Don't count your eggs before they resorb: Early collection of ovarian follicles influences estimates of Lake Trout fecundity in Yellowstone Lake.

Objective: Understanding recruitment dynamics is necessary to predict population‐level responses to exploitation, management actions, or anthropogenic influences. Fecundity is commonly used as a metric of recruitment dynamics and can guide successful management of fisheries. However, an individual female's fecundity is not constant over time; females resorb ovarian follicles to regulate fecundity as they approach spawning. This suggests that sampling for fecundity too early may produce inaccurate estimates of relative fecundity. In Yellowstone National Park, suppression of invasive Lake Trout Salvelinus namaycush reduced the abundance of mature fish by 92% between 2012 and 2022. The continued efficacy of this suppression effort requires accurate assessments of reproductive potential of the population that remains. Methods: We sought to determine whether the timing of ovarian follicle collection affected estimates of mean relative fecundity. We collected ovarian follicles from female Lake Trout, which are autumn spawners, between mid‐August and early October in 2021 and 2022. The number of ovarian follicles per sample was counted to obtain estimates of relative fecundity for each female. Result: We observed a 13% decline in estimated mean relative fecundity between individuals that were sampled before mid‐September and those that were sampled after mid‐September. Conclusion: Our data support strategic timing of fecundity sampling to best capture the true reproductive capability of a population, which is a key metric used in models that guide adaptive management of fishes. Impact statementFemale Lake Trout undergo fecundity reduction through follicular atresia, reducing the number of ovarian follicles that may be spawned, which suggests that fecundity estimates based on fish that are sampled too early in the reproductive cycle will be inaccurate. [ABSTRACT FROM AUTHOR]