Summiting the steepness mountain: Current and alternative approaches in determining steepness for West Coast rockfish stocks

Chantel Wetzel1
1NWFSC
October 2, 2018 9:00 (PST): FSH 203

Summiting the steepness mountain: Current and alternative approaches in determining steepness for West Coast rockfish stocks

Productivity, termed steepness, in West Coast rockfish stocks has historically been difficult to estimate within stock assessment models. Since 2007, West Coast Estimating the rockfish assessments have used a regional meta-analysis prior to estimate or fix the steepness parameter. However, the information about the steepness parameter has varied among assessments for the same species over time and the number of species included in the meta-analysis has varied since 2007 resulting in variability in the prior mean for steepness being applied in assessments for rockfish species for each West Coast groundfish assessment cycle. The assumption regarding steepness is highly influential in the assessment model and the majority of assessments for rockfish species have been unable to estimate steepness resulting in the parameter being fixed at the prior mean. This was a major issue in the 2017 Pacific ocean perch assessment where the model results were deemed implausible when steepness was fixed at the prior mean. The steepness value applied in the final model was determined using a new approach for West Coast rockfish stocks where a model-ensemble approach with alternative values of steepness were equally weighted. This presentation will examine the approach that was used in the Pacific ocean perch assessment, explore the results of this approach being applied to other rockfish stocks, and the potential challenges to this approach.

Posted in Fisheries Think Tank.

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