Stock Assessment and Management Implications of Fishing Gear Selectivity

Stock assessments and resulting fishery management decisions are highly sensitive to estimates of selectivity and efficiency of fishing gear.  Selectivity and catchability of fishing fleets and resource surveys are typically estimated parameters in stock assessment.  However, stock assessment models can produce unrealistic estimates of catchability and inaccurate forms of selectivity.  The implications of estimating ‘dome-shaped’ selectivity (i.e., large, old fish being less vulnerable to the fishing gear than younger, smaller fish) when the actual selectivity is ‘flat-topped’ (i.e., large, old fish being fully vulnerable) produces overestimates of adult biomass and underestimates of fishing mortality, and vice-versa.  Inaccuracies in estimates of catchability (the product of availability and efficiency) are inversely proportional to the resulting bias in stock size estimates.  Selectivity and catchability are not directly informed in most stock assessments.  However, field observations of relative selectivity and efficiency can be used to directly inform those important parameters and to help avoid unrealistic estimates of selectivity patterns and catchability.  Several examples from New England groundfish demonstrate the implications of selectivity and catchability on assessment and management, and recent examples are provided to demonstrate how field observations can improve both stock assessment and fishery management.