Science topics
Tuna biology - Naomi Clear
The Scientist
I am a fisheries scientist working in a group of about 20 CSIRO
scientists who study tunas and billfishes. My work focuses on the
biology and ecology of these fish and specifically it contributes
to our understanding of the age and growth, life history, movements
and behaviour of tuna and billfish stocks caught around Australia.
I spend time working at sea each year on commercial and scientific
fishing vessels. During these trips I collect data, take samples
and tag and release hundreds of fish. I also spend time in our laboratories
examining biological samples and I analyse biological data and interpret
the results. My work also involves communicating my science to other
scientists, government funding bodies and the fishing industry;
this can be in the form of oral presentations, written reports or
scientific papers.
Naomi's collaboration is with artist John
Lendis.
The Research
Tunas and billfishes are migratory, open-ocean fishes (known as
pelagic species). The species studied by CSIRO scientists include
southern bluefin tuna (see image 1), yellowfin tuna, bigeye tuna,
broadbill swordfish and the marlins. They are particularly interesting
fish to study because of special adaptations that make them extremely
efficient swimmers and predators.
Southern bluefin tuna (SBT) are caught in the southern Pacific,
Atlantic and Indian Oceans. The fish is prized as sashimi and attains
high prices on the Japanese markets. To manage the SBT stock a worldwide
quota is set, which is divided between different fishing nations
(see image 2).
Part of the research conducted by CSIRO involves assessing the
size of the SBT stock. Population assessments are made using information
from reported catches and monitoring through markets with the aim
to create a statistical model of the stock. The models estimate
how many fish of each age group are being caught, and conversely
how many are left. There has been a decline in the SBT stock since
fishing began in the 1950s so our work involves making recommendations
about sustainable catch rates that will allow the stock to recover.
The stocks of SBT have declined since fishing began in the 1950s
Estimates of the stock size using various methods don’t produce
the same answers. This is because of uncertainties in the models
so CSIRO research includes specific studies to answer questions
raised by these uncertainties. The more the uncertainties are reduced,
the more confident scientists can be with prediction of how the
stock has declined and likelihood of its recovery. Two of the uncertainties
in the SBT population models are the estimation of ages of fish
and fish mortality (how many fish from each age group die each year).
Age and Growth
Knowing the ages of a representative sample of the stock can reduce
some of the uncertainties. Ages of individual SBT can be determined
directly using the ear bones, called otoliths. In SBT otolith growth
varies seasonally like the growth rings in a tree and by counting
these rings under a microscope the age of the fish can be determined
(see image 3).
Tagging
The number of tagged fish recovered from tag-and-release programs
indicates how many die due to fishing and natural mortality. CSIRO
has tagged over 130,000 SBT; about 10% of these have been re-caught
and the tags recovered. Tagging studies also provide valuable information
about where fish travel and how fast they grow (see image 4).
The traditionally used "spaghetti" tags can only provide
information about the fish at the time of release and time of recapture.
Archival tags are data-storage tags that provide extra information
on fish movement and behaviour between the time of release and recapture
(see image 5).
For more information on archival tags and other research conducted
by CSIRO on tunas and billfishes, visit the pelagic
ecosystems web site.
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