Firstly, PSAT tags are designed to break free and
float to the surface if their depth has not changed
for several days (this duration can be pre-set).
Therefore, if a tagged fish dies and sinks to the
bottom in reasonably shallow water, or if it dies and
floats, the tag will release after the pre-set time
and start transmitting, revealing the fate of the fish.
Alternatively, if the tagged fish dies and sinks in very
deep water, the tag will automatically release if it
reaches a pre-determined depth below which it is
fairly certain the species never attains in normal life
- say 1200 metres or so for most species. This fail-
safe setting is designed to prevent the tag from being
crushed should the fish sink to extreme depths, but
also helps reveal when a mortality has occurred.
Lastly, if the tag is swallowed by a predator, and
importantly, later regurgitated (or maybe even passed
through the other end), and floats to the surface,
the data will show this in several ways. One is by
recording an extended period of permanent darkness,
since these tags normally detect day and night. The
temperature of the tag also provides another clue as
the stomachs of all large fish are at least a little, and
sometimes a lot warmer than the surrounding water,
especially at depth. Thus, if the data shows that
the light suddenly disappeared and the surrounding
temperature was warmer than it should have been at
a given depth, then it can be assumed that the tag
must have been inside another animal.
A further clue is that predators, especially sharks,
will not show the same patterns of daily ascending
and descending of a tuna or a marlin. Therefore, the
depth profile is a further indication that the tag was no
longer on the released fish and was instead most likely
inside a predator. The only problem with this scenario
is that it might be possible for a predator to bite the
tag off and swallow it without actually damaging or
killing the fish bearing the tag.
If the tag fails to report at all then there is no way
of knowing what really happened. Some researchers
have interpreted non-reporting tags as mortalities,
leaning towards them being explained as predation
events in which the tag was swallowed and either
damaged or never regurgitated. However, equally
plausible in such cases is tag failure of some kind.
Bio-fouling (marine growth) may also prevent the tag
from releasing, or weigh it down so it is unable to
reach the surface to transmit data.STUDIES TO MEASURE SURVIVAL
At around $5000 each, it is simply not practical for
PSAT tags to be used in a controlled experiment
specifically to estimate mortality of fish released in any
condition. Although some mortalities of PSAT taggedTracking tags have
now proven that
even exhausted
marlin that are
upside down at the
boat can survive if
revived and sent off
swimming upright,
like this enormous
blue caught by Trish
Simpson off Kona,
Hawaii.“A blue
marlin
that was
completely
exhausted
when tagged,
survived the
experience.”
bluewatermag.com.au 53Life after release