BLUE MARLIN
Of all the billfishes, blue marlin have been studied in
more detail with respect to their reproduction than
other species. According to early Japanese studies
of larval distribution, blue marlin apparently have
extensive spawning areas throughout the tropical
and sub-tropical Pacific, Indian and Atlantic oceans.
Even so, spawning adult blue marlin do appear to be
patchily distributed, being mainly found in seasonal
aggregations around isolated islands.
After dissecting many fish landed at the Hawaiian
International Billfish Tournament over the years, it
was revealed that nearly 80% of the blue marlin
landed during summer are males, mostly less than
90kg in size. Female fish caught at the same time
cover a much greater size range, from as small as
22kg to over 600kg.
A recent study based at the tiny remote Japanese
northern Pacific island of Yonaguni also indicated an
extended spawning season from May to September.
That study found the smallest mature male blue marlin
were only about 30kg, or just one year old, while the
smallest mature females were around 50kg and three
years old. A 400kg blue marlin may produce as many
as 150 million eggs, each a little over 1mm in diameter,
very similar to my own findings with black marlin.
BLACK MARLIN
The egg counts from spawning female black marlin
- numbering up to 220 million eggs – certainly are
remarkable. However, as is the case for most of the
other billfishes, no studies on the actual frequency of
spawning have been undertaken on black marlin.
If we were to estimate the average number of eggs
spawned each ‘batch’ per gram of body weight, then
a grander black (or blue) marlin would be spawning
between 23 and 45 million eggs each batch, quite
likely every one to two days.
Although it’s not known what proportion of those
eggs would be fertilized each time, given a strong
evolutionary imperative, it can be assumed that
fertilization rates are probably high. This means that
the number of larvae produced by just one female
black marlin in a single spawning season could be in
the billions.
SMALLER PELAGICS
It isn’t only the large fish that produce big numbers of
eggs either. Female dolphinfish in captivity have been
shown to produce around 100,000 eggs every second
day for several months on end. Spawning takes place
at the surface, nearly always at night, with fish usually
forming in pairs to spawn.
Female dolphinfish only grow to about half the
maximum size of males, thought to be due to the
constant production of eggs which consumes much
of their available energy. Adding to their reproductive
capacity is the fact that dolphinfish mature at a
surprisingly small size.
One study off Florida showed that female dolphinfish
show signs of maturing at a length of only 35cm, and
are all fully mature at 55cm (about 2kg). Similarly,
the smallest maturing males average about 42cm
long, with all fish of both sexes being found to mature
within their first year of life.
Meanwhile, at the far end of the spectrum of pelagic
fish, the humble slimy mackerel is another prolific
spawner, releasing around 70,000 eggs every 1.3 days.PELAGIC SHARKS
As already noted, oceanic sharks all give birth to live
young, although the development of the embryos inside
the mothers’ body may vary. A study in the US collected
and summarised data from around the world on any
reliable observations of these sorts of variables. For 30
pregnant mako sharks recorded, the number of pups
found ranged between 4 and 28, with an average of
12.5, while the average weight at birth was about 4kg.
In contrast, while the number of tiger shark pups
produced per litter varies considerably, average
numbers appear to range between 30 and 50, with one
litter of over 80 noted. Tiger sharks are born at 50 to
75cm long, which is quite small for live-bearing sharks.
Relatively few pregnant white sharks have been
examined, with numbers of pups ranging from 2 to 14,
although the counts at the lower end of this range are
thought to be possibly due to spontaneous abortion of
most of the pups during the act of capture. Detailed
inspection of a 5.15m pregnant white shark caught in
Japan revealed eight full-term embryos, ranging from
135 to 151cm long and from 21.3 to 32.4kg in weight,
easily the largest babies of any fish.MORTALITY VARIATIONS
Ever since that life-changing trip to study black marlin
reproduction all those years ago, I have never ceased
to be amazed at how prolific all of the tunas, billfishes
and other gamefish and baitfish really are. With eggs
from spawning aggregations of fish numbering in the
billions, even seemingly small changes in mortality
rates of larvae may be manifested as large increases
or decreases in numbers of fish over time, even
without the intervention of humans.
At the same time, our knowledge of the life cycles
of these fish can and does improve information used
in determining ideal catch rates, hopefully to ensure
that extraction by fishing doesn’t exceed natural
production of future generations of pelagic fishes.Prior to coupling, the
mating ritual of mako
sharks can get very
rough – as shown
by the fresh bite
wounds on the back
of this large female
caught in southern
NSW.“The number of larvae produced by just one female black
marlin in a single spawning season could be in the billions.”
bluewatermag.com.au 77Next Generation