Green water in larviculture - An experiment with natural phytoplankton in tanks for first feeding of halibut larvae (Hippoglossus hippoglossus L.)
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At 232 day degrees, halibut larvae were transferred from indoor tanks to 1.7 m^3 outdoor tanks for first feeding. The number in each tank was approximately 750. Three tanks were continuously given algal suspension ("green water") and supplied nonenriched Artemia instar II. Six tanks were given filtered deep water ("clear water"). Three of the six were supplied nonenriched Artemia, and three were supplied Artemia prefed in green water. Feeding incidence at day 3 was 47 % in green water and 0 % in clear water. Larval growth was significantly higher in green water compared to clear water, while no significant difference was found between the clear water groups given prefed and nonenriched Artemia. The mean myotome heights for all groups were 0.75 - 0.78 mm at day 7. At day 14 and 21, the mean heights were 1.49 and 1.86 mm in the green water group and 0.84 and 1.05 mm in the clear water groups. The survival rates were also much higher in green water. Out of a total of approximately 2250 halibut larvae in the green water tanks, 684 larvae were found alive at the end of the experiment. Corresponding numbers for the clear water tanks were 57 out of 4500. Preliminary results indicate no nutritional effect of the algae.