Does size matter? A cautionary experiment on overoptimism in length-based bioresource assessment

Abstract

Recently, there has been considerable progress in the development of neurolipofuscin-based age determination methods for crustacean stock assessment. Initial applications to lobster and crab fisheries suggest some important method-related differences between conventional length-based assessment parameter estimates and those derived with the new aging technique. Here, for the first time, we aim to clarify the basis for and implications of some of these discrepancies using an experimental fishery context. We estimate von Bertalanffy growth parameters (k and l∞), longevity (tmax), total and natural mortality (Z and M, respectively), maximum sustainable relative yield-per-recruit (MSY'/R), and the exploitation rate that produces MSY'/R (EMSY'/R) for a freshwater crayfish (Pacifastacus leniusculus) population by length–frequency analysis and tag–recapture (length-increment-at-length) and compare these results with simultaneous neurolipofuscin demographic estimates. Our central finding is that the length-based approaches are largely blind to the biological reality of asymptotic postmaturational growth, with the consequence that longevity is prone to underestimation, late growth trajectories and mortality rates to inflation, and sex differences to misjudgment. This inherent bias is likely to lead to pervasive undervaluing of potential yields and overly optimistic target exploitation rates that will heighten the risk of growth and recruitment overfishing. Neurolipofuscin offers a means of identifying and overcoming this important problem.