Smolting Under Time Constraints: Is There a Seasonal Decline in Smoltification Size of Subyearling Pacific Salmon?

Optimal timing of smoltification and downstream migration of juvenile salmonids requires matching their physiological preparedness with the timing of optimum ecological conditions. Directional climate change is expected to increase water temperatures and decrease stream flows that will weaken the synchronicity of environmental cues such as photoperiod, temperature and stream flow. Thus, stream conditions will deteriorate earlier in the spring-summer season. The costs of not being physiologically prepared for emigration when conditions decline are significant, and fitness is known to decrease for late migrants. Many smoltification studies focus on salmonid species that can delay smolting for another year if unprepared and point to a fixed threshold size. I propose that fish from populations with subyearling migration have potential for a plastic response to seasonal progression by smolting at smaller body sizes. I studied the effects of seasonal constraints on the timing and size at smoltification of subyearling ocean-type Chinook salmon from Southern Georgian Bay. I varied growth rates in a hatchery setting under a natural photoperiod and compared body size and timing of smoltification over the migration season. A seasonal decline in size at smoltification will indicate that later in the season, individuals prioritize physiological preparedness and emigration over continued growth.