Southern Bluefin Tuna

Why

Some things are just best served cold, revenge, beer and tuna. Most fish species are ectotherms, this means the temperature of the water regulates where they can inhabit or swim.

However, there are a small number of fish that are known as endotherms and they have the ability to control their internal body temperature, meaning they can swim across a wide range of water temperatures.

These species are usually highly migratory and include tuna, some sharks and billfish. Because of this unique ability to elevate their body temperature, it means as fishers we need to be conscious when catching and dispatching tuna that their bodies continue to warm.

• Making sure anything touching the fish is wet, whether this is the net, your gloves or bare hands it will reduce the chance of removing slime from the fishes skin. This slime protects the fish from bacteria.
• Using a knotless landing net to support the fishes body weight
• Cradling the fish when holding rather than grabbing them by the tail, will prevent spinal damage.
• Resting them on a wet foam matt to prevent damage to the meat
• A wet towel over the eyes to minimise stress

Ike Jime Process

Brain spike: Spiking is a quick and humane way of dispatching a fish and the first step to the Ike Jime process.

Some things to keep in mind:

• You’re aiming for the soft circle in between the eyes and the spike should enter the center of this circle and be inserted until the end of the spike reaches the same level as the eyes.
• Move the spike once it has reached eye level to ensure you destroy the cranial cavity.
• Avoid using a knife as your hand may slip which could cause a bad injury.
• A screwdriver will work as a brain spike if you do not have the correct tool.

Bleeding

Bleeding a fish will ensure the meat has flavour and isn’t to rich as well as moving unnecessary mass, meaning a quicker cooling process.

How to do this:

• Make a cut behind the pectoral fins on both sides.
• Make sure the incision is a maximum of 2-3cms deep, any deeper than this and the blood will run back into the meat, undoing the bleeding process.
• This is because the artery is directly under the surface of the skin.
• Pressure bleeding is another option, however if you are not familiar with the process this should be avoided as you can flood the meat with saltwater.

Pithing

Even after a brain spike a tunas nerves will continue to twitch casuing the muscles to react and generate heat. To prevent the muscles from creating heat, running a wire down the spinal column will destroy the spinal cord, stopping these reactions and heat production.

How to do this:

• Using a knife or a coring tool, cut a wedge out around the spiking entrance.
• This will open up the the entry point to the spinal cord.
• Run a thick cord, or wire down the spinal cord to the end of the tail to deactivate the nerves.
  Tip: you will see the fish twitch as the cord is passing through the column, when it reaches the end, the meat will relax and soften.

Gut and Gill

Gutting and gilling not only removes the components wihtin a fish that have the cpaacity to cause spoilage, but it also removes mass from the fish allowing the cooling process to be moree effective.

How to do this:

• Traditional method of making an incision along the centre of the stomach and removing the guts from the cavity and cleaning it with saltwater. Then remove the gills by lifting the plate and cutting them away from the head.
• To keep a fish whole you can actually make a small circle incision around the intestine and cut this away from the skin, than gill using the traditional methods and pull the gills. As you pull the gills the guts will come out as one piece with the gills leaving a whole gutted and gilled fish.

Chilling

When we say tuna heat, they heat so intensely they actually will actually cook from the inside out, which is called burnt tuna syndrome and results in a brown flesh.

A study conducted in 2017 looked at the body heat of tuna and the time it takes to reduce their internal body temperature after a fight.

To do this, a 35kg Southern Bluefin Tuna was caught using a rod and reel in 16°C water and the internal body temperature was measured immediately and was 27°C.

The fish was then placed in a saltwater ice slurry for 6 hours.

Go on take your guess, you think we got the core temperature to 0? Guess again! After being in a -1°C slurry the fishes core temperature was still 13°C. This fish was only 35kg in size, imagine chilling a 100kg barrel!

Things to keep in mind when chilling fish:

• Let the fish cool and chill for a 48 hour period before processing. This will allow the proteins to set in the meat.
• A saltwater slurry should use the ratio of 1kg of ice to 1kg of saltwater.

How much ice do I need?

• The rough rule is 1kg of ice to 1kg of meat, I know right now you’re thinking how in the world am I meant to carry 100kgs of ice onboard. Well, you’re not.
• The common misconception is that you come in from your days fishing and begin the filleting process. But, to get the best quality meat leaving your fish chilling for up to 48 hours allows the meat to set, making it easier to fillet and also drops the core temperature to near 0°C.
• This is where the 1:1 rule with ice comes into play, as that is the amount of ice required over the 48-hour period to be continually added to the slurry, as the fish will melt the ice through body heat.
• Or, when on land, befriend a mate who has a cool room! 1:1 is also the ratio of ice to saltwater when making a slurry, this will maximise the chilling potential of the slurry.

Use our calculator below to work out how much ice you need for your fish: