Growth stability after the collapse of alewives in Lake Huron and direct size-at-age comparisons between stocked and wild lake trout

The 2003 collapse of alewives (Alosa pseudoharengus) has allowed lake-wide recruitment of wild lake trout (Salvelinus namaycush) for many consecutive years in the main basin of Lake Huron, but the loss of this major pelagic prey fish also raised concerns about the carrying capacity of the lake for lake trout rehabilitation. In this paper, size-at-age data for individual lake trout from the 2003 and more recent year-classes were analyzed, and the additive effects of age, year-class, and sampling year were estimated and compared using a linear-mixed model. After declines to a very low level following the collapse of alewives, lake trout size-at-age recovered during the late 2000 s and early 2010 s, and then stabilized, although the new level was not as high as that in the pre-2003 time-period with abundant alewives and rainbow smelt (Osmerus mordax). Lake Trout size-at-age also did not maintain a meaningful difference between northern and southern Lake Huron. Furthermore, the total length and body mass of wild lake trout were smaller at ages 2–4 than that of stocked lake trout. The differences became insignificant at ages 5–6 and negligible at ages 7–19. The direct size-at-age comparisons between stocked and wild lake trout were different from the comparisons of growth parameters based on the von Bertalanffy growth function, suggesting that it is important to analyze actual size-at-age measurements and not to fully rely on the growth-parameter comparisons.