it is much more widely accepted that most yellowfin
mature at 25 to 30kg in weight – or about 100cm
long. Full maturity is reached by about 45kg, or
three years old. Most spawning occurs in the tropics
between the latitudes of 15° north and 15° south
in the three major oceans, again with no specific
spawning grounds.
Like skipjack tuna, yellowfin spawn at least once
every two days in the spawning season, producing an
average of 1.5 to 3.5 million eggs, which works out at
about 60 to 70 eggs per gram of body weight.
As with most pelagics, yellowfin spawn at night,
between around 10pm to 3am, preferring water
temperatures of 24 to 25°C. Development into
reproductive state typically occurs rapidly and at
regular intervals, followed by periods of rest.
It is calculated that over her lifetime, the average
female yellowfin will have cast many tens of millions
of potential offspring to the mercies of the currents. It
is at this critical stage in the life cycle of the yellowfin
when slight increases or decreases in mortality willdetermine how good or bad fishing will be when
this ‘year class’ of fish grow to a fishable size (this
is termed ‘recruitment’ of fish into a fishery). Good
years and bad years for catches are closely linked to
the strength of these recruitment events, not just for
yellowfin, but for most pelagic species.SOUTHERN BLUEFIN TUNA
In contrast to skipjack and yellowfin tuna which
spawn over extensive areas, the southern bluefin
tuna has a remarkably discrete spawning ground,
confined to a small area off southern Indonesia. This
was proven decades ago by larval sampling surveys
which found concentrations of southern bluefin larvae
occurring exclusively in this area. Spawning occurs
from September to March, with peak larval catches in
January/February.
Adult fish with ripe ovaries and testes are also found
on these grounds during the same period, and recent
studies have shown that the average age of mature
fish on the spawning grounds is very old, with most of
the tuna exceeding 20 years.
In a by now familiar story, southern bluefin spawn
on average every 1.1 days, with a 105kg female fish
releasing around 6 million eggs each time – or 57 eggs
per gram of body weight.ATLANTIC BLUEFIN
Like the southern bluefin (and the northern Pacific
bluefin tuna), the Atlantic bluefin tuna also has highly
restricted spawning grounds, occurring in two areas in
this case: the mouth of the Gulf of Mexico and inside
the Mediterranean Sea. The existence of these two
widely separated spawning grounds led to the early
assumption of distinct western and eastern stocks
of Atlantic bluefin, and subsequent studies using
electronic tags have proven this hypothesis.
All tunas are highly fecund, producing huge numbers
of eggs over the course of a spawning season. In
the case of Atlantic bluefin tuna, mature females
each produce in excess of 10 million eggs in a given
spawning season. Being long-lived fish, they will
potentially produce perhaps hundreds of millions of
potential offspring each year.Above:For enough
individuals to survive
and perpetuate the
species, baitfish must
be prolific spawners.
The ovaries of this
large slimy mackerel
could release around
70,000 eggs every
1.3 days.
Right:Male
dolphinfish grow
larger than the
females, since
producing around
100,000 eggs
every second day
for several months
on end consumes
much of the female’s
available energy.“The smallest
mature male
blue marlin
were about
30kg, or just
one year
old, while
the smallest
mature
females were
around 50kg
and three
years old.”
76 facebook.com/BlueWatermagazine
Next Generation