Incorporating Time-Varying Catchability into Population Dynamic Stock Assessment Models

Abstract

Catchability is an important parameter in many stock assessment models because it relates an index of abundance to stock size. We review the theory and evidence for time-varying catchability, its effects on stock assessment estimates, and methods to include time-varying catchability in stock assessments. Numerous studies provide strong evidence that time-varying catchability is common in most fisheries and many fishery-independent surveys and can be caused by anthropogenic, environmental, biological, and management processes. Trends in catchability over time can cause biased estimates of stock size and fishing mortality rates in stock assessment models that do not compensate for them. Methods that use descriptive and functional relationships have been developed to incorporate time-varying catchability in stock assessment models. We recommend that the default assumption for stock assessments should be that catchability varies over time and that multiple methods of including time-varying catchability should be applied. Additional studies are needed to determine relative performance of alternative methods and to develop methods for selecting among models.

Keywords:

• index of abundance
• relative abundance
• catch per unit effort
• catch rate
• density-dependent catchability

ACKNOWLEDGMENTS

We thank J. Bence, A. Chester, C. Porch, M. Prager, an anonymous reviewer, and participants of the SEDAR workshop on time-varying catchability for helpful comments and discussions that improved this manuscript. Partial funding for this work was provided by the Southeast Fisheries Science Center of the National Marine Fisheries Service and NOAA Chesapeake Bay Office under award #NA06NMF4570298. This is contribution number 4351 of the University of Maryland Center for Environmental Science Chesapeake Biological Laboratory.