Christine Stawitz1
1SAFS
November 08, 2016 9:00 (PST): FSH 203
How do changing somatic growth rates affect estimation of management quantities in fisheries stock assessments, and can such growth changes be detected?
A substantial body of work suggests that somatic growth in marine fish is plastic, and thus growth rates may change substantially in response to both extrinsic (i.e. climate) and intrinsic (i.e. competition) factors. Additionally, such changes in somatic growth rate may substantially affect population dynamics of marine fish. However, these changes in growth rate are not incorporated into many fisheries stock assessment models and, consequently, management advice. Growth rate changes are difficult to incorporate in management models, since such models are already highly parameterized and often contain temporal variation in observational parameters which are confounded with somatic growth parameters, such as fisheries selectivity. In this analysis, we evaluate the detectability of temporal changes in somatic growth and how such changes may impact fisheries management quantities. We examine these questions using a simulation framework, employing an operating and estimation model closely modeled on stock assessment models for North Pacific groundfish and the Stock Synthesis 3 (SS3) program. Simulated variation in population processes, including growth, recruitment, and fishing, is derived directly from empirical estimates. We test, first, if regime-like growth shifts can be detected using realistic quantities of time series data. We find regime-like patterns in growth rates are not detectable in population models without many decades of high-quality composition data, even when process error is low. Secondly, we find such changes, while difficult to detect, can introduce substantial bias into management reference points, particularly current depletion levels. This suggests more work is needed to increase estimability of temporal growth changes and incorporate them into fisheries stock assessment models.