GROWTH, DEVELOPMENT, AND SKIN ABNORMALITIES OF HALIBUT (HIPPOGLOSSUS-HIPPOGLOSSUS L) JUVENILES KEPT ON DIFFERENT BOTTOM SUBSTRATES

Atlantic halibut (Hippoglossus hippoglossus L.) juveniles were reared for 6 months in units with bottom substrates of smooth PVC, silicone r idges on smooth PVC, or a layer of sand lying over smooth PVC, Mortali ty was highest among the halibut reared in units with a bottom substra te of sand. This may be attributed to poor water quality in this syste m. Growth rates were highest for the halibut reared in units with sili cone ridges. After 3 months the juveniles reared in units with a smoot h bottom had developed external skin lesions on the caudal peduncle an d on the mid-body area of the abocular side, Simultaneously there was hyperpigmentation of the outer margins of the body and head region on the abocular side. The external skin lesions and patches of hyperpigme ntation increased in size and number as time progressed. By the end of the experiment the numbers of halibut with external skin lesions and hyperpigmentation of the abocular side were significantly higher among st fish held in the units with a smooth bottom than amongst those rear ed in the other two systems. The coincidence of external skin lesions and hyperpigmentation with reduced growth suggests a negative effect o f these skin problems on growth rates in halibut.