An increase in lactic acid in the muscles has also
been shown to increase lipid oxidation, so get your
tuna in quickly, kill it immediately and then bleed it.
CORING AND PITHING
Another process recommended to improve the eating
quality of your tuna is destroying its spinal cord. This
prevents further lactic acid and heat build-up, and
minimises bruising.
Even when the tuna is brain dead, nerves in its spinal
cord can still work and generate muscle activity. You
may have noticed that even after a fish has been
killed it still does some violent shaking as its muscles
spasm. This muscle activity generates heat and lactic
acid, and the fish’s movements can bruise its muscles
against the deck, so to prevent this we do something
known as pithing.
We first create a hole in the skull – termed ‘coring’
- using a special tool like an apple corer or cookie-
cutter. The hole should be made at exactly the same
spot the tuna was spiked, twisting the tool to cut a
round hole through the top of the tuna’s head. The
tool and the plug of flesh inside – like an apple core –
is then removed, creating an easy access hole down
through the brain to the spine.
A flexible wire cable (or heavy mono leader) is then
passed down through the hole in the head, through the
brain and into the open end of the backbone. It is then
pushed right down inside the tuna’s spine almost to
the tail, totally destroying the nerves in its spinal cord.
Once the spinal cord is destroyed, the tuna’s muscles
totally relax and all twitching stops.
USE AN ICE SLURRY
The final step is chilling your tuna, and if you are
serious about great tasting meat then you use an ice
slurry, which is a mixture of ice and water. This has
been scientifically proven to cool fish very much faster
than ice alone.
When a fish is immersed in a slurry its heat is
drawn out all over, whereas using ice alone means
the effective chilling is limited to only where the ice
makes contact.
To make a slurry I generally use freshwater ice mixed
with saltwater. There should always be ice floatingin
the slurry, so whenever the ice has all melted you
must add some more.
Remember that when you make a slush ice using
seawater, the ice needs to cool down the seawateras
well as the fish. If you’re fishing in a cool climate with
a water temperature less than 18°C, to effectively
chill a tuna of more than 100kg I suggest having
at least 60kg of ice on board to initiate the cooling
process until you get back to port. If you’re fishingin
the tropics you will need a lot more ice than this,as
well as an insulated vinyl fish bag to cool and store
your fish in.
My Whittley 6.5m Sea Legend has an excellent
fibreglass chill tank beneath the deck which works
very well with a saltwater ice slurry to hold smaller
tuna. However, the underfloor fishboxes found in
many aluminium boats are not insulated and so donot
hold a chill well, the metal transferring a lot of heatin
from the outside water.COOL FROM WITHIN
To effectively chill the tuna’s internal muscle – where
a lot of the heat is – you must open its gut cavity.
Therefore, gill and gut your tuna first before puttingit
in the ice slurry. This is a very quick process if you know
how, which you can watch on video at my website:
http://panaquatic.com/projects/fishing-for-tuna.Below (L-R):
To prevent the
tuna’s muscles
twitching after death,
which continues to
generate heat and
lactic acid, the tuna’s
central nervous
system is deactivated
by ‘pithing’ its spine.
To do this, a hollow
coring tool cuts an
access hole to the
brain and spine.
Then a flexible
cable or length of
heavy leader mono
is inserted and run
down inside the
spine, almost to
the tail.To maintain prime
eating quality,
the tuna’s natural
body heat must be
removed as quickly
as possible after
death. This is best
done by immersing
the tuna in a slurry
made from lots of ice
mixed with seawater.
The tuna must be
first cleaned, so the
iced water has easy
access to the core
of the fish where
the most heat is
contained. As ice
melts, it must be
topped up.bluewatermag.com.au 77From boatside to plate