bleeding from the gills apparently died, 63% of deep
hookedfish died and 9% of fish deemed to be in good
condition died. However, this study is somewhat of an
outlier anomaly in the literature on billfish survival,
in that the pressure release devices for the tags –
glassimplosion links designed to crush at depths of
either350 or 1500 metres – were only ever used in
thissingle study. This caused some doubts regarding
some of the tags that came to the surface having
beeninterpreted as mortalities. These tags may have
released while fish were simply undergoing normal
divingbehaviour to depths greater than 350 metres,
ratherthan the result of dead fish sinking.
There have also been equivocal results from a
large-scale black marlin tagging exercise off the
GreatBarrier Reef in which 67 fish were tagged with
PSATs between 2002 and 2009. Data was received
from42 tags, from which a post-release mortality of
about 25% was surmised. This figure included some
non-reporting tags that were assumed to be shark-
relatedsince a significant proportion of the confirmed
mortalities were clearly caused by shark predation on
fishimmediately after release. Subsequent re-analysis
of theresults from this work indicated that 12% (15 of
53)of black marlin had died soon after release, eight
of which had been predated, presumably by sharks.LANDMARK META-ANALYSIS STUDY
Manyother experiments using acoustic and PSAT tags
on billfish for various reasons have been conducted
around the world. However, due to high tag costs
mosthave only been able to use quite small samplesizes of less than 10 fish, thus extrapolating survival
rates can be statistically problematic. Therefore, in
an effort to apply more robust statistical methods
to these studies overall, an international group of
billfish scientists undertook what is known as a
meta-analysis of the combined data. This takes into
account many important variables, including predator-
related mortalities, where short-term predations are
attributed to angling-related causes, but longer-term
ones are assumed to be natural mortalities.
In this landmark study, data on post-release survival
was derived from 46 separate electronic tagging
studies, across which six billfish species had been
tagged: black marlin, blue marlin, longbill spearfish,
sailfish, striped marlin and white marlin. The data
came from 400 pop-up satellite tags and 64 acoustic
tracking tags that had been deployed on fish tagged
across many different regions. The main fishing gear
used in the studies was rod and reel (32 studies,
82% of tags), commercial longline (11 studies, 11%
of tags), and even harpoon to place tags on free-
swimming fish (3 studies, 7% of tags). The overall
results can be summarised as follows:
Not surprisingly, post-release mortality varied
considerably among all these studies. In fact, 25
studies, representing 33% of all tags, indicated zero
mortality. However, overall, the estimated mortality
across all six billfish species was 14%. Fish that died
did so within a few days of release (usually within a
day), with mortalities due to sharks also contributing
to this figure. While care should be taken in breaking
the numbers down for each species, nevertheless“Fight
intensity may
affect stress
levels more
than fight
duration.”
With pop-up satellite
tags costing around
$5000 each, most
tagging studies
involve a small
sample. However,
analysis of 46
separate electronic
tagging studies,
involving six species
of billfish, showed
that around 90%
of sailfish survived
release – higher still
when caught with
non-offset circle
hooks or lures.56 facebook.com/BlueWatermagazine
Life after release