Using Length-Based Methods to Estimate Mortality Rates of Larval Pacific Lamprey

Addressing the ongoing decline of Pacific Lamprey Entosphenus tridentatus across its range requires an understanding of mortality during multiple life history phases. However, the unique biology of lamprey precludes the use of traditional methods for estimating mortality (e.g., catch curves). Issues include a lack of ageing data, size-specific gear selectivity, and the metamorphosis and outmigration of larger larvae. We used length-based methods to make inferences about natural mortality of larval Pacific Lamprey collected in the Willamette River Basin, Oregon. We used a mixed-effects model that incorporated data from repeated measures of length-at-age for individual fish, size-selectivity curves, larval to metamorphosed outmigrant transition probability, and length frequency information from our surveys to estimate the likelihood of different levels of natural mortality. Sensitivity analysis suggested an annual mortality of ~15%, which is exceptionally low for a larval fish. These estimates provide a baseline to which other assessments of Pacific Lamprey populations can be compared, and may be used to implement and evaluate management actions for these fishes. In addition, our approach further develops length-based methods that can be utilized in population analyses with fishes that display similar life histories or ageing-related challenges.