Variations in growth in haemoglobin genotypes of Atlantic cod A.K. Imsland 1 , A. Foss 2 , G. Nævdal 3 , T. Johansen 3 , A. Folkvord 3 , S.O. Stefansson 3 and T.M. Jonassen 4 1 Akvaplan-niva Iceland Office, Akralind 4,201 Ko ´pavogur, Iceland (Phone: +354 562 5800; Fax: 354 564 5801; E-mail: ai@akvaplan.niva.no); 2 Akvaplan-niva Bergen, Nordnesboder 5, 5005 Bergen, Norway; 3 Department of Biology, University of Bergen, High Technology Centre, 5020 Bergen, Norway; 4 Stolt Sea- Farm ASA, Bredalsmarken 15/17, 5836 Møhlenpris, Bergen, Norway Accepted: 24 November 2004 Key words: genotype dependent growth, haemoglobin genotypes, optimal temperature, quantitative trait locus (QTL) Abstract In the present paper are described the growth properties of three different haemoglobin genotypes of juvenile Atlantic cod (Gadus morhua) reared at 7, 10, 13 and 16 °C. In addition one group was reared under ‘‘temperature steps’’ i.e. moved successively from 16 to 13 and 10 °C. The genotype Hb-I(2/2) displayed the overall highest growth rate in the temperature range 13–16 °C, whereas the Hb- I(1/1) genotype showed the highest overall growth at the lowest temperature (7 °C). Accordingly, we found a significant interaction between genotype and temperature. The differences in growth were largest when cod were reared under the temperature step regime where the Hb-I(2/2) genotype displayed 17 and 24% higher growth than Hb-I(1/1) and Hb-I(1/2), respectively. Optimal temperature for growth (T opt. G) varied between the genotypes with the genotype Hb-I(1/2) displaying the highest (mean ± SE) T opt. G (14.5 ± 1:0.8 °C) and Hb-I(1/1) the lowest (12.5 ± 0.2 °C). The biological significance of this link between biochemical genetic variation and physiological properties might be the influences on growth pattern, ultimate size and age at first maturity. Introduction In Atlantic cod haemoglobin is characterised by three different and common genotypes called Hb-I(1/1), Hb-I(1/2) and Hb-I(2/2). Mark et al. (1984a,b) and Nævdal et al. (1992) indicated hae- moglobin dependent growth rate in Atlantic cod, although in other studies no such dependence has been found (Jørstad and Nævdal 1994; Glover et al. 1997). If present haemoglobin dependent growth rates might be correlated with differences in functional properties of the haemoglobins as Karpov and Novikov (1980) and Brix et al. (1998) have reported specific temperature depen- dence of oxygen dissociation curves for cod hae- moglobins. A similar system has been reported for turbot, Scophthalmus maximus Rafinesque and in both species the Hb-I(2/2) genotype has the highest oxygen affinity at all tested tempera- tures, followed by Hb-I(1/2) and Hb-I(1/1). For both species temperature sensitivity of O 2 binding for haemoglobin is low but increases with increasing pH, and it has been hypothesised that the temperature insensitive haemoglobins might be an adaptation to variable temperature condi- tions in the distribution area of the species. Fish typically show a rapid increase in relative growth rate as the temperature rises, passing through a peak at optimum temperature (T opt. G) and falling rapidly at temperatures beyond T opt. G (cf. Brett 1979; Cuenco et al. 1985; Ims- land et al. 1996; Jonassen et al. 1999). A com- mon finding in studies examining the relationship of temperature and size on growth is that T opt. G Fish Physiology and Biochemistry (2004) 30: 47–55 Ó Springer 2005