A mixed score for Japanese yellowtail

Published on
September 14, 2009

Last week, SeafoodSource visited a yellowtail farm near Uwajima in Ehime Prefecture, Japan, in conjunction with a Japanese Ministry of Agriculture, Fisheries and Forestry promotional event. This presented an opportunity to see if the sushi sustainability ratings by various organizations are accurate.

Among the most popular sushi sustainability guides is Monterey Bay Aquarium’s Seafood Watch program. It recommends that consumers “avoid” yellowtail farmed in Japan due to its “high reliance on wild-caught fish to create feed and serious concerns over disease and water pollution.”

The Blue Ocean Institute gives Japanese yellowtail, or hamachi, its lowest rating and says yellowtail farming “can cause substantial water quality impacts in areas of intensive farming. The fish is farmed in net cages with little or no treatment of effluent, which has been linked to localized pollution and damaging red tide episodes. Farmers also feed raw fish to large individuals, increasing waste accumulation and further degrading water quality. The biggest concern in farming practices is the dependence on wild juveniles.”

The operation visited was indeed stocked with wild-caught fingerlings. Youhei Yamada, part of Yonkyu Corp., which supplies the pen operators with fingerlings and feed, was unaware of any catch limit on fingerlings. He said that they simply obtained estimates from the pen operators of the numbers they would require in the coming year, and the quantities were filled.

However, professor Makoto Nakada of Tokyo University of Marine Science and Technology wrote in his 2008 paper “Capture-based aquaculture of yellowtail” that the fingerling stocks are estimated to be greater than 100 million fish and current regulations limit the harvest to 25 million fish, which he estimates to be the level at which the wild population will not only be maintained but will increase.

“When the number of fingerlings caught has been insufficient, yellowtail fingerlings have been imported mainly from the Republic of Korea,” he wrote.

Thus, the orders for fingerlings are always filled, either by domestic harvest or import, so that the catch limit poses no practical limitation on the scale of fish farming. To the industry’s credit, it has brought the fingerling mortality rate down to less than 2 percent since the limitations were initially imposed, thus maintaining production with a smaller harvest of fingerlings.

Feeding fish to fish has been cited as a threat particularly to Japan’s domestic sardine stocks. At the pens visited, younger fish are fed on pelleted feeds of different sizes, according to the size of the fish. For some fish, such as sea bream, the content of the pellets is identical. However, in the case of yellowtail, the oil content is increased as the fish grow during the nearly three years to harvest.

Japanese feed manufacturers produce several different feeds for yellowtail, but all now include considerable soy protein content. Taking the “Expander Pellet” line of feed produced by Chubu Feed Products Co. as an example, the contents of the feed are 60 percent fishmeal, 20 percent soy protein, and 20 percent minerals and other supplements, such as taurine. Japanese researchers have found that taurine must be added to offset soy’s lack of this necessary amino acid. Horse mackerel and South American sardines and are the major sources of fishmeal.

“Farmers have achieved feed conversion ratios of 1.2 during the production of one-year-old fish,” wrote Nakada. “Using the same type of feed, satisfactory growth has also been achieved during the second year, providing that water temperatures are optimal. For yellowtail larger than 3 kilograms, raw fish is preferred to extruded pellets. Currently, the production cost for yellowtail moist pellets or formulated feed is less than that of raw fish.”

This matches the pattern observed at the site. Smaller fish were fed pellets using a feed mixing machine with a dispensing tube, and were able to consume the pellets thoroughly with little waste. The pollution issue with younger yellowtail would relate mainly to fish feces, rather than wasted feed.

For older yellowtail, a slurry of chopped mackerel, Pacific saury and juvenile Japanese sand lance (Ammodytes personatus) was fed. In contrast to the pellets, unconsumed feed did spread to the water outside of the pens. It attracted small fish and gulls, but these consumed only a small portion of the floating feed.

Yamada said that the adult fish fatten faster on a diet of fish than on pellets. As they near harvest size, the farmers may vary the feed to bring them to market faster or slower, depending on current market prices. A finishing diet of oily fish also improves the flavor of the finished product. Weather also plays a part, as fish do better on the richer fish diet in colder weather.

This farm SeafoodSource visited is located by Hiburi Island in the Hoya Straight, where tidal flows ensure adequate water circulation to prevent waste build-up. In the 8-square-meter pens at this site, 4,500 yellowtail fingerlings were stocked. About 50 such pens were located at this site.

As for disease prevention, erythromycin and ampicillin cases could be seen on the docks and are presumably used for treatment. A bath of oxytetracycline for treatment of sea bream could also be seen. Treatment was generally on an “as-needed” rather than a routine preventive basis, according to Yamada.

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Contributing Editor reporting from Osaka, Japan

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