marlin do not possess any gill rakers at all (comb-like
projections in front of the gills that can filter small
food items). Therefore, the only conclusion possible
was that the marlin must have been actively picking
these tiny morsels one-by-one from the water column.
As studies became more sophisticated, gut analysis
expanded to more than one species at a time and
became known as the study of ‘feeding ecology’, or
more accurately ‘trophic ecology’. A relatively recent
major study by CSIRO off eastern Australia, covering
multiple pelagic predatory fishes, has certainly lived
up to the name of a true trophic ecological study – and
what a study it was.GROUND-BREAKING STUDY
In a nutshell, the aims of this large and ambitious
project were to better understand how predators
share the pelagic ecosystem, and to look for groups
of species that were associated together as ‘trophic
assemblages’. The study also examined differences in
feeding times and places, and even aimed to estimate
how much food was consumed on a daily basis by
individuals of each of these species.
Field work for the study spanned the period from 1992
to 2006. Paid observers on Australian longline vessels
fishing off eastern Australia collected stomachs from
a cross section of their routine catches. The stomachs
were frozen at sea and then thawed back in the lab
so that long-suffering technicians could sort through
the mush and record what the fish had been feeding
on. Fish were caught over a huge area, ranging from
southern Tasmania to the Coral Sea, with the highest
concentrations of activity being recorded off Tasmania,
New South Wales and southern Queensland, extending
well beyond the Australian 200-mile zone.
During this epic study – which took a further four
years to analyse and publish – 3562 stomachs were
collected from 10 species of pelagic fish. These are
shown here with the number of stomachs inspected in
brackets: Southern bluefin tuna (1363), yellowfin tuna
(763), swordfish (642), bigeye tuna (184), lancetfish
(149), blue shark (147), dolphinfish (118), striped
marlin (99), albacore (71) and shortfin mako shark
(26). Because Australian commercial fishermen must
release all blue and black marlin, those species could
not be sampled.STOMACH CONTENTS
One initial interesting finding was that only about 80%
of all of the stomachs examined contained any food
at all, while conversely, 20% were completely empty.
However, that ratio was not the same for all predatory
species. Looking at the bar graph included, we can
see that the percentage of empty stomachs was leastA scientist sampling marlin stomach contents
at a gamefishing tournament in NSW.“In Hawaii, examination of blue marlin revealed
they had fed on tiny boxfish, triggerfish, squid,
prawn killers and crab larvae.”
Prey species found in east-coast predators36 facebook.com/BlueWatermagazine
Diets of pelagic predators