Catchability

Catchability is a parameter used in fisheries science to describe how easily individuals from a population can be captured by a particular fishing method or gear, under a given level of fishing effort. In its common form, catchability is represented by the coefficient q in the equation C = q E N, where C is catch (numbers or biomass), E is fishing effort, and N is population abundance. When q is constant, CPUE (catch per unit effort) is proportional to N, making CPUE a proxy for abundance.

Catchability is not fixed; it varies across time, space, gear types, and target species. Factors influencing catchability include gear design and efficiency, fish behavior and distribution, habitat structure, schooling, regulatory restrictions, and changes in fleet composition or proficiency. Environmental conditions can also alter catchability by affecting catch rates or visibility.

Estimation and application: catchability is typically estimated within stock assessment models, either as a constant parameter or as a function of effort, time, region, or environmental covariates. Methods include analysis of CPUE data, depletion approaches, tag-recapture studies, and integrated assessments that combine catch, effort, and abundance indices. Understanding q helps relate observed catches to true abundance and to forecast responses to management actions.

Limitations: assuming constant catchability can bias results; heterogeneity among individuals, spatial structure, gear selectivity, and imperfect detection can distort estimates. Therefore, estimates of catchability should be treated with care and routinely updated with new data in adaptive management.