Sensitivity of predicted cohort size and catches to errors in estimates of fishing mortality in the terminal year

Abstract

Abstract Horbowy, J. 2008. Sensitivity of predicted cohort size and catches to errors in estimates of fishing mortality in the terminal year. – ICES Journal of Marine Science, 65: 1227–1234. Formulae for the sensitivity of projected cohort size and catches to errors (bias) in estimates of fishing mortality in the terminal year were developed. Assessment models allowing for random errors in the observed catches as well as models in which catches are treated as exact were considered. The formulae were applied to a Gulf of Riga (Baltic Sea) herring assessment to show how well they estimated prediction errors and to evaluate the effect of assessment errors on predictions. The errors propagate quickly with time, and the higher the fishing mortality, the bigger the projection error. The errors in predicted catches are somewhat lower than the errors in predicted cohort sizes. The formulae developed show that with moderate error in estimated fishing mortality (20%), the errors in predicted cohort size can reach 100%, and the errors in predicted catches may be 50% for fishing mortality estimated at 1.0 in the terminal year and the status quo prediction. As the Gulf of Riga herring case demonstrates, the overall error in predicted stock size and catches may be lower when terminal fishing mortality is underestimated at some ages and overestimated at others (cancelling effect).