The IUCN/SSC Shark Specialist Group
Shark News 9: June 1997
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Burning the Candle at Both Ends
Steve Branstetter, Gulf and South Atlantic Fisheries Development Foundation, USA
Sharks and batoids (rays) have a reproductive strategy that is very
different from the vast majority of most fishes. In general, bony fishes
spew thousands of eggs and sperm into the water column where
fertilisation takes place. For the larvae produced, there is a substantial
mortality. Conversely, sharks produce a limited number of young after
carrying the pups internally for a lengthy gestation period, which is
analogous to mammalian reproduction. These precocious young
receive no further parental care or protection, but they are of a size and
developmental stage that allows them to avoid and bypass many of the
natural mortality forces that affect their bony fish counterparts.
Litter of near-term milk shark Rhizoprionodon acutus pups. Pregnant females and newborns regularly appear in Sabah fish markets, presumably taken as bycatch in fisheries targeted at other species. Photo: Sarah Fowler, Sabah Darwin Project.
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For example, viviparous species such as sandbar and blacktip
sharks have a one-year gestation period, and a two-year reproductive
cycle. Initial maturation and final reproductive tract development are
apparently timed to occur just prior to the mating season. For most
warm-temperate and sub-tropical species, pupping usually occurs in
summer. However, some tropical sharks do not exhibit a particular
seasonality to their cycle.
In the case of viviparous species of the Northwest Atlantic, four to
eight eggs are ovulated (often in pairs) in June. They pass into the
female reproductive tract where they are fertilised and encapsulated,
and pass on to the two horns of the uterus, usually with equal numbers
in each horn. The embryos are 10-20 mm in length by July, clearly
visible, and attached to the egg yolk. By October the embryos are
nearing half their birth length, and look very similar to miniature
adults. By this time, a pseudoplacental connection has formed between
the embryo and the uterine wall, most of the yolk is resorbed, and the
embryos receive nourishment directly from the mother through the
placental connections. By January, the embryos are nearly full length,
but still need considerable development before being born. Birth
occurs in May and June. After birth, the mother leaves the 'nursery'
ground, and does not mate again for a year. This allows her to rebuild
the energy reserves needed during pregnancy. Thus, the gestation
period is one year, and the entire reproductive cycle is two years.
By contrast, other species, usually smaller and faster growing,
have a one year reproductive cycle in which new ova begin developing
during the final phases of pregnancy. Shortly after pupping, the female
mates and starts the cycle again.Because the pups are carried internally, there is a trade-off in the number of young that can be produced and their size at birth. Some
species, such as the sand tiger, produce only two very large pups per
cycle. By contrast, the blue shark may produce as many as 50 young
at a much smaller size. More commonly among familiar shark species,
the number of pups per litter ranges from six to twelve. In some cases,
the number of young increase as the mother grows older; for example,
her early pregnancies may only produce two or four young, but later
in life she will produce a full complement of eight pups.
The young may or may not utilise nursery grounds, year-round or
seasonally, during their first or first few years. By doing so, they avoid
being preyed upon by larger fishes, including their own species. The
length of time that they use these nurseries is somewhat dependent
upon their growth rates. It has been suggested (by this author) that it
may be necessary for an individual to attain a size of approximately
one metre in order to successfully avoid or deter predators. In some
cases this may be the maximum size of the species, and, in those cases,
reproduction and growth rate are the highest.
These differences highlight the various strategies used to
successfully maintain species and populations. Whether the young
use a nursery or not, they are susceptible to mortality from both
natural and man-made (fishing) sources. The juveniles and adolescents
tend to remain in shallower coastal waters, whereas the larger adults
may be found in deeper coastal waters, but this nearshore existence
makes the juveniles more vulnerable to a myriad of fishing efforts.
They may be targeted by recreational and commercial fishers, or they
may be inadvertently taken in recreational and commercial efforts
that target other species. In some fisheries, in certain areas, the
bycatch of young sharks may actually dominate the catch.
Large-scale shark fisheries have never been known to be
successfully sustainable. In part, this is because of the sharks' general
life history. It has been estimated that some shark species may only
have a capacity to increase their population by 2% annually; at least
for those species that have been mathematically modelled to determine
such values, the maximum ability to increase may be only 10%-15%
annually. Thus, given the biological constraints of the resource to
rapidly replenish itself because of low fecundity, slow growth, and
late maturation, populations have little natural flexibility to withstand
excessive fishing mortality. Even if a directed fishery (no matter the
source) were properly managed to allow for a sustainable harvest of
adults, there would be little flexibility in that management strategy for
other sources of mortality, such as bycatch of the juveniles. Thus,
juvenile survival is critical to continuation of strong populations, and/
or sustainable fisheries.
There is a growing concern about the exploitation of shark
resources world-wide, especially because the primary product is only
the fin, and there are increased efforts to apply management to various
fisheries that target or interact with marketable sharks. Just as important,
however, will be management to address fisheries that interact with
juvenile sharks as a bycatch. Failing to address mortality on both the
young and the old is analogous to burning a candle at both ends.
There may be more immediate benefits, but in the long run ... well,
in the case of sharks, there is no 'long' run.
References
Bonfil, R. 1994. Overview of the world's elasmobranch fisheries. FAO Fisheries Tech. Paper No. 341. FAO, Rome, Italy. 199 pp.
Branstetter, S. 1990. Early life-history implications of selected carcharhinoid and lamnoid sharks of the Northwest Atlantic. NOAA Technical Report, NMFS 90: 17-28.
Pratt, H.L., Jr. 1990. Shark reproductive strategies as a limiting factor in directed fisheries, with a review of Holden's method of estimating growth parameters. NOAA Technical Report NMFS 90: 97-109.
Steve Branstetter, Ph.D., Program Director
Gulf & South Atlantic Fisheries Development Foundation,
Lincoln Center, Suite 997
5401 West Kennedy Blvd,
Tampa, FL 33609, USA.
Email: steve.branstetter@worldnet.att.net
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