fishwaspredateduponbya shark,whichwefigured
outprettyquicklybytheoddbehaviourof thetracked
fish,despiteseeingnoindicationof anattackonthe
surface.REMARKABLERECOVERY
On the other hand, we also had an early result
suggesting that marlinare actuallyquite robust fish.
Forexample,oneof thefishwetagged,estimatedat
around440kg, hadbeen tail-wrappedfor 30 minutes
beforeit couldbebroughtto theboat.Ontheleaderit
cameto thesurfacebellyupandprettywellmotionless.
Withonlyoneremainingacoustictagonboard,I made
thecallto tagthefishandseewhathappened.It waslaterdiscoveredthatthefishin factsurvived.
Having descendedtoa depth of 140 metres, it held
itself in the current for two hours before resuming
normal behaviour patterns over the following 24
hours.Aroundthesametime,myfriendandcolleague,
DrKimHolland,whowastrackingrod-and-reelcaught
blue marlin in Hawaii with acoustic tags, related a
verysimilarstoryto meof a fishthatwascompletely
exhaustedwhentagged,butwhichalsosurvivedthe
experience.Sofar,sogood.
Theseearlystudieswereprimarilyaimedat revealing
theverticalswimmingbehaviourof healthyfish,which
tendedtomeanthatgenerally– althoughnotalways- only fish considered tobe in good conditionwere
tagged.Nevertheless,asshown,theseearlyexercises
gaveusinformationonsurvivalof at leasthealthyfish
afterrelease.
Thoseearlysignswereverypromising,in thatmost
billfish being tagged did survive, contrary to what
some were assuming based on very low recapture
ratesof billfish fromconventionalplastictags.Atthe
same time, these experimentsgave indications that
unseenpredationbysharksafterreleasewascertainly
a factorin survivalof releasedfish.However,thereal
breakthrough in seriously measuring post-release
survival ofgamefishwas thedevelopmentof pop-up
archivalsatellitetags– orPSATsforshort– thatcan
beprogrammedto revealthefateof fishafterrelease.
SATELLITE TAGBREAKTHROUGH
Pop-up satellite tagswere notoriginally designedto
revealwhetherornottaggedfishdied afterrelease,
but their use for that purpose was realised and
improved through time. PSAT tags themselves are
essentially mini data-logging computers that record
temperature,depthandlightlevelsat regularintervals
whileattachedto a fish.Thisinformationis storedand
thenrelayedviasatelliteoncethetagdetachesitself
fromthefishandcomesto thesurface.Howthencan
thistellusif thefishsurvivedandresumeditsnormal
activities,oralternatively,if it died?“The real breakthrough in measuring
post-release survival of gamefish was the
development of pop-up archival satellite tags.”
Dr Julian Pepperell
preparing a pop-
up satellite tag to
be deployed on a
billfish. This is an
earlier model ‘Mark
10’ PSAT tag which
was used on many
studies around
the world.A next-generation ‘MiniPAT’ pop-up satellite tag made by Wildlife
Computers, which superseded the Mark 10 tag. It is smaller
and more compact and can store more data recorded at more
frequent intervals than the earlier models, thereby revealing the
movements of the fish in far greater detail.52 facebook.com/BlueWatermagazine
Life after release