Norwegian researchers have found a possible explanation for why the deadly intestinal disease DGS attacks cod larvae. Could producers soon be rid of this bottleneck in cod aquaculture?
Periodically, vast numbers of cod larvae die during fry production. First the larvae become lethargic, then lose their appetite, then their gut fills with fluid and they eventually die. The problem occurs without warning. In some seasons there are few deaths, in others the mortality is high. Researchers label the mysterious condition distended gut syndrome, or DGS.
Professor Ivar Rønnestad and his colleagues at the University of Bergen's Department of Biology are collaborating with SagaFjord Sea Farm to crack this problem. They have studied two hatching seasons of cod larvae. In spring 2008, larva production was a success; the fish were well formed and remained healthy. The next hatching season, however, in autumn of the same year, went badly as the larvae began showing signs of DGS.
"Collaborating with the Institute of Marine Research in Bergen, we used MicroMatrix technology to compare healthy and diseased larvae," explains Professor Rønnestad. "Several genes involved in the metabolism of nutrients were affected in the DGS larvae. Previous research has shown that these genes are involved in redox balance, cell renewal and inflammatory response."
Oxidative stress Although the researchers are not prepared to draw any final conclusions yet, they believe they have found clear indications that cod larvae are affected by oxidative stress. As Professor Rønnestad describes it, "When the body metabolises nutrients, several types of free oxygen radicals are produced. If there are too many of them, they can start undesirable processes within the cells. One of our theories revolves around fat turning rancid."
"The larvae of cod and other marine production fish consume a feed (rotifers) enriched with polyunsaturated fat, which can easily go rancid and cause harm to the body. It is possible that the oxidative stress taking place leads to the inflammatory response we see in the intestine and liver in cod fry."
More knowledge needed We need to find out more about this, asserts the University of Bergen professor. "We know that among other things, certain minerals and vitamins are important in counteracting rancidity, yet at levels too high they can be harmful."
Professor Rønnestad would like to see more research conducted on oxidative stress. More knowledge is needed about interactions between feed, bacteria, digestion and metabolism in fish larvae.
"We still don't fully understand why DGS problems only occur periodically. A number of processes need to be studied in greater detail if we are to make cod larva production more predictable. There are just too many cod producers in Norway that have to call it quits because of these problems," laments Professor Rønnestad. "It would be of great benefit to the production of cod as well as other fish species if we could somehow get this challenge under control."
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