Assessing fish growth changes under an ecosystem regime shift: The approach of linear mixed-effects modeling with application to Lake Huron lake trout

Linear mixed-effects modeling has been used to characterize fish growth changes over time, but early studies did not fully consider the implicit relationship among the three major factors of fish age, year-class, and sampling year in model development. Our objective is to resolve this issue and develop a linear mixed-effects model to clearly assess how lake trout (Salvelinus namaycush) growth has responded to the ecosystem regime shift in Lake Huron. We found that, from the late 1970s through the early 2000s, lake trout weight-at-age in northern Lake Huron had large variation with no apparent temporal trend, whereas the weight-at-age in southern Lake Huron steadily declined but remained larger than that in northern Lake Huron. The difference between northern and southern Lake Huron almost disappeared prior to the 2003 collapse of alewives, which accelerated the weight-at-age declines during the 2000s in both regions. By 2010, lake trout growth had a modest recovery and then stabilized. Our model fitted and explained variation in lake trout weight-at-age between years, regions, and recruitment origins, with the fixed age effect representing the general growth trajectory, the fixed year effect as the year-specific growth index, and the random year-class effect as the cohort-specific growth index.