MethodologyScientific basisData and methods
In 2008, a benchmark procedure was applied to the stock assessment.
The advice is based on an analytical assessment, which takes into consideration catch data and eight surveys (acoustic surveys of adults and juveniles, larval survey, and 0-group survey), of which five surveys are continuing. The assessment is conducted using a VPA based model from the TASACS package. As agreed at the benchmark, the VPA analysis is restricted to the years 1988–2009, which is regarded as the period representative of the present production and exploitation regimes. For the period before 1988, the results are derived from the 2006 assessment, which are consistent with this year’s assessment.
The present assessment is an updated assessment, using the models, configurations and procedures agreed at the benchmark. The benchmark also examined cases where individual survey points might be excluded from the assessment model. Based on the agreed examination procedures, the 2009 value from the Norwegian herring larval survey on the Norwegian shelf (survey 8) was excluded from the assessment. This data point was excluded as the abundance estimate was anomalously low and the average size of the larvae was large suggesting that the survey was late relative to spawning. A similar rationale was used in the past to exclude the 2003 data point from the Norwegian larval survey. Uncertainties in assessment and forecast
There is uncertainty about recent recruitment estimates. A source of uncertainty is caused by the lack of coherence in some of the survey information for the youngest ages. In addition, the catch in 2008 from a few directed fisheries by Norway and Russia in the 3rd and 4th quarter contained relatively high numbers of 1 and 2 year olds (year classes 2007 and 2006). The available surveys, however, indicate that these year classes are not strong.
There is uncertainty in the estimate of SSB and fishing mortality related to the exclusion of the 2009 survey point of the Norwegian herring larvae survey on the Norwegian shelf. Including the 2009 survey point would have resulted in the estimate of SSB being about 10% lower.
The international ecosystem survey in the Nordic Seas in May is the most important survey in the assessment and will remain so in future assessments. It is important that this survey is maintained and that the vessels participating in this survey have access to the survey grounds. As well it is essential to maintain good geographical survey coverage to avoid increases in assessment uncertainty and maintain the integrity of the assessment.
While discarding of this stock is estimated to be low, an un-quantified amount of slippage is known to occur, thus it has not been possible to account for slippage in the assessment.Comparison with previous assessment and advice
The perception of the state of the stock and its exploitation has not changed markedly. Compared to last year, the SSB for 2008 is estimated to be about 5% higher and the fishing mortality in 2007 to be about 3% lower (see Figure 18.104.22.168).
The basis for the advice is the same as last year.