It was abundantly clear that these great ocean-
roaming fish were indeed gathering at this location
each year with a single age-old purpose: to meet and
spawn, thereby ensuring the reproduction of their
species into the future.VITAL STATISTICS
Studying the reproduction of fish is vital to
understanding their life cycles. This in turn helps
inform fisheries managers on the dynamics of fish
populations, potential effects of fishing, and ultimately
the sustainability of stocks.
There are a number of aspects that need to be
considered in order to gain a better understanding
of the reproduction of pelagic fish populations. These
include the location and extent of spawning grounds,
length of spawning seasons, numbers of eggs or
offspring produced by each fish, survival rates of
larvae and young fish, and sex ratios of fish in
different parts of their range.
It’s also important to consider the size and especially
the age at maturity of female and male fish, and
the number of seasons over which individuals might
spawn in their lifetimes. For pelagic sharks, factors
such as location of mating and pupping grounds,
gestation period and time between pregnancies could
also be added.REPRODUCTIVE STRATEGIES
Fish have evolved an incredible array of reproductive
strategies aimed at ensuring that mating occurs,
eggs are fertilized and young are produced which will
eventually replace the previous generation. However,
in the case of pelagic fish, two dominant strategies
have emerged: one for bony fish and one for
cartilaginous fish (sharks and rays).
Excluding sharks for the moment, nearly all of the
well-known pelagic fishes are what are known as
‘broadcast spawners’. This means that they produce
very large numbers of eggs that are fertilized external
to the body. The fertilized eggs – usually no bigger
than a pinhead – then float very near the ocean’s
surface as the larvae develop inside. Perhaps becausefloating eggs are so prone to predation by zooplankton
and even specialist seabirds, embryonic development
is very rapid, with eggs hatching after only a few days.
The resultant larvae – no more than three millimetres
in length and highly active – then proceed to hunt and
eat whatever they can, growing quickly as they swim
through the gauntlet of small predators, but inevitably
sustaining extremely high mortality rates. The main
exceptions to this general rule in the open ocean are
the flying fishes, whose eggs are heavier than water
and would sink to the depths if female fish did not
adhere them to floating seaweed.ONE-IN-A-MILLION
The strategy of broadcast spawning and the ensuing
enormous mortality of larval and post-larval pelagic
fish in their first few months of life is an excellent
illustration of the old proverb about not putting all your
eggs in the one basket. This is not only because of the
sheer numbers of larvae that might insure against a
mass mortality event, wiping out a generation of tiny
fish, but also because broadcast spawners spread their
production of eggs over relatively long periods of time.
After my eureka moment discovering the huge black
marlin’s ovaries packed with tiny eggs, I decided to
take some small samples back to the lab for analysis.
Eventually we were able to estimate the number of
eggs in those 30kg egg sacs, which for female fish
weighing between 360 and 590kg was a staggering
65 to 220 million eggs! It has also subsequently been
proven that all of the tunas, and most likely billfishes
and other pelagics such as wahoo and dolphinfish,
spawn a portion of those eggs every one to two days
for periods of weeks or even months on end.
Given that each pair of fish only really need to produce
two offspring that survive to maturity and reproduce,
the number of eggs produced by oceanic fishes every
day shows just how precarious life must be for the
untold billions of larvae that hatch on a daily basis.SHARKS’ DIVERGENT SYSTEMS
The second reproductive strategy that has been adopted
by the cartilaginous sharks and rays, including those thatTiger sharks
produce much
larger litters of
young than other
sharks, with
between 30 and
50 pups of 50 to
75cm born
at once.“The sex
of many
fish species
can only be
determined by
cutting them
open.”
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