Thus to preserve the fish meat quality, one would have to land the fish quickly and prevent it from struggling too much. When gaffing fishes for food, always aim for the head or the lower jaw so as to prevent damage to the flesh or contamination. Also keep the fishes in a single layer.
Stunning.
Stun the fish immediately after it comes on deck to eliminate scale loss and bruising. It is easier to stun the fish when it is still on the gaff or the hook remover. Club the fish with a modified bat, mallet, or lead filled steel pipe on the soft spot right above the eyes.
Brain Spiking (optional).
Another option is to immobilize the fish immediately after, or instead of stunning, by destroying the brain. (Spiking the brain is a required procedure for production of sashimi grade tuna worldwide.) In addition to immobilizing the fish, brain destruction helps stop the production of heat and acid, and the loss of energy rich compounds.
To spike a fish, position yourself so that you are balanced with the fish positioned on its belly, and the spiking tool (an ice pick or a sharpened screwdriver) firmly in one hand and the fish's tail in the other. Do not attempt spiking without complete control of your balance, the spiking tool, and the fish. Place the spike at the soft spot above the eyes at a 30 degree angle to the horizontal. Push the spike quickly into the skull maintaining the 30 degree angle while holding the tail with your other hand. Move the instrument from side to side to destroy the brain. The fish will shudder, all the muscles will flex, the mouth will open, and the pectoral fins will flare. After one or two seconds the fish will go limp if done properly.
If not done properly, the fish can shudder violently creating the potential for personal injury, and further scale loss and bruising to the fish. With slippery conditions this procedure can be a challenge, but after several attempts brain destruction is swift and thorough.
Bleeding.
Bleeding improves the appearance of uncooked tuna loins and may help initially to reduce fish temperature on deck. It is essential for sashimi grade fish. Fish should be bled for 10 to 15 minutes after stunning (brain spiking) and then immediately chilled. Bleeding is most efficient when done immediately after the fish is landed, and when the heart is left intact to take asvantage of its pumping action. If possible, orient the fish head down and spray with water to prevent clotting. Bleeding can be accomplished in at least three ways; use the one easiest for you. If one cut does not produce blood, try one of the other methods. Using more than one cut may promote more efficient bleeding.
Pectoral Cut:
This cut is the most common in tuna fisheries worldwide. With the fish on its side measure 1.5 to 2 inches (about the width of two fingers) from the base of (under) the pectoral fin along the midline. Make a shallow cut about 1/8 to 1/4 inch wide and 1/4 inch deep along the raised ridge near the midline using a clean, sharp knife with a narrow blade. If this cut is made too deep or too wide, usable flesh can be destroyed and reduce the fish's value. Flip the fish over and repeat the cut on the other side. We found this cut to be very effective with albacore.
The gill arch cut is the one most commonly used. With the fish on its side, lift the gill cover and sever the gill arch and/or insert the knife behind the gill through the gill membrane, and cut up toward the spine, severing the blood vessels at the top of the gills.
This cut involves cutting the blood vessel between the heart and the gills. It can be done in either of two ways, depending on the preference of the buyer. With the fish on its back or side cut the "V" shaped nape between the gill covers and the body of the fish to the artery just below the surface. An alternative that leaves the head firmly attached to the body is to make a shallow cut just inside the point of the "V" of the nape, lift the artery with your finger, and cut. The heart is about three inches behind or inside the point of the "V". Take care not to sever the heart or you will loose the pumping action the heart provides. We recommend finding the heart and the artery running between the heart and the gills in your first fish or two, and find what is easiest for you.
Chilling fishes
Chill fish first in slush ice, a mixture of two parts ice and one part seawater before icing. Fishes will cool four to five times faster in slush ice than on ice because the fish are completely surrounded by the chilling medium, and the rate of heat transfer in a liquid is about 25 times faster than in ice. The slush ice tank should be insulated, with a tight fitting, insulated lid, and maintained so that ice is always present. This will require that ice be added periodically depending on catch rates, weather conditions, and the extent of insulation. Add fish to the slush ice tank as they are landed - not all at once. In addition to a substantial loss of shelf life and quality that result from fish left sitting on the deck, slow, inefficient cooling will result from overloading the chilling system.
A mixture of 2 parts ice and one part seawater will maintain a temperature of 0°C. Because fish begin to freeze at temperatures below this, there is no risk of partial freezing and related quality loss.
Ice and freshwater will tend to float on the surface of the slush ice mixture, creating sharp temperature differences, even in a shallow tank. Warm temperature pockets can also become trapped between fish. Agitate the mixture periodically with a pump, by bubbling air in the tank, or manually with a paddle to minimize these potential problems.
Transfer the fish to ice storage when the backbone temperature reaches 10°C and continue to cool the fish to 4°C or below. Holding fish in slush ice for more than 12 hours may lead to excessive water absorption and bleaching. Proper management of the chilling system requires that the internal temperature of the fish be measured periodically throughout the fishing trip. This can be done simply by using a piercing probe meat thermometer, available through most refrigeration equipment suppliers. There are also a number of digital, battery operated, piercing probe thermometers available on the market. When measuring the temperature place the probe close to the backbone on one side of the fish about two inches behind the pectoral fin (the thickest part). After some experimentation, the proper chilling times can be determined for certain slush ice mixtures, fish sizes, loading density, etc.
How you ice your fish after they are removed from the slush tank varies by vessel. If possible, ice fish in single layers in each available bin. Before adding a new layer of fish, gently pack down the ice with a shovel to eliminate any air pockets that may have formed. Cover each bin with an ice blanket. The amount of ice required to chill fresh tuna will vary with a number of factors such as the length of the trip, catch rate, and the extent of insulation in the fish hold and the slush ice tank. In general, proper chilling will require about two pounds of ice per pound of tuna. Each operator must determine the amount of ice they require based on vessel characteristics and fishing pattern.
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