ON THE DEVELOPMENTAL PATHWAY TO NONANADROMY IN SOCKEYE-SALMON, ONCORHYNCHUS-NERKA

The evolution of nonanadromous forms of salmonids has been linked to a daptive similarities between migration to the sea and migration to lak es, and to inhibition of smoltification by rapid development rates and early maturation. We examined the seasonal cycle of seawater adaptabi lity in a landlocked (10 000 years) population of kokanee, Oncorhynchu s nerka, to determine if the typical anadromous cycle persists in the absence of recent gene flow from sockeye salmon, as would be expected if components of smoltification are beneficial to a lacustrine existen ce. We also monitored the effects of maturation on seawater adaptabili ty, to determine if they had an inhibitory effect on smoltification, a s would be expected if a developmental conflict was responsible for th e evolution of nonanadromous forms. Maturation was measured through th e growth of gonadal tissue and the increase in circulating testosteron e and 11-ketotestosterone. Landlocked kokanee showed the seawater adap tability cycle typical of sockeye salmon, and maturation had no inhibi tory effect on seawater adaptability until near, or at, full maturity. We conclude that at least some components of smoltification must be b eneficial to a life in lakes to account for the continued maintenance of the cycle. Further, rapid development and maturation on their own d o not appear to account for the evolution of nonanadromous populations of O. nerka.