Journal of Fish Biology (1997) 51, 750–759 Growth, maturation and reproductive investment in Arctic charr C. E. A  F. A. H  Fish Behaviour and Ecology Group, University Field Station, Division of Environmental and Evolutionary Biology, University of Glasgow, R owardennan, Glasgow, G63 0A W , Scotland (Received 20 July 1996, Accepted 1 M ay 1997) Size and rates of growth in a cohort of 1+ Arctic charr housed in standard conditions were tracked over 12 months (December to December) and mature and immature males and females compared retrospectively. In both sexes, maturing fish were larger than non-maturing ones. In males, this size differential was the result of differences in growth in winter and early spring, but not in the remainder of the study period. In females, size differentials resulted mainly from growth rate differences immediately prior to breeding. In females but not in males, gonado- somatic index was predicted by growth rates in the months leading up to maturation, and among the females that matured, faster growing fish produced more eggs. Lipid reserves in July were correlated negatively with growth during the previous 7 months and, in females only, lipid reserves were significantly lower in maturing fish than in non-maturing fish, indicating that mobilization of lipid energy reserves in maturing fish had commenced by this time. Variation in investment in gonadal tissue, measured as gonadosomatic index, was not explained by variation in July lipid reserves for either males or females. However, July lipid reserves were negatively correlated with egg number, so females investing more in ova exhibited greater depletion of lipid reserves. These results are discussed in the context of the relationship between body condition and the onset of maturation in salmonids, relative investment in reproduction and sexual differences in the cost of reproduction.  1997 The Fisheries Society of the British Isles  1997 The Fisheries Society of the British Isles Key words: maturation; growth; reproductive investment; Salvelinus alpinus; Arctic charr. INTRODUCTION It is well established that the onset of the annual cycle of gonadal development in many temperate fish species is activated by photoperiod and temperature regimes (Scott, 1979). However, it is commonly found that not all individuals within a cohort exposed to appropriate environmental conditions mature in a given year. Among those that do, the extent of investment in gonadal tissue may be highly variable (Adams & Thorpe, 1989a). It is a matter of considerable fundamental and applied interest to identify the factors that determine which individuals within a population or cohort mature in any given year and the extent to which individuals invest in gonadal tissue. The most thorough investigations of individual variation in response to the external cues that control maturation have been carried out on the Atlantic salmon Salmo salar L. There is now considerable evidence that growth rate, or some other measure of energy acquisition, is critical in determining the onset and extent of gonadal development in this species. Thorpe’s (1986, 1994) model defining the relationship between seasonality, growth and onset of sexual maturation has now been tested extensively. Acquisition of surplus energy (in the form of both lipid deposition and body growth) in the winter prior to 750 0022–1112/97/100750+ 10 $25.00/0/jb970476  1997 The Fisheries Society of the British Isles