We conducted a statistical catch-at-age (SCAA) assessment, also known as an integrated assessment, of the Chinook salmon Oncorhynchus tshawytscha population in Lake Huron's main basin to describe the dynamics and assess population abundance since the species began being stocked in 1968. The purpose of this assessment was to determine whether recent declines in Chinook salmon recreational harvest were indicative of an overall decrease in population abundance, and to estimate the magnitude of that decline. Data sources integrated into the assessment model included recreational harvest and effort, returns to the Swan River weir by year-class, age composition of recreational harvest (overall and of mature fish), and proportion mature by age. Parameters estimated included time-varying age-0 natural mortality rates and recreational fishing catchabilities, age-specific recreational fishing selectivities, and the parameters of a logistic function relating maturation probability to fish age and weight. The fitted model did a reasonable job of predicting recreational harvest, Swan River weir returns, age composition of recreational harvest, and proportion mature by age. The model predicted that abundance of age-1 and older Chinook salmon has declined by approximately 98% since the mid-1980s, with an estimated abundance of age-1 and older fish of approximately 65,000 in 2009. The primary cause for this decline has been a substantial increase in age-0 natural mortality rates, which peaked at an instantaneous rate of around 6.3 in the late 2000s, although fishing mortality rates have also been quite high. Initially, declines in survival were associated with increases in overall predator abundance in the lake. The most recent and large declines were associated with a decrease in the abundance of alewives Alosa pseudoharengus. We suspect that declines in age-0 survival reflect both direct effects of food limitation and increased predation mortality by larger fish, and are consequences of decreased abundance of other prey fishes.
