Change-in-ratio estimates of lobster exploitation rate using sampling concurrent with fishing

Abstract

We present a continuous change-in-ratio (CIR) method for estimating lobster exploitation rate using data from monitoring traps continuously sampled during fishing. The exploitation rate is estimated by fitting a nonlinear model to ratios of exploited catch over total catch (exploited plus an unexploited reference class) as a function of the cumulative exploited catch. Confidence intervals are obtained by bootstrapping. The method is applied to data collected by nearly 100 lobster fishers who sampled monitoring traps in fishing areas of Nova Scotia, Canada, from 1999 to 2001, and to simulated data. Best estimates are obtained where the exploited and the reference length classes are adjacent and narrow. A method to predict the impact of season length restriction on exploitation rate is presented. Simulations demonstrate that the method displays some robustness relative to departures from the model's assumptions. Exploitation rate estimates decline for length classes in which the minimum legal carapace length has been increased. The continuous CIR method can provide daily, local, and length-specific estimates of exploitation rate. For similar sample sizes, continuous CIR estimates are better than CIR estimates based on pre- and post-season sampling. A continuous CIR method is cost efficient because the data can be collected during regular fishing activity.