OPPORTUNISTIC FEEDERS
As you will have noted, while pelagic fish mainly eat
fish and squid, the study also highlights the variable
nature of predatory fish preferences in space and time.
Scientists often describe predatory fish as being
‘opportunistic’, which simply means that they take
the opportunity to eat a wide range of prey species,
depending on where and when they find themselves.
However, this doesn’t necessarily mean that fish will
eat everything that passes by, including that tasty-
looking lure. In fact, it is more likely that predators
will tend to focus on the most abundant food source
in an area. The reason for this is that hunting different
prey species requires different strategies and it is much
more energy efficient to use one strategy at a time.
While this study did a remarkable job in analysing the
food web of the open ocean off eastern Australia, it is
highly likely that results would be different if the same
species of fish were caught closer to the coast or in other
oceanic systems. Again, this would further confirm the
opportunistic nature of feeding in pelagic fish and also
likely show that each species’ behaviour and preferences
maintains ecological separation, as found here.
WHAT THEY FREQUENTLY EAT
Stomach content studies show what a fish has eaten
in the past day (or even less), but only if the species
among the slush can be identified. However, there is
another method that can indicate fairly broadly what a
fish has been eating in recent weeks or months, even
if it has an empty stomach. What’s more, this method
doesn’t only rely on access to dead fish, but can also
be successfully used on tissue samples taken from live
fish prior to release.
Known as the stable isotope method, it involves
detailed chemical analysis of tissue – usually muscle
- to measure the levels and ratios of specific isotopes
of carbon, nitrogen, sulphur and other elements.
The isotopes from food items are incorporated into
the muscle of the predator, thus reflecting the prey
species that it has been consuming since each species
tends to have a specific ‘signature’ ratio. In practice,
the method isn’t very good at detecting individual
prey species, although it is able to provide a very
good idea of the main sources of food and where in
the marine food-web a fish is placed.
I have recently been involved in two such studies,
one on tiger sharks and one on billfish. The tiger shark
study compared dietary levels of sharks from Ningaloo
and Shark Bay in Western Australia with sharks from
inside the Great Barrier Reef (GBR) and deep water
off southern Queensland and New South Wales (the
tiger shark samples from NSW came from the routine
sampling at gamefishing tournaments).ADAPTING DIET TO HABITAT
Without going into too much detail, analysis of carbon
and nitrogen isotopes showed that tiger sharks from
Shark Bay and the GBR reflected seagrass habitats. It
also showed that they were on the top of food-webs
in those habitats, whereas the isotopes in deep-water
tiger sharks off Queensland and NSW indicated diets
that relied on pelagic food-webs, but also showed
that they were not at the top of the food-web in that
environment. That is not surprising as it is known that
tiger sharks are quite generalist feeders, although
the technique did show that the species is highly
adaptable depending on habitat.
The billfish study has not yet been published, but
analysing similar isotopes from tissue samples mainly
collected at tournaments showed there was a high
level of ‘trophic overlap’ among blue, black and
striped marlin, as well as swordfish. The proportions
of fish and squid varied characteristically among each
species, with differences also noted between juvenile
and adult black and striped marlin.MATCH THE HATCH
Since the days of Isaac Walton, anglers have tended to
follow the old maxim of matching the hatch. In other
words, always present the quarry with an imitation
of the most abundant natural food at the time. Wise
words, but in many situations this presents a dilemma
since you first have to catch a fish in order to open its
stomach to see what it has been feeding on to know
what to use to catch the fish.
Perhaps in the future someone will invent a portable
machine that will instantly analyse the stable isotopes
in a finclip of a released fish to help inform the
educated angler what bait or lure to use. But hang on,
you’d still have to catch the fish in order to take the
sample... Aah, the joys of fishing.Left: These
copepods and a
small triggerfish
were taken from the
stomach of a bigeye
tuna, demonstrating
that at times these
large gamefish will
feed on tiny prey.
Centre left: When
brought alongside
off Kona, Hawaii,
Trish Simpson’s
400kg blue marlin
regurgitated this
sizeable and partly
digested tuna.“Predators
will tend
to focus on
the most
abundant
food source
in an area.”
bluewatermag.com.au 39Diets of pelagic predators