Vegetable-rich diet takes blame for kingfish mortalities.
An outbreak of enteritis among juvenile yellowtail kingfish being reared in Australia by the Clean Seas Tuna company has devastated a substantial portion of stock the company was hoping to sell to support its ongoing hatchery-based tuna program. Dr Craig Foster, Clean Seas’ recently-appointed chief executive officer, said in iterview from the company’s Port Lincoln base, that the outbreak of enteritis among the hatchery’s young kingfish off spring began about two months after the juveniles were put out into sea cages.
The enteritis, which is similar to gastro-enteritis in humans, is causing weakening of the fish so that they don’t grow as well as they should. The sickness is thought to be tied to the increased vegetable content of the new diets the fish were weaned on to in order to cut back on the use of fish oil and fish meal. Those diets were supplied at company request by two different feed companies, said Foster, so it’s not tied to any one diet or supplier. Unfortunately, he said, the enteritis had also led to a secondary, coccidial (parasitic) infection and inflammation in the kingfish’s intestines, which had pushed the mortality rate up to the point that it was claiming as much as 3% of the juvenile fish in the pens each week. Foster said that rate had not continued every week and there had been some improvement through alteration to the fish’s diet. He acknowledged though that the combined outbreaks had wiped out some 35% or so – around 200,000 – of Clean Seas’ stock of 700,000 kingfish that it had put into ocean growout facilities earlier in the year as 5g fish. Foster said the enteritis became noticeable when, after two months in the ocean facilities, the juvenile fish had achieved only about half the growth rate that had been expected. “We were down to about 25% fish meal and substituted about 75% of the fish oil,”
Foster told this publication, “but we’ve taken a more conservative position with the fish meal and we’re not substituting fish oil at all at the moment. “We’ve removed all non-fish meal and brought up the fish meal to 40% now.” Foster said that obviously there’s still more that has yet to be learned about vital ingredients in yellow kingfish diets, that has already been learned with oil and meal substitutes for salmon. He said that lesson had already been learned with both juveniles and adult kingfish in Japan, which now produces some 150,000 tonnes of kingfish a year, mostly by growing out wild-caught juveniles. “I think we still have a lot to learn about how to manage the feed and husbandry of these small fish, especially in the first few months after introduction to the ocean,” said Foster.
Clean Seas has been putting them into the ocean at about 80 days after they hatch in the company’s on-land facilities. “They’re only small fish (when they go in) so in some ways it would be more ideal to put them in as larger fish,” he said. “We’re certainly looking at doing that next year.” He said it takes them 18 months or so to grow to 3.5kg market size, at the lower end of the preferred 3.5-4.5kg market range. And Foster also acknowledged that Clean Seas had also reduced its harvest to-market rate to reflect the impact of the problems with the two outbreaks in the juveniles’.
Reposted from Hatchery International July/August p 35