J. Northw. Atl. Fish. Sci., Vol. 25: 115–123 Growth Rate and Sexual Maturity in Cod ( Gadus morhua) and Atlantic Halibut ( Hippoglossus hippoglossus) O. R. Godø Institute of Marine Research, P. O. Box 1870 Nordnes N-5817 Bergen, Norway and T. Haug Norwegian Institute of Fisheries and Aquaculture N-9291 Tromsø, Norway Abstract Variation in growth and reproduction dynamics of fish populations substantially affects production in wild as well as domesticated fish populations. Identifying the intrinsic and environmental factors which influence the dynamics of these traits is important for stock assessment methodology and management of wild fish populations as well as for improving production in aquaculture. In this paper the relationship between growth and maturation of cod ( Gadus morhua) and Atlantic halibut ( Hippoglossus hippoglossus ) is described using data from spawning locations along the Norwegian coast. Both species show an inverse relationship between immature growth and age/size at first spawning. Long term trends may be connected to climatic changes as well as to the increased exploitation experienced in this century. Short- term variation may be caused by abrupt changes in the conditions for growth, e.g. due to temperature shifts and/or variation in prey abundance. Key words : Atlantic halibut, cod, growth, maturity, Norway Introduction Traditionally, cod ( Gadus morhua) and Atlantic halibut ( Hippoglossus hippoglossus) are important commercially exploited species in Norway. Overexploitation of wild stocks and recent advancements in marine aquaculture have made domestication more viable (Haug, 1990; Tilseth, 1993). The age/size at onset of maturation and growth rate are important stock parameters required for reliable assessment and prediction of com- mercially exploited fish species. These parameters are also of crucial importance in aquaculture and set the limits on the commercial potential of the species. The scientific literature has not reached a consensus over whether organisms mature at a fixed age or at a fixed size. According to Stearns and Crandall (1984) they probably do neither, in that selection seems to operate on the shape of age/size maturation trajectories. Over some portions of these trajectories, organisms mature at a fixed size. Over other portions, organisms mature at a fixed age. In a study of several flatfish species, Roff (1982) suggested that age could be more important than size for early maturing species and vice versa for late maturing species. According to Stearns and Crandall (1984) it is only by considering maturation events occurring across a wide range of growth rates that one can perceive the shape of the whole trajectories. Growth rates of fish are density-dependent, and vary with factors such as temperature, food availability and exploitation (see Beverton and Holt, 1959; Bromley, 1989). The heavily exploited stocks of northeast Arctic cod and Atlantic halibut in Norwegian waters have exhibited considerable variation in growth this century. Jørgensen (1990) demonstrated a considerable decrease in both the age and size at onset of maturation which occurred simultaneously with increased fishing pressure. Reduced age at maturity following overexploitation http://journal.nafo.int