First, prepare the deck.
1. Prevent fish from jumping: Once the fish is on the deck, it should not be allowed to jump, as this can cause congestion in the body. After bruising, muscle damage can increase by up to five times, especially in the abdominal area. To minimize this, place the fish on a carpet and hold its eyes with your hands, or gently hammer its head without breaking it, then immediately perform bloodletting.
2. Cut the tail: Use a sharp knife to cut between the third and fourth sections of the dorsal fin, typically at the penultimate segment. For larger fish, to avoid the tail becoming too thin, cut between the fourth and fifth sections instead.
3. Remove the posterior blood vessels of the pectoral fin: In the rear of the pectoral fin of tuna, there are two blood vessels—artery and vein. Carefully cut them separately, making a 5 cm deep and 4 cm long incision. The depth must be just right; if cut too deep, blood may seep into the flesh, causing waste. Even if the fish has already died in the water, these steps must still be completed.
4. Cut the head artery before the heart: Cut the artery leading to the heart, then rinse the fish with seawater to prevent body temperature from rising. Next, perform blood drainage by cutting an opening in the gill lining and using a water pipe to inject seawater. Continue until almost no blood remains, checking the tail to determine if it's fully drained.
5. Insert pins at the back of the head: This step should be done after the bleeding process. The goal is to destroy the fish’s brain cartilage and ensure death. Insert the pin into the inner part or depression of the head, or above the eye.
6. Remove the viscera and mucus: Use a sharp knife to make an incision from the anus to the pectoral fin and remove the internal organs. Rinse thoroughly with seawater. Be sure to remove any tissue attached to the inner wall of the abdominal membrane. Also, cut along the edge of the gills and remove some gill tissue. When removing the gills, cut as much as possible from the back side. Push the blade along the medial side of the gill until it touches the bone. Make a straight cut from the top of the gills to improve the appearance of the fish. After removing the gills, use a small spoon to take out the meat from the gill bones, but avoid scratching the gill flesh.
7. Wash the fish: Scrub the fish body from tail to head, then immerse it in seawater. Transfer it to the preparation room and then to the ice-freezing room. Keep it cold to meet high-quality and high-value standards for commercial sale.
Second, the chilling process.
1. Prepare bilges and bulkheads: Cover the bilges and bulkheads with 30 cm of ice. When placing the fish, leave 30 cm of space and fill it with ice.
2. Pack ice from the mouth to the abdomen: Fill the fish’s mouth and abdominal cavity with ice for sashimi quality.
3. Arrange the fish: Lay the fish flat, keeping a 30 cm gap between each fish. Place ice in the middle and maintain a 30 cm layer of ice between each layer. Stagger the fish in each layer to form a triangular pattern. Turn the heads and tails upside down. Once covered with ice, the process is complete.
4. Knock and re-ice: On the second day, break up the ice from the previous day, add more ice, and repeat the process.
5. Control the temperature: Maintain the chilled fish tank between 0–2°C, with a maximum of 5°C.
6. Shelf life: The chilled fish can last up to 12 days, with a maximum of 14 days. Beyond 14 days, the quality will gradually deteriorate.
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