The Role of Male Choice in the Assortative Mating of Anadromous and Non-Anadromous Sockeye Salmon (Oncorhynchus Nerka)

Assortative mating between the anadromous (sockeye salmon) and non-anadromous (kokanee) forms of Oncorhynchus nerka was documented in Pierre Creek, Babine Lake, British Columbia. Sockeye males of all sizes mated almost exclusively with sockeye females. Only a small proportion of the smallest sockeye males (acks) were observed courting kokanee females. Kokanee males either courted kokanee females or acted as sneaks to sockeye pairs. We experimentally tested the hypotheses that assortative mating occurred because of: a) male intrasexual competition for the largest (sockeye) females; b) male choice dependent on male size; and c) male choice dependent on form. Our results demonstrated male competition was not the cause of assortative mating as has previously been suggested. Kokanee males only preferred sockeye over kokanee females when direct access to kokanee females was denied. Male choice was dependent on male size as has previously been demonstrated within kokanee, but appeared to also involve a form component. Kokanee males preferred kokanec females over the much larger and fecund sockeye females. The evolution of assortative mating between sockeye and kokanee fits models of sympatric speciation. An initial degree of assortative mating between forms is predicted by their environmentally induced great size difference. This promotes genetic divergence between the forms, which has been documented, accompanied by the evolution of premating isolating mechanisms. However, behavioural divergence is not complete in Pierre Creek. Kokanec males restricted from direct access to kokanee females prefer to sneak on sockeye pairs over kokanee pairs.