Ž . Aquaculture 203 2001 51–67 www.elsevier.comrlocateraqua-online Growth, gonadal development and spawning time ž / of Atlantic cod Gadus morhua reared under different photoperiods Tom Hansen a, ) , Ørjan Karlsen b , Geir Lasse Taranger c , Gro-Ingunn Hemre d , Jens Christian Holm b , Olav Sigurd Kjesbu c a ( ) Institute of Marine Research IMR , Matre Aquaculture Research Station, N-5198 Matredal, Norway b IMR, AusteÕoll Aquaculture Research Station, N-5392 Storebø, Norway c IMR, P.O. Box 1870 Nordnes, N-5024 Bergen, Norway d Fisheries Directorate, Institute of Nutrition, P.O. Box 185, 5002 Bergen, Norway Received 9 August 2000; accepted 5 March 2001 Abstract Ž .Ž . Individually tagged Atlantic cod Gadus morhua L . mean body weight 459 "6g were reared in 20-m 3 tanks supplied with running sea water. During the first year the fish were exposed Ž . to one of four photoperiod regimes: Natural light LDN , continuous light from June 23, 1993 Ž . Ž . LL , natural light from June 23, 1993 and continuous light from December 22, 1993 LDNrLL Ž . or continuous light from June 23, 1993 and natural light from December 22, 1993 LLrLDN . In July 1994, the LDN and LDNrLL groups were terminated. The LL and LLrLDN groups were reared on until May 1995. From July 1994 to the termination of the experiment in May 1995, the LL group was reared under continuous light and the LLrLDN group was reared under natural photoperiod. Cod reared under natural photoperiod spawned in the period between January and April. Photoperiod manipulation changed the incidence of sexual maturation, spawning time, fecundity and egg size. Cod that were transferred from natural photoperiod to continuous light in December spawned earlier, having lower fecundity and smaller eggs than the cod reared under natural photoperiod. Oocytes of females reared under continuous light were arrested in the cortical alveoli stage, and even in their second year on continuous light very few females matured. When transferred to natural photoperiod in December, females ovulated within 4–5 months, approximately 3 months delayed compared to the natural photoperiod group. These cod had a higher fecundity and smaller eggs than natural photoperiod cod. When reared on for another year under natural photoperiod these cod spawned again the next year at their normal spawning time, e.g. 9 months after their first spawning. ) Corresponding author: Tel.: q 47-56-36-60-40; fax: q 47-56-36-61-43. Ž . E-mail address: Tom.Hansen@imr.no T. Hansen . 0044-8486r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0044-8486 01 00610-X