by Matthew Holmes, Features Editor, International Aquafeed,

 

Japanese amberjack (Seriola quinqeradiata) culture began in 1927 in the Kagawa prefecture of Japan where wild juvenile amberjacks were first reared in shore enclosures.

Waste accumulation and poor water quality led to this type ofculture becoming swiftly obsolete, and so refined commercialproduction began in the 1940s, which expanded rapidly in the 1960s. By 1970, amberjack production was exceeding 43,000 tonnes, reaching a peak in 1995 of nearly 170,000 tonnes. The industry hit a record height in the late 1990s, where production reached between 132,000 and 160,000 tonnes.

 

Plentiful production

Japanese amberjack features in the fisheries of the Western Central Pacific Ocean, from Japan and the eastern Korean peninsula to the Hawaiian Islands,but its farming occurs primarily in Japanese waters. The Republicof Korea is the only other country reporting production to the Food and Agriculture Organisation (FAO).

The aquaculture production of Japanese amberjack constitutesover 50 percent of the total farmed marine finfish production inJapan, a surprisingly high amount when considering the rarity ofthis fish in the Western market. Amberjack can be eaten as sashimi or grilled when sold as fillets. It has a firm white, mild tasting flesh.

The common name of Japanese amberjacks varies with size. Those that weigh under 50g are called "mojako", whilst those weighing between 50g and 5000g are named "hamachi". "Buri" is the name granted to the amberjack which weigh in at over 5000g.These fish spawn along the 200 mile contour in the East China

Sea, juveniles migrating north towards Hokkaido, where theyfeed for three to five years until reaching sexual maturity, whenthey migrate south for spawning.

Adults of 70-80cm approach the western coast of Kochiprefecture, Japan, in March-April. From season to season, various sizes can be caught in different parts of Japan.

The optimum rearing water temperature for Japanese amberjack is 20-29C and the optimum salinity is 30-36 percent.

Aquaculture of amberjack is primarily dependent on seed supply from the wild, although imported seed is also availablefrom the Vietnam and the republic of Korea. Soon afterspawning, larvae less than 15mm long are brought near the coastby the Kuroshio Current, where they are caught in fine mesh nets,and sold to fry specialists.

Although artificial propagation of Japanese amberjacks has been successful, the number of juveniles produced through induced breeding has not yet reached a level where it can make asignificant contribution to the demand of juveniles for aquaculture.In fact, there remain some problems in larval rearing: feeding is particularly critical, as imbalanced larval feed leads to heavy mortalities. Efforts are being made to improve this situation.

The design of suitable larval feed by using mass-produced foodorganisms, such as rotifers and brine shrimp nauplii fortified with n-3 highly unsaturated fatty acids (HUFA) and formulated feeds, may soon make the production of healthy fry in large numbers possible.

Amberjack farming in Japan began at the Ado pond in Shikoku 90 years ago. After peaking in the late 1990s the level of production has now levelled off and continues to maintain itself at a steady level.

Commercial Japanese amberjack culture occurs, primarily, in nylon netting alongside the occasional use of metal sea cages. Net-pens are the most common tools used,as their steel frames facilitate harvesting in a much more efficientmanner. A cheaper alternative is also the non-steel net-pen, an appliance which is inexpensive whilst also proving strong enough to withstand severe tides and typhoons. Harvesting operations through this method can, however, be challenging and arduous. The size and number of cages used in farming varies, depending upon the size of the operation and environmental conditions.

A relatively small-scale production site may have five cages of 10 x 10 x 8m, while a relatively larger production site may often have over 20 cages of 18 x 22 x 8m or 12 x 12 x 12m. Even larger pens, up to 50 x 50 x 50m are in use, particularly, to grow larger-sized amberjack.

As net-pen culture has developed over the past few decades, increasingly larger pens are now used, and the frames have transformed from simple wood to metal and often reinforced plastics. Larger net-pens are now desirable to produce high- quality meat with the correct fat levels.

Wild Japanese amberjack juveniles are reared in 5x5x5mnet-pens and are sold to growers when the fish have grown tobetween 50-100g.

 

Categorisation

The first task of the fry specialists is to grade the larvae intosmall, medium, and large categories. A failure to grade early can result in high mortality rates due to cannibalism.
After grading, the larvae are stocked into floating nylon net-pens.
In 5 x 5 x 5m net pens the stocking rate of 0.5-10g amberjack ranges from 10,000 to 30,000 and the harvest size ranges from 20-200g with average survival of 90 percent.

It is important to feed wild caught juveniles with good qualityfeed while the fish are on the collecting boat, to avoid growth-related problems in the later grow-out phase.

A typical floating raft (30 x 30 x 15m) can be used to rear25,000 amberjacks with an average body weight of approximately 2kg.

The stocking density is also dependent on cage-site conditions, such as temperature, dissolved oxygen, mesh size and water exchange rates.

Depending on water temperature, smaller amberjack (mojako)can usually be stocked from April through to July. Fingerlings of8-50g stocked in June reach 1.0-1.5kg by December. The fish thatremain in the cages until the following farming season usually reach an impressive size of 2-3kg.

 

Sustainable resources

Water temperatures can be subject to variations in the areas where Japanese amberjack culture is carried out. In each region, the farmers have developed a characteristic method of rearing by considering this local factor.

For young amberjack (hamachi), the ranges for economical andmaximal growth are 17-30 oC and 22-27 C, respectively; foradult amberjack (buri) they are 15-30 C and 20-26 C.

Japanese amberjack culture used to depend on locally available trash fish, such as pacific sandeel (Ammodytes personatus), anchovy (Engraulis japonicus), chub mackerel (Scomber japonicus), sardine (Sardinops melanostictus), and pacific saury (Cololabis saira). Japanese amberjack culture expanded due to the massive catches of the low-cost fish used for feeding, suchas sand-lance and sardine which are readily available in the East China sea.

The availability of freezing equipment has made it possible for the farmers to feed minced frozen sardine to their amberjack, creating an affordable and readily available food source for their livestock. In recent years, however, there has been a decline in the sardine resources caught around Japan and the cost has therefore increased. This has forced many farmers to shift to the utilisation of formulated feed.

 

Pellets versus raw fish

Formulated feed production has increased dramatically in the recent years and the number of extruded pellets is now roughly 40 percent of the total food used for Japanese amberjack production.

Feeding amberjack extruded pellets for the first year of culture,during the growing season, has become very popular amongstfarmers. The use of raw fish or moist pellets is still somewhatcommon when water temperatures are reduced in cooler seasons. Feeding is crucially important because feed costs represent about half of the total budget of the average farmer.

With improved knowledge on nutritional requirements it has become possible to produce moist pellets and formulated feeds for Japanese amberjacks which also prove to be sustainable and environmentally friendly. Yet, despite these successes, farmers are still facing problems in culturing larger fish on extruded pellets.

Larger fish (>3 kg) prefer eating raw fish to extruded pellets, and it is difficult to attain daily feeding rates of two percent onextruded pellets, especially during winter.

The feeding frequency of amberjacks range from five times perday to once every three days, depending on body weight, growing stage, and seawater temperature. Food consumption drastically reduces at temperatures below 17 oC, particularly when a dry diet is fed.

 

Weighing the whoppers

Most Japanese amberjack growers target a market size ofapproximately 2-5kg, while some even raise the fish to aremarkable 7-8kg.

The average harvest sizes are 6kg in 19-20 months in high- temperature areas, 5-6kg in 27 months in medium-temperature areas, and 3.5-4.5kg in 27 months in low-temperature areas. The size of amberjack at harvest, and the length of the grow- out period, not only varies according to the mean annual water temperature but also on the desired market size. In order tomaintain optimal product quality, the fish are fasted beforeharvesting. The main purpose of this is to evacuate the ingestedfeeds, as they may contribute to rapid deterioration in fleshquality.

During harvest, Japanese amberjacks are collected from the net cage by scooping them up with nets attached to long steel pipes at both sides. Fish that have been collected in the canvas netting are then subsequently released onto a selection stand, where they are size-graded and counted.

Japanese amberjacks are slaughtered individually on a rubbermat instantly after harvest. The fish are then packed in corrugatedboxes under ice, or placed into a tank submerged in chipped ice,before the harvested fish are transported in individual containerson refrigerated trucks. To maintain freshness for a longer periodthe fish are killed immediately after being taken from the waterand completely bled.

 

An optimistic future

Recently consumers have shown greater interest in fresh fish and are ready to pay higher prices for premium products.Consequently, there is growing business interest among Japaneseamberjack producers to supply fresh fish directly to the end-users,avoiding the complicated wholesale network.

Japanese amberjack meat is mostly eaten raw as sashimi and a relatively small proportion of the total production is being consumed in soups or via grilling.

The meat can be served as sashimi when cold-stored for nomore than three days (the actual maximum storage time depends on rearing conditions and post-harvest treatments).

In retail shops Japanese amberjack is mostly sold as fillets, whereas in supermarkets both whole fish, as well as fillets, areavailable for purchase.

In aquaculture operations, fry and feed together account for most of the total production costs of amberjack. On average, half of the total annual farm expenditure goes towards feed procurement. The feed conversion rate for farmed Japaneseamberjack, using raw fish, is about 7-8:1. The market price of Japanese amberjack fluctuates widely, depending on the availability of captured fish, and is also dependent on harvest size. Generally, wild-caught fish fetch more than farmed fish and the larger fish fetch a higher market price.

 

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