A historical review of selectivity approaches and retrospective patterns in the Pacific halibut stock assessment

• Retrospective bias in stock assessment results for Pacific halibut has been identified during three historical time-periods of analysis.
• In each case, amelioration of the bias was achieved through modification of the selectivity assumptions in the population dynamics model.
• Investigation of more flexible assumptions for the modeling of selectivity may be warranted for other species whenever evidence of poor model behavior is present.

The Pacific halibut stock assessment has proven to be a particularly challenging application for the estimation of selectivity. Contributing factors include: extremely pronounced temporal changes in length-at-age, a steep vulnerability curve for commonly used hook sizes, a minimum length limit, relatively late (∼age 6–10) appearance of fish in survey and fishery data, and geographic heterogeneity in demographic parameters coupled with pronounced spatial trends in population abundance over time and significant ontogenetic migration over the stock range. Historical stock assessments have variously modeled selectivity as a function of length or age, employing nonparametric forms in attempting to account for these various factors. Despite these efforts, a strong retrospective bias in model results occurred during three separate time periods; each of which ultimately required modification of the selectivity parameterization to ameliorate that bias. This paper provides a summary of historical approaches, and the methods employed to address the most recent retrospective pattern.