Alternative life-history strategies in male Atlantic salmon (Salmo salar L.).

In this thesis I looked at the choice of mating tactics in male Atlantic salmon by determining the fertilisation success of individuals adopting different mating behaviours. In this species, as in most salmonids, males can either become sexually mature early in the life cycle, as small freshwater juveniles called parr, or undergo a sea migration before maturing. The alternative behaviours are thought to be contingent on environmental and social circumstances, although some researchers see them as mixed evolutionarily stable strategies. Here, using the charomid cloning method I isolated eight highly polymorphic minisatellite DNA markers and, using two that showed heterozygosities above 80%, fingerprinted more than 2,000 salmon embryos and measured for the first time the fertilisation success of mature parr and anadromous males in a wild Scottish population. The percentage of eggs fertilised by mature parr was estimated to be between 35% and 40% in this population, more than double the two previous estimates reported in semi-natural environments. In mating experiments in captivity I manipulated the number and body size of parr. Pairs of migrants were allowed to spawn in an enclosure with mature parr. Analysis of paternity revealed a strong correlation (r2=0.95) between parr size and reproductive success. The results also provided further support to a hypothesis predicting negative frequency dependent reproductive success in this species. The results in both the experiments and the field study are used to develop an optimisation model for understanding life history variation in this species. The model assumes that the choice of mating tactics is contingent on body size. General agreement between the results and observations in wild populations is taken as supportive of the important role that environment plays in male mating tactic choice in Atlantic salmon.