DIET PER SIZE OF PREDATOR
So do big fish eat big prey? The short answer from
this study was that yes they do – although there were
plenty of exceptions to the rule. The length of prey
compared to the length of the predator that ate them
ranged from 5% for albacore and lancetfish to 15% in
swordfish and blue sharks.
In between, dolphinfish, striped marlin, yellowfin,
bigeye and southern bluefin tuna ate medium-sized
prey – although there was a big spread and all of
these predators also ate a fair amount of smaller prey
items less than 40mm long.
The findings showed that with increasing predator
size, the maximum size of their prey significantly
increased, while the minimum size of prey tended to
stay pretty constant.DAILY RATION
One of the questions I’m often asked is how much does
a marlin/shark/tuna eat each day, and how long does
it take these fish to digest their food? Fortunately, the
CSIRO study was able to estimate the daily ration for
each species of predator – that is, the percentage of
body weight consumed each day.
For most of the predators, this figure came out at around
1 to 2% of body weight per day. The main exceptions
were lancetfish, which ate around 6%. For blue and
mako sharks it was around 9% of their body weight per
day, although the shark data was highly variable. These
figures would translate, for example, to a 100kg striped
marlin eating 1 to 2kg of food per day, but a mako of
similar size consuming around 9kg per day.
Daily rations of 1 to 2% of body weight for mostof the predators are actually less than estimated for
other parts of the world. The authors suggest this
result might be due to the less productive waters
off eastern Australia compared with other areas with
more upwelling of nutrients.
The estimated digestion rates in the study were
remarkably consistent, with all predators taking
between three and five hours to evacuate all food in
their stomachs at any one time, a factor that no doubt
contributed to the high percentage of empty stomachs
for many of the top predators.NICHE SEPARATION
One of the aims of the study was to see if and how
this suite of pelagic predators might be dividing up the
available resources and reducing competition among
the different species.
Using some sophisticated mathematical techniques,
the results indicated that the feeding behaviour and
diets of each species of predator were in some way
separate from each of the other predators. To quote
directly from the paper: “... although striped marlin and
dolphinfish fed mainly during the day, the latter occupied
a shallower depth distribution and fed on smaller prey.
Bigeye tuna had a similar depth distribution to albacore
but fed day and night, whereas albacore fed mainly in
the daytime. Dolphinfish, like albacore, fed mainly on
small prey. However, dolphinfish were mainly caught in
surface waters while albacore were caught in deeper
waters. In fact, all possible comparisons indicated at
least one way in which each predator separated itself,
if not trophically then behaviourally”.A baitball of Jack
mackerel (aka
cowanyoung) off
Port Stephens,
NSW. This fish
is a type of
trevally and is an
important prey
species to many
pelagic predators
off Australia’s
south-east coast.“With
increasing
predator
size, the
maximum size
of their prey
significantly
increased,
while the
minimum size
of prey tended
to stay pretty
constant.”
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Diets of pelagic predators