A Review of ICES Assessment Data and Advice for Selected Fish Stocks

A review has been undertaken to examine the history of ICES advice on several fish stocks that are important to Scottish fishermen, and in particular to examine: The extent to which estimates of the abundance of the stocks have changed over time; and, how well the predictions of changes in the abundance of the stocks matched the changes subsequently estimated to have taken place. The review is not intended as a criticism of ICES but rather is intended to provide the fishing industry with a better understanding of some aspects of ICES advice.
Changes in Estimates of Spawning Stock Biomass
Three of the four stocks covered by this review – North Sea cod, North Sea whiting, and West of Scotland whiting – have seen large revisions in the estimates of their abundance (spawning stock biomass; SSB) over the last decade, generally due to changes in the assessment methodology used.
These revisions may not be readily apparent to many stakeholders but they highlight the uncertainties inherent in the stock assessment process and the extent to which the assessments are sensitive to changes in the assumptions made in the assessment models. Such uncertainties and sensitivities are generally not clearly articulated in the ICES Advice reports and may not be fully appreciated by stakeholders. This may create a risk either of excessive confidence being placed in estimates of fish stock abundance, or of confidence in the assessment being eroded.
Predictions of Future Spawning Stock Biomasses
Comparison of the changes in spawning stock biomass predicted by ICES with the actual changes subsequently estimated to have occurred provide a mixed picture of the reliability of the predictions (that is, how well the predicted changes in abundance match the subsequently estimated actual changes).
While the predictions made for the North Sea and West of Scotland haddock stock appear to have been relatively good, those for the North Sea whiting stock have been very poor. The results for the North Sea cod stock were more mixed but may suggest that confidence in the reliability of the predictions for this stock should be limited also.
The same may be true of the West of Scotland whiting stock, although that judgement is only subjective. Given the importance of the predictions of future changes in stock size to the assessment process, the apparent poor reliability of the predictions must be a matter of some concern.