Caitlin Stern, Alaska Department of Fish and Game

Indices of abundance are key inputs to stock assessment models, influencing model outputs that shape fisheries management decisions. Design-based methods for producing these indices from fishery-independent surveys come with potential drawbacks, including highly variable estimates when spatial dependence in survey catch is present yet excluded from calculations, and biased results when survey sampling deviates from survey design. Implications for stock assessment models include the potential for better predictive ability when using model-based rather than design-based indices. The stock assessment for red king crab (Paralithodes camtschaticus) in Norton Sound, Alaska, uses design-based indices of abundance based on three fishery-independent surveys. These surveys cover different years and areas, and within a survey the locations sampled sometimes varies among years, suggesting a need for spatiotemporal standardization to improve abundance estimation. Here, I evaluate the effects on key outputs of the stock assessment model of using design-based survey indices versus a spatiotemporally standardized model-based index of abundance. Fits to size composition data were similar for models using design-based versus model-based indices, while fits to standardized fishery catch per unit effort indices were poorer for models using model-based indices. Retrospective patterns were less extreme for models using the spatiotemporal model-based index, indicating an improved ability to predict stock biomass in the terminal year. The estimated scale of the population was lower across the time series when using the model-based index. While estimated stock status was comparable or higher when using the spatiotemporal model-based index, model-estimated abundance was reduced, as was recommended harvest.