Estimating Skipped Spawning in Lake Trout in Lake Superior: Detection and Implications for Stock Assessment

We found that some lean (12.2%) and siscowet (58.0%) lake trout (Salvelinus namaycush) in populations in southern Lake Superior do not reproduce each year (i.e., display “skipped spawning”) even though they have reached puberty. We measured (histologically) skipped spawning in lake trout in southern Lake Superior and assessed its impact on reproduction by developing length-based and age-based ogives (i.e., probabilities of being mature). In populations in which skipped spawning is negligible, a maturity ogive can be considered equivalent to a reproduction ogive (probability of being reproductive). However, in populations in which skipped spawning is present, such as in lean and siscowet lake trout in southern Lake Superior, maturity and reproduction ogives are very different. We evaluated the influence of skipped spawning on a key stock assessment quantity, spawning stock biomass, by applying maturity and reproduction ogives to simulated populations. Our population simulations indicate that spawning stock biomass can be overestimated (8.3% for leans and 475% for siscowets) when skipped spawners are assumed to be a part of the spawning stock. An inflated reproductive potential of the population could allow for excessive fishing if quotas are based on these biased spawning stock biomass estimates.