2015 Assessment of Greenland Turbot (Reinhardtius hippoglossoides) in the Bering Sea and Aleutian Islands
Analyses of new data (namely size and age composition data for 2013 – 2015) made available in September 2015 exacerbated a data conflict with the NMFS EBS Shelf and Slope trawl surveys necessitating unexpected model configuration changes to resolve what are clear structural misspecifications. The EBS shelf survey provides a reasonable index of young fish but as they age, they clearly exit the survey area. The EBS slope survey provides an index of adult fish and typically occurs every other year (except that the 2014 survey was dropped so the most recent data is from 2012). Initial runs of the 2014 Model with the new data (Model 14.0) suggested that recent high recruitment estimates (2007-2010) are closer to average, likely reflecting a change in the availability of these fish to the Shelf survey gear. Re-weighting shelf and slope survey composition data to better account for shifts in distribution relative to survey gear appears to improve model diagnostics while acknowledging that Greenland turbot are distributed to a large degree outside the survey areas and are affected by thermal conditions (shifting further north in warmer years in the EBS).
To simplify data conflicts, a model in which the ABL longline data were removed was evaluated with the justification that data were aggregated by sex and fit poorly. The lack of fit was likely due to the high degree of sexual dimorphism found in this species (bimodal size distribution when aggregated). Another factor in defense of omitting these data was that whale depredation in recent years in the EBS for Greenland turbot specifically has increased substantially and likely affects the reliability of this as an index. We adopted the naming convention proposed in September 2015 so that “Model 14.0” represents the configuration and data types used in the model accepted in 2014 with 2015 data additions.