INTERNATIONAL COUNCIL FOR EXPLORATION OF THE SEA. C.M.1990/F:45 Induced deformities on larvae of the Atlantic halibut (Hippoglossus hippoglossus L.). A new experimental approach. Jelmert, A. and Naas, K.E. Institute of Marine Research, Austevoll Aquaculture Research Station, N-5392 Storeb0, Norway. ABSTRACT: 2 days before estimated hatching, halibut eggs from one egg batch were transferred to sterile polystyrene plates with 6 wells. Each well contained 10 ml of sterile seawater and one single egg. Chorion of the eggs was removed within 1 day after hatching and 5 ml of the seawater was renewed. 4 temperatures(range 0.9°C- 10°C), 4 light intensities(range 0.5 lux- 150 lux), 4 levels of 0 2 concentrations (range 1ppm - 8ppm) , and 4 levels of H 2 S concentrations (range 0.002-0.2mgjl Na 2 S) were chosen as stressors. Each treatment was replicated with 60 or 48 eggsjlarvae. Overall survival in the control groups was somewhat lower (33% at 286 d 0 and 45% at 215 d 0 ) than the rest of the egg batch reared in a "silo"(33% at 235 d 0 ). However, prevalence of healthy larvae was similar in control groups and silo(67 and 68 %, respectively). The percentage of healthy larvae were significantly lower (p<0.05) for low 02, H2S and light groups compared to control. Low [0 2 ] caused higher prevalence of yolk sack oedema, abnormal jaw articulation and head deformities (p<0.05) than in control. Elevated temperature (10°C) did not increase prevalence of deformities compared to controls(7.5°C and 5.5°c; P=0.05). INTRODUCTION. The atlantic halibut is viewed as a promising species for cold water aquaculture, and have been subjected to extensive research efforts both in Norway and in U.K. the later years. As the biology, and especially the ontogeny of the halibut was little studied, a strategy of simultaneous basic and applied research have been chosen. It is known that several factors have effects on the developing fish larvae(Rosenthal and Alderdice 1976). We have the impression that answers to many of the basic problems concerning larval development have been sought in experimental units not suited to give these answers. We strongly felt the need to develop an experimental design were hypothesis could be tested.