In iantaylor-NOAA/Lingcod_2021: Estimate the Status of Lingcod off the US West Coast in 2021

The base-model configuration was developed with the goal of balancing parsimony with realism while fitting the data. To achieve parsimony, some simplification of the model structure was assumed relative to known processes, which may impact the interpretation and fit to specific data sets. For example, a clear break between the northern and southern stock at Cape Mendocino is unrealistic but we do not currently have the resources necessary to add spatial dynamics to the stock assessment or estimate the level of overlap between the stocks.

Patterns of sex-specific selectivity were apparent in the data, particularly for the fishing fleets. Unfortunately, we were unable to configure the model in such a way that the model fit all data sources equally as well as the base-model configuration when attempting to account for these patterns. Numerous attempts at sex-specific selectivity were explored shortly before and during the stock assessment review. However, overparameterization proved an insurmountable problem. There are more than 50 selectivity parameters in the base model (with selectivity equal for females and males), so adding sex-specific offsets or separate parameters caused estimation problems and implausible selectivity patterns. Options explored included sex-specific offsets for the height of the peak selectivity as well as offsets for the position of the peak and the descending slopes. Therefore, new research to develop an theoretical basis for sex-specific differences in selectivity or availability using differences in spatial distribution or behavior may be a more fruitful path for future selectivity setups than relying simply on fits to data and other model diagnostics.

Uncertainty in parameter estimates are quite large relative to recent assessments because of the choice to estimate both natural mortality and steepness. Recent work has shown the utility of estimating both parameters with respect to management reference points, and although estimates provided in this document are imprecise, we predict that the estimated reference points are less biased than if the model would have been configured with one or more of these parameters as fixed inputs rather than estimated. Estimating both parameters led to counter-intuitive differences in estimates of natural mortality between the southern and northern areas. Hopefully, future work on parameterizing selectivity will lead to more precise estimates of male and female natural mortality given the life history of this species, specifically the nest-guarding behavior of males.

{asis, opts.label = "north"} The internal estimates of total catch including dead discards for some recent years are higher than those provided by the Groundfish Expanded Multiyear Mortality (GEMM) reports. This may be the result of the status-quo data weighting approach which applies the same sample size multiplier to the discard and retained length composition data within any given fleet, in spite of the sample sizes often being unbalanced among these sources relative to the magnitude of the catch. Alternative models were explored during the assessment review, whereby the input sample sizes for the discard data were adjusted as an ad-hoc refinement to the data weighting or the uncertainty associated with the discard rate was reduced to force the model to fit these rates more precisely. Neither sensitivity had much impact on estimates of population trajectories or stock status so this issue was shelved for future research.

iantaylor-NOAA/Lingcod_2021 documentation built on Oct. 30, 2024, 6:42 p.m.