for yellowfin tuna and striped marlin (about 15%
empty) and most for albacore, makos and swordfish.
In fact, close to one-third of the swordfish stomachs
contained no food. This could be due to some species
regurgitating the contents of their stomachs while
hooked, but probably also related to the rate of
digestion of these high-energy fish.
In order to compare diets of juvenile and adult
fish, the analyses were based on fish measuring less
than or greater than 100cm in length. However, as
anglers are mainly interested in larger fish, I will
focus here on the data for fish over 100cm. Also note
that diets were reported in terms of wet weight of
items in the stomachs. In other words, the stomach
contents would be removed and weighed before being
separated into identifiable groups, each of which
would also be weighed in turn.
MAJOR PREY TYPES
Looking first at the major groups of prey for each
predatory species (see the series of charts), some
major differences were apparent. For example, fish
very much dominated the diet of yellowfin tuna, striped
marlin, dolphinfish, and especially mako sharks.
In contrast, squid were by far the dominant prey
items of swordfish, blue sharks and albacore. Fish also
constituted more than 50% of the diet of lancetfish,
southern bluefin and bigeye tuna, with squid also an
important component.
Crustaceans (small items such as crab larvae and
squillas) were present in small quantities in most
species, with yellowfin tuna consuming the highest
amounts (2.85% of their total diet).
MAIN PREY FAMILIES
Considering the dominant prey groups across all the
predators, the trevally, tuna and flying squid families
were the main groups eaten by most predators. The
exceptions to this include lancetfish, albacore and
dolphinfish, each of which focused on a range of prey
in other families.
The main member of the trevally family found in
many stomachs was the Jack mackerel (cowanyoung),
especially in southern bluefin tuna, but also significant
in striped marlin, yellowfin tuna and blue shark. Tuna
species constituted another significant component in
the diet of many predators, particularly mako sharks,
striped marlin and yellowfin tuna.
One problem of the study was determining exactly
which species of tuna were the primary targets, due
to rapid digestion. For example, members of the tuna
family made up over 90% of the diet of mako sharks,
of which only about 20% could be identified to species
level – these being yellowfin tuna. In the case of
striped marlin, skipjack tuna was the main identifiable
prey species, although the stomachs also contained a
lot more unidentified tuna remains.
One family of fish that formed a significant part of
the diet of at least some predators was the pomfret
family, Bramidae. Ray’s bream was the most common
of these, constituting one-third of the diet of blue
sharks and nearly 20% of the diet of larger southern
bluefin tuna.
Many other families of fish were found in the
stomachs of most of the predators. Although these
did not usually represent large volumes of the
diet by proportion, it still indicated that they had
been targeted and eaten with some degree of
frequency. Such groups of prey and the predators
that mainly fed on them (in brackets) included:
flying fish (dolphinfish), sauries (yellowfin tuna,
dolphinfish), small swordfish (dolphinfish, striped
marlin), leatherjackets (dolphinfish, striped marlin)
and pufferfish (dolphinfish, bigeye tuna, striped
marlin, yellowfin tuna).
Similarly, while the dominant squid species in all
predators’ diets were from either the flying squid or
giant squid families, many other families of squid and
pelagic octopuses were also represented in the diets
of a large number of predators. However, these were
mostly in smaller proportions by weight due to their
generally smaller body sizes.
Lastly, cannibalism was found to be rare in all species
except for lancetfish, in which their own kind made up
around one-third of their total diet.Above left: Gamefish
can be very selective
and will focus on a
single prey species
if one is abundant,
but generally they
are opportunistic,
as demonstrated
by these stomach
contents from a
15kg dolphinfish
caught in 80m off
Port Stephens, NSW.
The mix of prey
included a juvenile
dolphinfish, trevally,
octopus, pufferfish,
leatherjacket and a
nautilus with its shell.
Above centre:
Squid, particularly
species of flying
squid, were shown
to be an important
prey type for many of
the pelagic predators
studied in the survey
aboard Australia’s
east coast longline
fleet.Lancetfish are an
oceanic mid-water
predator.“Fish
very much
dominated
the diet of
yellowfin
tuna, striped
marlin,
dolphinfish,
and especially
mako sharks.”
bluewatermag.com.au 37Diets of pelagic predators