Auke Bay Laboratory (ABL)

Groundfish Assessment Program

Performance of Modern Age-Structured Stock Assessments with High Survey Measurement Error

The objective of AFSC bottom trawl surveys in Alaska is to obtain biomass estimates for all major Alaska groundfish species. However, many species of rockfish (Sebastes spp.) are patchily distributed and therefore are imprecisely estimated in these multispecies surveys, which are based on a stratified random design. This same general problem pervades the stock assessments of many fish species worldwide.

In our study, the stock assessment model for Pacific ocean perch (Sebastes alutus) in the Gulf of Alaska was used to explore the consequences of survey imprecision and other uncertainties in components of this type of model. The characteristics of the Pacific ocean perch assessment can be generalized to other long-lived, iteroparous fish species with uncertain survey biomass estimates. The results of the 2003 stock assessment model served as the "true" values, and simulated data sets were constructed in five experiments to answer the following questions:

1. What is the effect of measurement error in survey biomass estimates on stock assessment results?
   
2. What is the effect if the catchability coefficient changes over time from either gear changes or environmental changes?
   
3. Does adding an additional biomass index increase model precision?
   
4. How sensitive are model results to applying arbitrary weights to different data sources such as fishery length distributions and survey ages?
   
5. How sensitive are model results to prior distributions (a distribution representing prior knowledge about a parameter that influences estimation) imposed on key parameters?

The five simulation experiments yielded the following general answers to these questions:

1. High measurement error (coefficient of variation equal to 50%) rendered the stock assessment inaccurate and imprecise.
   
2. A catchability trend in the biomass index was undetectable and created a large bias in biomass and parameter estimates.
   
3. The addition of a second, more precise biomass index with a shorter time series improved performance of the model. Examples of an additional index could be a hydroacoustic index or a dedicated rockfish survey.
   
4. This type of assessment was robust to various data weightings, indicating that the stock assessment scientist could merely give each data source equal weight.
   
5. The prior distribution for natural mortality needed to be precisely specified, while the prior distributions for the catchability coefficient and recruitment variability could be relatively uninformative.

Overall, the study showed that the high measurement error in the survey index (for species such as rockfish) can render stock assessment intractable, data weighting was less important than expected, and prior distributions on parameters (except natural mortality) could be uninformative.

By Dana Hanselman

2005 Sablefish Longline Survey

The AFSC has conducted an annual longline survey of sablefish and other groundfish off the coast of Alaska since 1987. The survey is a joint effort involving the ABL and the Center’s Resource Assessment Conservation Engineering (RACE) Division. In February 2005, a new longline survey contract was awarded to two vessels over a period of 4 years. The Ocean Prowler will conduct the survey in 2005 and 2007, and the Alaskan Leader in 2006 and 2008. The schedule will be similar to previous years, with sampling in the Gulf of Alaska annually and biennial sampling of the Bering Sea (2005, 2007) and Aleutian Islands region (2006, 2008). The 2005 survey begins on 28 May and ends on 1 September.

By Chris Lunsford

>>>continued

AFSC Quarterly Research Reports Jan-Mar 2005