J. Fish Biol. (1983) 22, 153-1 57 Growth studies with fish-overcoming the problems of size variation M. JOBLING Institutt f i r Fiskerfag, Universiietet i Tromse, 9001 Tromse, Norway (Received I March 1982, Accepted 3 April 1982) The relationship between growth rate and fish size is described by the equation log, G,=a-0.4 log, W, where G, is the specific growth rate and W is fish weight. Since the intercept (a) represents the log, G, of a fish unit size, the relationship presents a method allowing comparison of data from experiments involving fish of different sizes. The appli- cation of the method is demonstrated by examining the effects ofenvironmental temperature on growth rates of cod, Gadus morhua, and it is suggested that the optimum temperature for growth of cod is 13-1 5" C. I. INTRODUCTION Alterations in the growth rate of fish are amongst the most sensitive indicators of environmental change and it is often desirable to conduct laboratory studies to obtain ' base-line ' values of growth which can then be applied in ecological studies. However, the frequent reliance on ' wild ' stocks for research material means that the numbers of fish available are frequently small and the size range variable. This can lead to problems in the interpretation of results since most studies have demonstrated that there is a gradual decrease in growth rate of animals with increasing size (Brody, 1945; Bertalanffy, 1957). In the current study the relationships between fish size and growth rates are examined. A method is suggested by which variations in fish size can be adjusted, allowing comparison of experiments in which weights are dissimilar. 11. MATERIALS AND METHODS The relationship between growth rate and fish size was examined using published data, recalculations were performed where necessary. As much of the published material relates to either relatively restricted size ranges or small numbers of replicates, experiments were carried out to examine growth rates of cod, Gadus morhua, of various sizes. Approxi- mately 100 cod, in the size range 128-1052 g were caught by angling, weighed and indi- vidually marked (Floy anchor tag, FD67). Fish were then transferred to a 50 m3 sea cage (Sterner 50, Skretting AS) and fed a moist diet composed of minced capelin and vitamin binder. Fish were fed 3 4 kg of the diet once per day and at other times, commercial pellet food (Ewos 4P/5P) was provided to excess by means of automatic dispensers. Fish were acclimatized to the experimental conditions for a period of 4 weeks and then reweighed. A number of fish failed to recover from the stress of capture and marking and lost weight during the acclimatization period. These fish were excluded from later analysis. The growth experiment lasted 4 weeks and during this time the fish were fed to excess. At the completion of the experiment each fish was weighed and specific growth rates of each individual calculated using the formula: log, w,-log, w, G,= x 100 t 153 0022-1 I12/83/020153 +05 1803.Oo/O 0 1983 The FisheriesSociety ofthe British Isles