Allozymeheterozygosity,dateoffirstfeedingandlife historystrategyinAtlanticsalmon I. D. MC CARTHY * †, J. A. S A ¤ NCHEZ ‡ AND G. B LANCO ‡ *School of Ocean Sciences, University of Wales-Bangor, Askew Street, Menai Bridge, Anglesey LL59 5AB, U.K. and ‡Universidad de Oviedo, Departomento de Biologı´ a Funcional, Area de Gene¤ tica, Julian Claverı´ a 6, 33071Oviedo, Asturias, Spain (Received 24 October 2001, Accepted 13 November 2002) The relations between allozyme heterozygosity, relative date offirst feeding and life historystrategy in juvenile Atlantic salmon Salmo salar were examined using eggs obtained from a 400 family cross (20 male  20 female adult Atlantic salmon). Multilocus heterozygosity, through its positive associations with the timing of first feeding and growth rate, was correlated with life history strat- egy in juvenile Atlantic salmon, albeit under genotype  environmental (temperature, food avail- ability) regulation. Under hatchery conditions, a10 daydifference was observed in the relative date of first feeding between early and late first feeding Atlantic salmon. Early first feeding Atlantic salmon exhibited a significantly higher mean heterozygosity, grew faster at ambient water tem- perature (April to November) and a significantly higher proportion adopted the early freshwater maturation (age 0þ years, male fish) or early migrant (age1þ years, mainly female fish) strategies compared to late first feeding Atlantic salmon. Elevated water temperatures over the winter (December to April, >105  C) provided additional growth opportunity allowing previously mature male parr (mainly early first feeders) and lower modal group parr (mainlylate first feeders) to adoptthe early migrant strategyby the following spring. # 2003 The Fisheries Societyofthe British Isles Key words: developmental rate; first feeding; growth; heterozygosity; life history strategy; Salmo salar . INTRODUCTION Intraspecific variation in life history strategy is found in many animal and plant species. Commonly adopted life history traits that exhibit variation include growth patterns, age and size at maturity, number, size and sex ratio of offspring, and age-specific or size-specific differences in reproductive investment (Stearns, 1992). Variation in any of these life history traits will lead to variation in fitness between individuals. The presence of a range of alternative life history strategies within a population, however, ensures that at least some individuals will achieve successful reproduction invarying environments (Stearns,1992;Thorpe et al.,1998). The lifecycle ofthe Atlantic salmon Salmo salar L. is characterized bygreat flexi- bility in life history strategy, both between and within populations, with variation found in the length of the freshwater ( i.e. age at smolt migration) and marine phases †Author to whom correspondence should be addressed.Tel.: þ 44 (0) 1248382862; fax: þ44 (0)1248716367; email:i.mccarthy@bangor.ac.uk Journal of Fish Biology (2003) 62, 341^357 doi:10.1046/j.0022-1112.2003.00026.x, available online at http://www.blackwell-synergy.com 341 # 2003 The Fisheries Society of the British Isles