Assessing fish stocks when catches are misreported: Model, simulation tests, and application to cod, haddock, and whiting in the ICES area

Abstract

Reporting of catches from several of the most commercially-important gadoid stocks in the North Sea and the west of Scotland areas (ICES Divisions IV and VIa) is believed to have become incomplete in recent years. This study uses a modification of a stock-assessment model due to Haist et al. (1993) (Canadian Special Publications in Fisheries and Aquatic Science, 120, 269-282) to explore the precision, accuracy and bias of estimates of parameters of management interest when catch information is incomplete but survey estimate of stock size and biological sampling of the age-structure of catches are known. It is concluded that the model can be used to estimate stock size, and hence the future catch for a known fishing mortality, with quite good accuracy even when catches are under-reported. However, estimates of actual catches and of fishing mortality were very imprecise and very inaccurate. Parameter estimates from the model had less bias, similar precision, and greater accuracy than a conventional 'tuned-VPA' calculation. The model was used to assess population parameters in stocks of cod (Gadus morhua) in the North Sea and off the west of Scotland, and of whiting (Merlangius merlangus) off the west of Scotland. Results from this analysis confirm the present belief that catches of west Scotland cod have been substantially misreported since 1991 but that reported North Sea catches are approximately correct. It is proposed that the model may be a useful addition to present methods used to assess these stocks, and the conclusion is proposed on the basis of the simulation experiments that in cases where misreporting is important, catch forecasts for a target fishing mortality will be more accurate than forecasts made for status quo fishing mortality.