Malformations remain a significant problem in commercial aquaculture production, and the persisting production of malformed fish represent practical problems and economic losses as well as an important animal welfare challenge. In Atlantic salmon (Salmo salar L.), the number of malformations have been reduced by controlling water incubation temperature during the embryonic stage. The Atlantic salmon has a long egg stage (60 days at 8°C) and a long yolk sac stage (45 days at 8°C) before reaching first feeding at a weight of approximately 0,2g. Thus, by using higher temperatures, considerable time could theoretically have been saved. However, results from repeated experiments show that increasing temperatures >8°C will induce malformations, both in soft tissues and in skeletal structures. The most vulnerable period was found to be before the eyed egg stage, i.e. during organogenesis. Similar results were found also in rainbow trout (Oncorhynchus mykiss), and in this species, optimal egg incubation temperatures were found to be 8-12°C, with an optimum at 10°C. The results thus indicate that the relation between temperature stress and embryonic malformations is a general response, and that all species may have their specific temperature tolerance range. Embryonic temperature is, however, not responsible for all malformations. Recent results demonstrate that high temperatures may induce vertebral malformations also during the juvenile stage (up to 60g size) in Atlantic salmon. Previous studies have furthermore demonstrated that there is a significant genetic variation in susceptibility to development of vertebral malformations, and also, ongoing research indicate that the combination of two or more adverse factors, including nutrition, may amplify problems. Thus, research on the causes of malformation in salmonids must continue. Some of the knowledge gained so far may, however, be relevant to the problems observed in other aquaculture species.

Dr. Grete Baeverfjord, DVM, Dr. scient. Senior Research Scientist, Fish Health at AKVAFORSK (Institute of Aquaculture Research).Dr. Baeverfjord is a veterinarian with a Dr.scient. degree (PhD) in comparative pathology. Her field of work is research in environmental and nutritional pathology and optimization of rearing conditions for aquaculture species, both salmonids and marine species. Main focus since 1997 on factors causing malformations under intensive rearing conditions.