A critical size and period hypothesis to explain natural regulation of salmon abundance and the linkage to climate and climate change

We hypothesise that salmon year class strength is determined in two stages during the first year in the ocean. There is an early natural mortality tha t is mostly related to predation, which is followed by a physiologically-ba sed mortality. Juvenile salmon that fail to reach a critical size by the en d of their first marine summer do not survive the following winter. In this study we describe our initial tests of this critical size and critical per iod hypothesis using data from ocean surveys of juvenile salmon and from ex perimental feeding studies on coho. Conservative swept volume abundance est imates for juvenile coho, and possibly chinook, indicate that there is high mortality in fall and winter during their first year in the sea. Studies o f otolith weight show that the length and otolith-weight relationship for y oung coho changes in the early fall of their first ocean year. Studies of g rowth and associated hormone levels in feeding studies show that slow growi ng juvenile coho are stunted and deficient in an insulin-like growth factor -I (IGF-I). Juvenile coho sampled in September had low IGF-I values, indica tive of poor growth. The results of these studies provide evidence for the general hypothesis that growth-related mortality occurs late in the first m arine year and may be important in determining the strength of the year cla ss (brood year). The link between total mortality and climate could be oper ating via the availability of nutrients regulating the food supply and henc e competition for food (i.e. bottom-up regulation). (C) 2001 Elsevier Scien ce Ltd. All rights reserved.