dwell in the pelagic zone, is live bearing of young and
birth at a relatively large size. We will note some specific
examples of this shortly, but first I want to compare
these two quite divergent reproductive systems.
Pelagic sharks and rays all copulate, ensuring
internal fertilization of eggs and live birth of young.
Interestingly, there are several different ways in which
the young develop inside mother sharks. In some
species such as the whale shark, embryos develop in
eggs inside the mother, hatching not long before birth.
In others, such as mako, white and thresher sharks,
small numbers of embryos continually eat other eggs
inside their mothers’ uteri (technically their siblings),
resulting in relatively few, but large young being born.
Lastly, the embryos of other shark species, whalers in
particular, attach to the mothers’ uterus via a placenta
in much the same way as mammals.
Newborn sharks can range from 40 to 60cm long, as
is the case for tiger sharks, to perhaps 120 to 140cm
long for white sharks, with numbers of pups ranging
from around 40 to perhaps 10 respectively.
Although sharks produce significantly fewer offspring
per breeding season than bony fish such as marlin,
by the time the survivors of those tens of millions
of marlin larvae have reached the size of a newborn
shark, there may well be similar numbers left alive.
HIGH TURNOVER HELPS
Marlin and most tunas and other gamefish are
generally far more prolific than sharks because in
good years, higher proportions of larvae will reach
adulthood. Importantly, most of the pelagic bony
fishes – with the exception of the bluefin tunas – also
reach maturity much more quickly than sharks and
rays, which means they have a much higher turnover
rate of biomass and are therefore able to withstand
fishing pressure much more readily.
I will now provide a summary of some information
on the reproduction of a few key pelagic fish and
sharks. Because tuna are so important commercially,
they have received the most attention with respect to
studies on their reproduction.
SKIPJACK TUNA
The skipjack tuna (striped tuna) was the first of the
tunas proven to not only produce huge numbers of
eggs, but to spawn every day or so for months on
end. In a breakthrough study in the early 1990s,
Dr Kurt Schaeffer, of the Inter-American Tropical
Tuna Commission, examined the ovaries of spawning
skipjack. He found that the number of eggs spawned
each day by a single female skipjack ranges between
about 100,000 and 2,000,000 (or 147 eggs per
gram of body weight per day). Considering the sheer
biomass of skipjack tuna in the tropical oceans this
really is a mind-boggling concept.
Given the fact that skipjack do not have specific
spawning grounds and reproduce over vast areas
when conditions are suitable, it’s no wonder that
this little tuna is sometimes called ‘the cockroach of
the sea’. It also explains why more than four million
tonnes of skipjack tuna is taken from the world’s
oceans every year while still being assessed as a
sustainable catch.YELLOWFIN TUNA
Although some studies suggest that yellowfin mature
at a size as small as 60cm in length (only about 4kg),Skipjack tuna are incredibly
fecund, with a single female
capable of spawning 100,000 to
2,000,000 eggs per day, every
day or so for months on end. This
baby skipjack represents prey
for a great many predators and
would have a minute chance of
surviving to spawn itself.This 465kg Atlantic
bluefin tuna would be
capable of producing
more than 10 million
eggs in a spawning
season, only a couple
of which would
survive to maturity.“The larvae
–nomore
than three
millimetres
in length –
eat whatever
they can.”
bluewatermag.com.au 75Next Generation