueter, Senior Scientist & Director,
Center for Shark Research, Mote Marine Laboratory,
1600 Ken Thompson Parkway, Sarasota, FL 34236 USA
Tel. (+1) 941-388-4441, fax. (+1) 941-388-4312
Email: rhueter@mote.org
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