eggs a female could produce in a given population,
divided by the number of eggs it could produce over
its lifetime in an unfished population.
The figure is based on fish size, which in a general
sense correlates with egg production. A five-year-old,
50-centimetre-long female, for instance, can produce
around 300,000 eggs. If she’s allowed to grow to 60
centimetres (perhaps seven years and older), she can
produce more than a million eggs, so longer-lived
females are essential to maintaining a sustainable
fishing stock.
An SPR ratio of 100%, Filous explains, “means
that this population is producing eggs at a rate that
would be like it’s virgin stock that has never been
touched by fishermen. And 0% means there’s no egg
production.”
The data showed that the average age of the
roundjaw bonefish in the waters around Anaa was
well below the species’ 20-year longevity, with males
surviving to eight years and females to 10. The SPR
metric hovered between 7% and 20%, a finding Filous
published in theJournal of Fish Biologyin 2019. So,
as many Anaans had already noticed, the numbers
weren’t just dwindling, they were unsustainably low.
To address the problem, the Island Initiative
isn’t talking about not fishing, says Bagnis, but about
leaving enough fish to repopulate. “It’s really man
and nature working together.”
The goal is to maintain a threshold above 20%,
ideally 30-40%. “The other way to think about 20%,”
says Filous, “is that means you can kill 80% of the
population’s egg production and still fish.” But it’s a
red alert if you dip below that. “My data supported
that it was pretty overexploited, and something
should be done.”
Filous delivered his findings to Bagnis, who
asked, “If we had the full support of the population,
what do you think should be done?”
As a rule, reef fish search for safe areas to spawn,
Filous explains. The upper section of the atoll is ideal,
because it has a deeper water passageway for themAccording to Filous, that’s because of the worms,
crabs and clams the bonefish feed on, all of which
thrive in the lagoon’s uniquely shallow waters. With
an average depth of just 1.5 to 3.5 metres – compared
with 20 to 30 metres in other atolls – sunlight is able
to freely reach the bottom.
“They’re well fed and well nourished, so they have
a distinct flavour that’s different than other bonefish,
and their flesh is softer,” he explains.
When Bagnis and McHugh formed the Anaa Atoll
Project, they invited Filous to investigate the viability
of a fly-fishing venture. After identifying the bonefish
as the atoll’s most abundant species, and therefore
critical for the islanders’ food security, Filous decided
to study it for his PhD research.
From 2016 to 2018 he tracked and tagged bonefish
to estimate their age, size and growth, reproduction,
fertility and natural mortality. He used underwater
computers that detect acoustic transmissions from
electronic tags within 500 metres. Filous surgically
inserted the tags, about the size of a small phone
battery, each with a unique beeping sound, inside the
body cavities of 40 fish. This enabled him to track and
correlate their movements with spawning and lunar
cycles. He did encounter a slight hitch: at some point
virtually every islander caught one of his tagged fish,
which meant retrieving and cleaning the tag, before
capturing another fish to track.
With the help of local fishermen, he also
attached ID tags to around 2500 other fish to gather
information on growth between captures, the timing
of their capture and the timing of spawning with
seasons and moon cycles.
To estimate the species’ natural longevity, in 2014
they had collected comparison samples from nearby
Teti’aroa, which had no permanent inhabitants – and
hence an unexploited fish population – and a similar
predator profile to Anaa.
From the data collected, Filous calculated a
spawning potential ratio (SPR) to evaluate the stock’s
health. The SPR calculates the theoretical number ofAustraliaNew ZealandFiji AnaaTimor-LesteKiribatiBrunei MalaysiaPhilippines
Federated States of MicronesiaMarshall IslandsNauruTongaSamoaTuvaluOpposite. Marine biologist
Alex Filous has been collecting
data on Anaa atoll (below)
for his PhD. His topic is
“fisheries science to support
conservation of bonefish
(Albula glossodonta)through
traditional community base
management”.GEORDIE TORR
Issue 86 COSMOS – 83SUSTAINABILITY BIOLOGY