Is individual variation in competitive performance of reared juvenile cod influenced by haemoglobin genotype?

To succeed in scramble competition for food an individual fish will have to have characteristics that allow it to respond rapidly to encountered prey. A trait such as metabolic rate, which has a positive effect on oxygen cons umption and growth rate, is likely to be positively correlated with the tra its that determine the speed of reaction. An important factor underlying me tabolic rare may be the transport efficiency of oxygen from the gills to th e: respiring tissue and this is mediated by the structure of the haemoglobi n molecule. In cod, two structures of this molecule exist due to polymorphi sm at the HbI* locus. An individual cod may be homozygous (HbI*1/1 or HbI*2 /2) or heterozygous (HbI*1/2). Evidence exists in the literature that HbI*2 /2 fish have higher growth rate and earlier maturation and higher transport efficiency of oxygen at low temperature. However, no study has examined wh ether this could be associated with fish behaviour. In a study reported her e we designed an experiment to test the hypothesis that fish with the HbI*2 /2 genotype have a higher motivation to feed and are better competitors tha n individuals with the other haemoglobin genotypes and that they will eat a larger share of the prey. We use prey capture success early in a feeding t rial and the rank of the first prey taken, as proxy variables for competiti ve performance. Randomly chosen one-year-old cod Gadus morhua L. in small g roups were tested experimentally for individual responses to prey offered s equentially. We analysed the effect on competitive performance of haemoglob in genotype, group, fish size, sex, maturation status and unobserved effect s using Components of Variance Analysis, which accounts for repeated observ ations from the same individuals. The most successful fish were usually amo ng the first to feed and tended to possess haemoglobin genotype HbI*2/2. Ot her factors such as body size, sex, stage of maturation and group also had effects which may modify the effect set by genotype. Our results suggest th at the link between HbI* genotype and growth is through feeding behaviour a nd it supports the idea that fish with HbI*2/2 genotype are better able to support an active metabolism. The results obtained are among the first on f ish that show that variation in feeding behaviour could be under genetic co ntrol.