Latitudinal cline of ocean dependence in a diadromous fish.

Diadromous fishes exhibit latitudinal clines of ocean dependency at inter- and intra-species levels. A pattern of ocean dependence at high latitudes and river dependence at low latitudes is explained by relative aquatic productivity. Such latitudinal productivity clines may induce geographical variations in life-history diversity within migratory phenotypes. We hypothesized that the lifetime ocean dependency of a regional migratory salmonid would display a latitudinal cline that increased at higher latitudes. Freshwater growth rate decreased with higher latitudes, whereas marine growth rate was independent of latitude. The percentage of adult weight gain at sea was higher at higher latitudes. Relative weight gain (ln(ocean weight gain/freshwater weight gain)) decreased to zero at lower latitudes, indicating no growth benefit of going to sea at the southern distribution limit. These latitudinal variations in life history within salmonid migrants are consistent with the intra- and interspecific patterns and provide insight into the origin of diadromous migration but raise the question of whether the current definition of anadromy may be insufficient to fully capture the complexity and continuum of river-ocean migrations.