Newslines
bluewatermag.com.au 13WHEN SATELLITE-TAGGED
FISH GET EATEN
Marlin are among the largest and fastest
fish in the seas, but they still face the
daily risk of predation. Although it’s a rare
occurrence in the IGFA Great Marlin Race
(IGMR), satellite tags have occasionally
been swallowed by predators.
While these events are very disappointing,
the transmitted tag data often yield good
insight as to the type of animal that preyed
on the marlin.
The data generated in the IGMR pop-up
satellite tags includes time, date, ambient
water temperature, latitude and longitude,
and pressure (used to calculate depth).
The temperature and depth data provide
valuable information on what part of
the water column the fish uses, and
the oceanographic structure of the water
column, while the light level data is used
to estimate the location of the fish and
develop a best-fit track.
DEPTH, LIGHT &
TEMPERATURE
Through analysis of the depth and light
data, it is often very easy to recognize
these predation events. For the most
part, billfish show very consistent diving
behaviour, so a sudden shift in diving
behaviour (shown by depth data) is
a good indicator that the tag is inside
another animal.
Another good indicator is when the
light data disappears. Prolonged periods
of darkness reveal that the tag is in the
stomach of an animal. Eventually, the
predator will pass or regurgitate the tag
and the light data will once again follow
the daily cycle.
While depth and light data are used to
identify predation events, the temperature
recorded from the predator’s stomach
reveals whether it is endothermic (warm-
blooded) or ectothermic (cold-blooded).
When a satellite tag is deployed on
a marlin, the temperature data remains
consistent with depth, so when the marlin
dives to deep water we’ll see the ambient
temperature decrease. However, when a
cold-blooded predator swallows a tag there
is little change in the relationship between
depth and temperature data, although the
temperature data will change a little slower
due to being inside the fish and a process
known as thermal inertia.
Warm-blooded predators, on the other
hand, generate and maintain heat within
their bodies so when one swallows a
tag the temperature data becomes more
independent of the depth data, and shows
less variation in temperature.
WARM-BLOODED SHARKS
Of the 361 pop-up satellite tags deployed
in the IGMR so far, only 20 reports have
been identified as predation events, with
the majority of predations carried out by
endothermic animals.
Although it’s not possible to determine
the exact species, common sense tells us
that in most scenarios the warm-blooded
animal large enough to prey upon alarge marlin would likely be a species
of lamnid shark, like a white or mako.
These advanced sharks maintain a slightly
warm body temperature even in cool
surrounding water.- Leah Baumwell
IGFA Conservation Coordinator
The blue line on this 24-hour graph shows
a 200kg black marlin’s diving behaviour
throughout the day. The changes in light
level are shown in pink.About one week after the 200kg black marlin
was released, the tag’s data shows the marlin
grew less active and took fewer and shallower
dives each day, suggesting that its health was
compromised. As shown at 2pm on this daily
chart, the tag was then swallowed by another
animal, indicated by the disappearance of light
and the changed diving behaviour.The active diving behaviour
and lack of light data continued
until the tag was presumably
regurgitated and floated to
surface, where it then beamed
its stored data up to a satellite.