Abstract
Polyandry and competition for spawning sites in a sedentary brown trout population was studied using four highly variable microsatellite loci. All loci combined gave average exclusion probabilities of 0.91 and 0.97 based on two reference samples of spawners. All eggs from 22 redds of this population were collected. For 10 of these redds, tissue samples of potential mothers, i.e. females that were observed to show spawning behaviour on these redds, were sampled. Only in four cases, the observed four-locus genotype of the putative mother conformed with corresponding egg-genotypes. For all four redds polyandry was detected encompassing two fathering males. We applied a sequential procedure of multiple paternity analysis that allowed the assignment of a single paternal genotype to each individual egg, and thus, to quantify the genetic contribution of different males. In each redd, one male fertilised the majority of eggs (60-92%) and the contribution of additional males ranged between 8-40%. The total number of eggs in the four redds represented 11-21% of the total fecundity of the mothers. This indicates that females in this population spread their eggs among several redds. Eight cases of incomplete redd superimposition were detected among all 22 redds analysed indicating strong competition for spawning sites between females.