Adult mortality and year-class strength of lake trout before and after alewife collapse in the main basin of Lake Huron

The collapse of alewives (Alosa pseudoharengus) and the lake-wide recruitment of wild lake trout (Salvelinus namaycush) distinguished two time-periods of lake trout rehabilitation in the main basin of Lake Huron. This paper compared lake trout adult mortality and year-class strength to explore the mechanisms for the rapid increase in lake trout abundance prior to the alewife collapse and the maintenance of lake trout status after the alewife collapse. The population metrics were derived independently from data for each of the two time-periods, using a recently described procedure of catch-curve regression with multiple years of data. Prior to 2000, the instantaneous total mortality rate was much higher than that for the post-2000 time-period. Year-class strength of stocked lake trout increased largely for those year-classes of the 1990s. The increases in year-class strength and decreases in adult mortality established the highest lake trout abundance at beginning of the post-2000 time-period. Wild lake trout first appeared with the year-classes of the 1990s, and the year-class strength really took off after 2000, prior to the 2003 collapse of alewives, while the alewife biomass was in steady decline during the 1990s before the population collapse. In contrast to wild lake trout, the year-class strength of stocked lake trout declined rapidly after 2000. Since the late 2000s, with the low adult mortality being maintained for both wild and stock lake trout, their year-class strength had similar dynamics although the variation for stocked lake trout appeared to be higher than that for wild lake trout.