Extracting abundance indices from longline surveys : method to account for hook competition and unbaited hooks

摘要

The most commonly used relative abundance index in stock assessments oflongline fisheries is catch per unit effort (CPUE), here defined as the numberof fish of the targeted species caught per hook and minute of soak time.Longline CPUE can be affected by interspecific competition and the retrieval ofunbaited or empty hooks, and interannual variation in these can lead to biasesin the apparent abundance trends in the CPUE. Interspecific competition onlonglines has been previously studied but the return of empty hooks is ignoredin all current treatments of longline CPUE. In this work we propose somedifferent methods to build indices to address the interspecific competitionthat relates to empty hooks. We show that in the absence of information aboutempty hooks, the relative abundance estimates have constant biases with respectto fish density and this is typically not problematic for stock assessment. Thesimple CPUE index behaves poorly in every scenario. Understanding the reasonsfor empty hooks allows selection of the appropriate index. A scientificlongline survey is conducted every two years in the Strait of Georgia, BritishColumbia by Fisheries and Oceans Canada. The above methods are applied to buildthe time-series of indices from 2003 to 2009 for quillback rockfish (Sebastesmaliger). Due to variation in the incidence of non-target species, the indextrend obtained is moderately sensitive to the choice of the estimator.