The Impact of Spatial Structure Assumptions on the Pacific Sardine Assessment

The results (and credibility) of stock assessments can be impacted by assumptions regarding spatial structure and movement. For the northern sub-population of Pacific sardine (Sardinops sagax caerulea), seasonal length-dependent migration has been described in the literature, as well as the possible presence of two stocks, rather than one. However, the current assessment, conducted using the Stock Synthesis 3 (SS3) framework, assumes a spatially-aggregated stock with constant growth across the entire coast and fleets with different selection patterns in the four areas included in the assessment (Ensenada, southern CA, northern CA and Pacific Northwest).

We propose a simulation-based approach using an operating model including several hypothetical scenarios of spatial structure and seasonal movement, on which the performance of SS3 can be evaluated to determine (1) how much error can arise because assessments of sardine are conducted using a spatially-aggregated stock assessment method when this assumption is violated, and (2) whether moving to a spatially-structured stock assessment could reduce this error. Specifically, our model will consider the impact of (a) the presence of a southern (Mexican) subpopulation in the area in which the northern subpopulation is usually found, (b) movement of sardine between southern California and the Pacific Northwest, and (c) the occasional persistent presence in the Pacific Northwest of Pacific sardine, on the performance of SS3. The focus of this presentation will be to introduce the project and preliminary results.