Growing tuna fish in aquaculture

 

Tuna farming began in Japan in the 70s of the 20th century, and in the last decade this area of ​​marine fish farming began to develop at a particularly rapid pace. The natural population is under severe pressure and breeding and growing tuna in aquaculture is the only way to preserve it in nature. Cage farming of tuna is developed in the Mediterranean - Croatia, Greece, Turkey, Italy, Morocco, Tunisia, Malta, Libya and France, Cyprus, as well as in Canada, Mexico, Japan and the USA; Southern bluefin tuna is farmed in Australia. Tuna farming is promising in offshore aquaculture. So, how is tuna fish raised, how is tuna farmed?

Content

• Tuna farming in Australia
• Tuna farming in the Mediterranean Sea
• Availability of juvenile tuna for captive rearing
• Methods of catching juvenile tuna for rearing
• Mortality rate of tuna from catch to stocking
• Trends in tuna aquaculture production
• Feed and feeding regimes for tuna during breeding
• Health and diseases of tuna during farming
• Farmed tuna harvesting systems

 

 

 

Tuna farming in Australia

How is tuna fish raised? Bluefin tuna (Thunnus maccoyi) is farmed in Australia. Fish weighing 10...18 kg are caught in purse seines in the sea from January to March and transported in them to towed cages. Depending on the fishing area, this takes from several days to several weeks. To calculate the total number of transported tuna, underwater chambers (bathyscaphes) are used, and for a preliminary assessment of the average weight of individual individuals, fish killed in the seine are used. On farms, tuna are placed for cultivation in floating net cages with a diameter of 40...50 m. They are fed 6 days a week, twice a day, with fish usually used in the fishery as bait, squid from frozen blocks, sardines, etc. Mortality of fish on farms is 3...7%. Divers regularly determine the number of dead fish in cages and check the condition of anchors and nets. After 3...9 months of cultivation, marketable bluefin tuna are sold fresh, chilled or frozen. Almost all farmed tuna fish is supplied to Japan.

Tuna farming in the Mediterranean Sea

Common tuna (Thunnus thunnus) is grown in the Mediterranean Sea (Croatia, Spain, Italy, Malta, Morocco). Juveniles are caught mainly with purse seines, traps and transported to floating cages in the open sea. In Malta, juveniles are placed in cages in May-July and kept in them until October-January. In Croatia, small tuna (weighing less than 10 kg) have been farmed for more than 2 years. Tuna weighing 10...200 kg are kept in large floating net cages with a diameter of 50 m or more, a depth of 15...25 m, and are fed with various small sea fish and squid. During the period of farming, the weight of tuna increases by at least 25%.

Let's take a closer look at how tuna is grown on farms.

 

Availability of juvenile tuna for captive rearing

Is tuna farmed artificially? Scientists in many countries are addressing the issue of how to artificially breed tuna, from caviar to commercial fish, but so far their successes have been modest; only in Japan have they been able to breed tuna on a farm.

Is tuna farmed? Tuna is grown in cages and cages on tuna farms (how tuna farms are organized in different countries will be discussed below), as a rule, fish caught in the wild are used for this, and therefore it is important to know where tuna spawns and where there are large concentrations of its young .

There are only two confirmed spawning grounds for tuna: the Gulf of Mexico in the Western Atlantic and the Mediterranean Sea in the Eastern Atlantic. Tuna spawn in the Gulf of Mexico from mid-April to mid-June, when each female (usually 8 years old) spawns approximately 30 million eggs.

The highest densities of larval bluefin tuna, a key indicator of spawning, occur in the northern Gulf of Mexico, with lower concentrations of larval tuna appearing off the Texas coast and in the Straits of Florida.

In the East Atlantic, tuna spawning occurs exclusively in the Mediterranean: usually from May to July in the Balearic Sea, South Tyrrhenian Sea and the Southern Mediterranean. Aggregations of juvenile tuna have been recorded in the eastern Aegean, southern Adriatic, Tyrrhenian Sea, Ligurian Sea and Balearic Islands, sometimes near the coast. The highest numbers of tuna larvae were observed south of Minorca and along the middle part of the Mallorca Channel.

  

Bluefin tuna in the South Atlantic belong to a separate southern population, the southern bluefin, whose spawning grounds are known south of Java, Indonesia. Schools of juvenile southern bluefin swim near the surface during their migration between spawning grounds and western Australia, where they concentrate in dispersal areas near Tasmania and New Zealand.

 

The Mediterranean bluefin tuna fishery is based on migration patterns.

In Croatia, the capture of juvenile tuna for artificial aquaculture purposes occurs in late spring and early summer. Specimens caught range from very small fish (less than 10 kg) to somewhat larger fish (20-80 kg). They are caught by Italian and Croatian purse seiners in the Adriatic Sea. Most of the fish for Murcia farms in Spain comes from Spanish, French and Italian seiners. This fish is caught in the Western Mediterranean, especially near the Balearic Islands; they range from small (20–80 kg) to medium (80–120 kg) and are mostly immature, but also include some larger sized mature fish. Bluefin tuna from Maltese farms is caught mainly in international waters, the main fishing season is from May to July.

Catching northern bluefin tuna (subspecies Thunnus Thunnus orientalis) in Mexico for commercial aquaculture is more difficult than in any other part of the world due to problems with the depth at which the fish can swim there. The main fishing period is from July to the end of August, but depending on the fishing locations, the season can last until November.

In contrast to the practice in other regions, juveniles weighing 150-500 g are caught off the coastal areas of Japan in spring and summer. Fishing activity in Australia is concentrated in South Australia, mainly at Port Lincoln, located in the South Australian Bight. The fishing fleet consists of purse seiners and a fishing rod fleet. Individuals have a mass range of 20-25 kg, although schools of 50 kg specimens are occasionally found. The fishing season lasts from December to May.

 

Methods for catching juvenile tuna for rearing

Tuna specimens intended for use in commercial aquaculture are caught using traditional fishing gear. The choice of fishing gear is very important: it must take into account the stress encountered during fishing and must provide specimens that can easily and quickly adapt to captivity without experiencing physiological stress that could have consequences for survival.

Tuna is prone to the accumulation of lactic acid, which is produced during muscle contractions after being caught.

Individual fishing with a fishing rod with one fishing hook is suitable for catching juveniles and sub-adults. Barbless hooks can be used to catch fish weighing several kilograms. However, the most physiologically suitable method for catching bluefin tuna for commercial aquaculture purposes is the purse seine. The purse seine fishery has become the most important supplier of live tuna (with minor amounts supplied by tuna pots) for commercial aquaculture. The fish are first caught using purse seines in the traditional manner and then transferred to transport cages by “swimming”.

Purse seine fishing is a very effective system that can be defined as a “capture fishery tool”, it is species selective and does not involve much cetacean bycatch in the Mediterranean Sea. This is the only system that allows the transfer of live fish into cages for captive aquaculture. It is therefore an important component of industrial tuna farming and is used throughout the world.

Another method of catching bluefin tuna is the use of tuna traps placed during the migration. As with purse seine fishing, it is important not to stress the animals. A floating cage is attached to the end chamber of the tuna trap. This system is especially suitable for catching specimens belonging to the “giant” class, weighing hundreds of kilograms. In Japan, juvenile bluefin tuna are caught in mid-water trawls.

Tuna mortality rate from catch to stocking

Farmed bluefin tuna are primarily caught by purse seiners. The transfer of live fish from the seine to the tow cages is carried out on the open sea by melting the fish from the purse seine into the tow cage (90 m in circumference), usually where the catch occurred. The transfer of live fish caught by purse seines to the tow nets is accomplished by connecting both nets either by "zipping" or "stitching" them together. The tuna is then carefully pushed into the tow cage, lifting the purse seine. Multiple purse seine hooks can be added to a single tow cage before it is towed to the rearing area. Traveling from the fishing area to the tuna farming area can take from several days to several weeks, depending on the location of the fishing area. In Mexico, towing distances can range from 90 to over 800 km.

Transporting adults weighing hundreds of kilograms to rearing or finishing cages requires vessels capable of towing large floating cages at speeds not exceeding 1-1.5 knots to avoid high bluefin tuna mortality due to lactic acid buildup. Care must be taken not to crowd the fish, but to create a tunnel into which they can swim without any manipulation. These maneuvers are difficult to perform, but result in schools of individuals in ideal condition that quickly adapt to captivity. The slow speed required for transport involves long return trips that can take weeks, and the fish must be fed along the way. This transportation is usually carried out by very powerful tugboats.

In Croatia, tuna is transhipped from purse seine fisheries to farms using smaller cages towed by trawler - over distances of up to several hundred miles if the fish are caught in the Adriatic Sea or other areas of the Mediterranean. Counting the fish caught in the net is usually done by divers, and cameras are used to count the fish as they move from the net to the tow cage. The average weight is tentatively estimated from the dead fish in the net. while cameras are used to count fish as they move from the net to the tow cage. The average weight is tentatively estimated from the dead fish in the net.

In Australia, schools of southern bluefin tuna are caught using seine nets and transferred via nets to specialized Bridgestone cages/pontoons. Up to 100–130 tonnes of southern bluefin tuna weighing 15–25 kg per pontoon are towed back at approximately 1.5 knots to farm areas adjacent to Port Lincoln, South Australia. This process can take several weeks and involves some feeding of the tuna. Mortality averaged 4%.

Mortality in rearing cages in Croatia was particularly high during the first month of adaptation (2.1%) and decreased significantly in subsequent months (0.6%). Stress-related mortality combined with fishing injuries Whether seine and transportation can cause a high mortality rate. Compared to later stages, younger juveniles appear to be more sensitive to stress.

In Japan, juveniles are caught in early spring by trolling in coastal waters. Newly caught juvenile fish react strongly to the stress of being caught and kept. Sometimes the jaws or other parts of the body of these fish are damaged. The skin is also delicate and easily damaged if mishandled, leading to a high mortality rate initially.

 

Trends in tuna aquaculture production

Global supplies of cultured tuna reached 20,000 tons in 2001. Bluefin tuna is farmed all over the world. Northern bluefin tuna is grown in the Mediterranean (Croatia, Spain, Italy, Turkey, Morocco, Tunisia, Malta), as well as in Canada, Mexico, Japan and the USA; Southern bluefin tuna is farmed in Australia. Some of these activities are sporadic or at an experimental level. In general, tuna farming is expanding, with new countries such as Greece and Libya, France entering the scene. 

Bluefin tuna aquaculture uses a similar tuna farming system around the world due to the fish's general behavior and environmental requirements. It is believed that a good tuna cage should be deep enough to ensure normal tuna behavior and avoid stress mortality caused by lightning during storms.

Tuna cages are usually large (average diameter 30–50 m) to provide sufficient space for large tuna to move around, and are floating (submersible tuna cages are also being tested). The circumference of tuna cages is typically 150 m and tuna stocking densities are typically around 2–4 kg/m3. Some companies now use much larger cages, with a circumference of about 270 m, giving a culture volume of 150,000 m3. There is already evidence that the larger the cage, the better the quality of the tuna produced. The cages should be moored in water of sufficient depth to provide good flow structure underneath the cage and to ensure that any accumulated waste underneath cannot interact with the water inside them. The recommended distance from the net to the bottom is at least 20 m.

The cages are usually made from high-density polyethylene, which is flexible yet quite durable. There are many versions of tuna cages and tanks. There are also cells consisting solely of networks without a surface framework.

 

Good water quality is critical to bluefin tuna farming, and suitable sites should eliminate any possibility of turbidity caused by runoff or mixing of bottom material. In Morocco, severe storms caused the death of large numbers of bluefin tuna kept in very large cages, as the fish's gills were found to be clogged with silt due to flooding in the area. Experience gained to date in tuna farming particularly highlights the importance of selecting suitable sites to ensure that the open sea dominates the dynamics of the water column in which the cages are located, thereby ensuring high transparency and high dissolved oxygen content, which maintains high physiological needs and constant energy costs for swimming.

Specific tuna cage designs vary greatly in the construction materials used, mounting systems, and net mesh size.

In Morocco, tuna is farmed in floating mesh cages measuring 120 x 40 x 30 m, moored in the open sea at a depth of 55 m. As of 2002, farms in Malta were raising tuna in a total of thirteen sea cages with a diameter of 45–60 m, which are anchored on anchored 1 km from the coastline. The cages are floating round cages made of high-density polyethylene (HDPE).

In Croatia, tuna cages are located mainly along the eastern and central coasts of the Adriatic Sea. A type of cage developed in Australia, although slightly modified, is used to house the tuna: a floating round cage has a diameter of 50 m and a net depth of 20 m. They are only partially fixed and can be moved from one place to another. Bridgestone or Dunlop floating cages (rubber hoses) are used in Cartagena, Spain. Old Japanese cages of the rectangular type, measuring about 70 x 40 m, attached to the seabed, are used in Ceuta, Spain.

Tuna aquaculture in Australia in 2002, based on tuna fishing, uses twenty leased areas of 20–30 hectares in wave-exposed waters 1–10 km from shore and at a depth of about 20 m. Polar-Cirkel HDPE cages with a diameter of 40 m are used and a total volume of 15,000 m3. Since 1996, larger cages (50 m diameter) with a total volume of 20,000 m3 have been used. The cell size varies from 55 to 70 mm. At Amami O-shima (Japan), the cages are supported by a buoy system that provides both buoyancy and structural integrity. The cages are rectangular in size, measuring 40 x 25 m, but are only suitable for use in protected water bodies.

Tuna-based aquaculture systems are considered to be in need of improvement. Design should focus on building larger cages that can withstand sea wave conditions with minimal maintenance and that can be moored in very deep water. The basic technology for constructing cages of this type exists and has been tested in Norway and other salmon producing countries; however, the financial implications and maintenance of cage mooring systems in very deep waters require careful evaluation. Solutions offered include a tension leg mooring system, increased automation and electronic monitoring transmitted via telemetry. Larger boats are required to service sea tiles.

Thus, daily hand feeding may become impractical, but fish health and water quality would definitely improve if it were feasible. However, repairs and cleaning of nets, as well as routine maintenance work, may become more difficult and costly if larger cages are installed in deeper locations.

 

Feed and feeding regimes for tuna during breeding

Information on feeding strategies, feed conversion rates etc. is limited and mainly relates to Australian tuna farming practices. Here the fattening period is short, from 3 to 10 months. Fat levels decrease during the winter months, which confirms that the summer produces higher quality fish.

Farmed southern bluefin tuna are fed bait six days a week, twice a day. Feed is usually supplied by placing frozen blocks of bait (sardine, herring, mackerel, squid) in a net inside each cage. Supplementary feeding is usually done by hand.

Typically during the growing season, food conversion ratios using bait are around 10-15:1. The average size of southern bluefin tuna increases by 10–20 kg, and the mortality rate is 3–7%. Conversion rates vary depending on the growing season (10:1 in summer and 17:1 in winter) and the size of the tuna - smaller tuna have a better ratio than larger tuna.

If the bait is low in protein and fat, the tuna's growth will be slowed and more bait will be needed to maintain the same level of production. The suitability of bait also depends on storage and the level of quality control. The characteristics of the bait used in the tuna industry vary considerably.

 

Composition of bait for feeding tuna

Parameter range

Crude protein (%DM) 49.4–75.3

Crude fat (%DM) 1.9–36.5

Free fatty acids (% DM) 2.9–53.4

Peroxide value (mg/kg DM) < 0.1–598.0

Southern bluefin tuna are an important mariculture species in Australia, but commercial feed development is limited due to a lack of detailed information about their nutritional requirements. Problems associated with suboptimal acceptance and palatability of pelleted feeds by southern bluefin tuna, production costs, and possible Japanese market resistance to fish raised on pelleted feeds were some of the reasons why farmers were reluctant to evaluate recommended commercial feeds for their commercial purposes. farms.

Australian researchers continue to work on extruded pellets for tuna feeding.

 

In the Mediterranean, tuna are also fed with bait. In Katavich, Vicina and Franicevic (2003a), tuna were first fed nine days after stocking. They were given raw, defrosted small pelagic fish, predominantly (87.9%) herring (Clupea harengus); the remaining food consisted of raw thawed sardines, Sardina pilchardus (6.7%) and cephalopods (5.4%). Food was distributed six days a week, twice a day, in the morning and in the late afternoon. The daily feed rate was gradually increased until the end of July, when tuna were fed a daily feed rate equal to 7.7% of their biomass. Based on feeding records covering a rearing period of 155 days, it was estimated that the tuna were fed a total of 399,005 kg of feed. The maximum daily feeding rate was 9.8%, and the average daily feeding rate was approximately 5.1% of the total fish biomass. Unfortunately, the FCR value is not specified in this article.

In Croatia, small pelagic fish (e.g. frozen herring Clupea harengus, sardine Sardina pilchardus, round sardinella Sardinella aurita and shortfin squid Illex coindetii) are used in the summer season at approximately 5-8%/day of estimated biomass. In 2001, approximately 15,000 tonnes of bait was used, both from the North Sea and locally. During the feeding period, bluefin tuna are typically overfed, with food conversion ratios ranging from 15 to 20:1. The greatest feed consumption occurs at 23-25°C, up to 10% of body biomass is fed daily; at 20°C this figure can be halved and daily feed intake should not exceed 2-3% of body weight at 18°C.

The submersible cage experiment was carried out in Italy, in the central Adriatic Sea, four nautical miles from Ortona; During the four-month rearing cycle, the daily diet consisted of sardines, squid (in small quantities due to their high cost) and mackerel at a daily rate that averaged 5% of the biomass within the cage. Tuna fed a total of

4,376 kg of live bait. At stocking, the average tuna weight was 3.8 kg; after 4 months he reached 10.8 kg. Depth did not affect feeding behavior: as soon as the cage was raised to the surface, the fish immediately responded to feeding in the same way as if the cage was permanently moored near the surface. Underwater feeding tests have shown that depth does not affect the fish's appetite.

 

In Malta, tuna is fed with Mediterranean frozen fish (such as herring, sardines and mackerel).

 

At an experimental farm in Morocco, they were fed "trash fish" - Atlantic mackerel, horse mackerel and short-finned squid. Two batches were kept in floating cages: the first batch consisted of 75 giant bluefin tuna (average body weight about 250 kg), the second - 106 juvenile bluefin tuna (average body weight approx. 55 kg). There are seasonal changes in the feeding behavior of tuna, which are particularly evident in smaller bluefin tuna. Higher feeding activity was observed in October-November and lower in June-July. Feeding selectivity has shown that giant bluefin tuna prefer squid and juvenile bluefin prefer Atlantic mackerel. Fish size was greatly influenced by sampling and mortality during the winter season. Bluefin tuna also showed a seasonal growth pattern.

 

 

Tuna farming experiment in Japan.

At the time of stocking, the tuna weighed 250 g, after 10 years it reached 110 kg, and after 15 years - 145 kg. The largest tuna at 16 years old weighed 177 kg and had a length of 229 cm. The tuna was fed with mackerel, horse mackerel, anchovies and cuttlefish. The development of captive rearing technology in Japan has been somewhat successful. If the supply of farmed juveniles (10 to 20 cm in length) is stable, commercial tuna farming will become practical. Feed ratios of 10:1 were achieved. The young are caught by fisheries and raised in large floating cages. When conditions are stable and ambient water temperatures remain above 15°C, mortality is low and growth is exceptional. Caught juvenile tuna weighs between 100 and 200 g, but can reach 6 kg in one year, 20 kg in two and more than 60 kg in four years, depending on food availability, water quality and temperature. In captivity, the diet of bluefin tuna is similar to that of yellowtail and consists of available resources (squid, small scombroids and trash fish). However, the efficiency of feed conversion is higher in young fish. The feed wet weight to weight gain ratio was approximately 8:1 during the early months of growth, but increased to approximately 12.5:1 by the time the fish reached 3 years of age.

One of the most important challenges in tuna-based aquaculture remains the development of feed that can replace bait. Raw fish diets have some inherent disadvantages, such as high rates of food processing, risk of contamination and disease, and variable quality; these issues have fueled research into feed production for farmed tuna. The industrial feeds developed so far are focused on wet and semi-moist feeds; however, dry food can also be used.

Artificial feed for tuna is also important because the supply of “trash” fish is not unlimited, and farms are already facing difficulties in obtaining it in sufficient quantities. Artificial diets should provide better feed conversion ratios and control meat quality and relative production costs in this type of aquaculture

 

Health and Diseases of Tuna Farming

Knowledge of microbial, nutritional and environmental diseases of farmed bluefin tuna is limited. However, adult tuna appear to be relatively resistant to bacterial infections, even when exposed to trauma and other factors that predispose them to such infections.

Young Pacific bluefin tuna are often infected with red sea bream iridovirus, but the disease never appears in individuals older than 1 year. Sometimes the mortality rate of juvenile tuna reaches 10%.

When artificial tuna farming began in 1990, little was known about the health of southern bluefin tuna. due to the size of the cells used and the tuna's susceptibility to stress. Water quality and overall cleanliness are known to be important to the health of tuna.

A parasitic blood fluke (Digenea: Sanguinicolidae) was discovered in farmed tuna in South Australia in 1997 and was subsequently described as Cardicola forsteri (Cribb et al. 2000). Southern bluefin tuna is a new host species for blood flukes, and C. forsteri is a newly described species. It was unclear whether the blood fluke was causing a major problem in the industry. Blood flukes are known to cause serious pathologies in some other maricultured species (eg, cultured sea bass (Latescalcarifer) in Malaysia). Overall, the pathology observed in farmed southern bluefin tuna was not considered severe enough to cause mortality.

In April-May 1996, significant mortality was observed in Boston Bay (Port Lincoln, South Australia), where approximately 75% (1700 tonnes) of fish died. This coincided with rough ocean conditions and strong winds. Clinical symptoms included distress, and some dead fish had large amounts of mucus leaking from their gills. Microalgae toxicosis, hypoxia, suspended solids asphyxiation, and hydrogen sulfide toxicity were considered possible etiological factors. The ichthyotoxic raphidophyte flagellum Chattonella marina was successfully cultured in Boston Bay (South Australia) and may have corresponded to such mass mortality in southern bluefin tuna (Marshall and Hallegraeff 1999).

Using oily bait as a source of tuna in captivity poses a number of problems. The presence of thiaminases and oxidized lipids in bait has been, or is likely to be, the cause of nutritional problems in tuna. Indirect problems can arise when trash fish (such as sardines) contain parasites; Appropriate freezing procedures reduce the risk because it kills all parasites. Benign parasitic infestations are more common in the summer when there are higher water temperatures, which is especially important for tuna farmed in southern latitudes. Parasitic worms are rarely found in the body and, although completely harmless to humans,

Special care must be taken when storing bait used to feed tuna in captivity. One study found that when stored in the refrigerator, sardines noticeably spoiled within two days. Significant peroxide levels were detected and oxidized odors and flavors were clearly evident after 4 days of refrigerated storage. When stored frozen, oxidation occurs after only one per month at a temperature of -20°C. Sardines that began to oxidize before freezing continued to oxidize during frozen storage. The oil in sardines has been shown to oxidize easily; To ensure the nutritional value of the captive tuna product, careful handling, cooling, freezing and storage procedures must be followed. Baitfish are also known to transmit important viral diseases such as viral hemorrhagic septicemia and sardine herpes virus.

During the early stages of Pacific bluefin tuna culture in Japan, morbidity and mortality were reported to be caused by thiamine deficiency [this is the most common form of vitamin deficiency in fish nutrition, and is especially common when raw aquatic animal products are used as feed, alone or in combination with other ingredients. This is particularly true where diets containing raw fish are not fed immediately after capture or production, as thiaminase may partially or completely inactivate the thiamine originally present]. Some fish contain particularly high levels of thiaminase. At that time, tuna in Japan were fed only Pacific saury (Cololabis saira) and/or Japanese anchovy (Engraulis japonicus), which led to a significant reduction in thiamine reserves in cultured fish

 

Farmed Tuna Harvesting Systems

Almost all commercial tuna aquaculture products are destined for the Japanese market, where they will be used primarily for the preparation of sushi and sashimi. To achieve the best prices in this specialized market, producers must harvest high-quality tuna; this requires special care when slaughtering fish and handling them during harvesting.

Criteria used to identify high quality tuna:

• high level of freshness;
• signs of rapid bleeding of tuna after killing;
• no flesh burns due to the formation of lactic acid;
• absence or very low levels of histamine; And
• high fat content.

 

It is clear that harvesting methods affect quality indicators such as freshness, fat content, color and shape.

When bluefin tuna reaches optimal weight and fat content and when the market price becomes favorable, tuna fishing begins. In the Mediterranean, tuna is ready to be caught by October. Some farms sell all of their tuna by December; others are delayed until February/March, depending on transport logistics. For example, if there is a problem with air transport, fish may have to be sold frozen rather than fresh. In general, the volume of processing in the frozen fish market depends on the daily capacity of blast freezing vessels. After freezing is completed, the ships transfer the fish to a refrigerated ship, which is then sent to the Far East.

When harvested, bluefin tuna are usually collected using a net from a small area where divers catch it by hand, as in Croatia and Australia.

Divers dive into the cages, grab tuna and immediately return to the deck of the floating structure. Here the tuna is killed with a blow to the brain [adult and giant bluefin are usually killed with a shotgun and then moved to a waiting boat where they are cored (a wire passed through a neural network). channel) on the slaughter table and left to bleed to death. If tuna is not handled correctly, the meat becomes “yake,” which means “burnt” in Japanese. This occurs when lactic acid accumulates and temperature rises due to muscle spasms that continue after death. To avoid this phenomenon, it is important to handle the tuna carefully to avoid ongoing muscle contractions. and kill the fish in the shortest possible time - this is done by paralyzing the nerve canal, which ensures good quality meat. It is also necessary to leave the fish bled to preserve the color and consistency of the meat. The bleeding process is especially important for fish that will be frozen as it allows the myoglobin to be converted into myoglobin; this gives the tuna its meaty color. The fish is then gutted and thoroughly brushed to remove any residue that could spoil the meat. During all these procedures, it is necessary to reduce the body temperature of the tuna in order to maintain the quality of the meat. Refrigeration lengthens the time until rigor mortis sets in, but this is also very dependent on the size of the fish.

Post-harvest activities depend on whether the tuna is exported to the Japanese market by plane or ship. Fish can be sent either fresh or frozen. In South Australia, 200 individuals are usually caught in one day. There is always a Japanese biologist present who controls the quality of the fish's muscles, and the fish will not be purchased if the muscles are burned. The bluefin tuna is then taken directly to Japanese (or Korean) boats, where the fish are cleaned and frozen to -60°C. It is slaughter that most influences the quality of tuna meat. Fish that were damaged during slaughter will have less pink or reddish flesh, which reduces their market price.

In the Mediterranean, electric shock techniques are used to obtain fresh fish. Two methods are used to stock fish for the frozen fish market. One method is to introduce a smaller net into the feeder cage to collect the fish, which can then be slowly removed (Figure 99). There is also a method that uses a small square cage with a V shape at one end; it has an open window through which fish can swim. When a fish swims into the cage, workers standing on a floating platform pull the net up and at the same time move to the V-end of the fishing cage to push it in. fish (Agius 2002).

Spanish farmers are experimenting with a major technological advance for the tuna industry: a new procedure for killing tuna. Electrofishing methods are already in use, but more research is needed to improve electroslaughter techniques and develop specially designed equipment. Currently, the diver selects a fish to catch and uses a gun to inject a small electric harpoon into it. The second diver checks to see if the harpoon has reached its target and then delivers an electrical shock of approximately 100 volts. This stuns or kills the fish. It would be possible to develop a new system that would eliminate the need for divers and minimize pre-mortem stress on fish by adjusting the electrical discharge based on their size.

In fact, pre-mortem stress is a very important factor in seafood quality. Research is currently being conducted on oxidative stress and it has been found that stressed fish are more vulnerable to disease due to the disruption of their antioxidant defense system. This is especially important where partial harvesting is the norm, such as in tuna farming. Automation can help develop effective quality assurance methods. According to Rock et al. (2003), each fish must be tagged after slaughter using an electronic transponder containing a chip capable of recording a variety of data such as weight, dimensions, catch details and processing instructions