Nis S Jacobsen¹, Jim Thorson¹ and Tim E Essington^2
1NWFSC
²SAFS
April 17th, 2018 9:00 (PST): FSH 105

Detecting mortality variation to enhance forage fish population assessments

Contemporary stock assessment models used by fisheries management often assume that natural mortality rates are constant over time for exploited fish stocks. This assumption results in biased estimates of fishing mortality and reference points when mortality changes over time. However, it is difficult to distinguish changes in natural mortality from changes in fishing mortality, selectivity and recruitment. Because changes in size structure can be indicate changes in mortality, one potential solution is to use population size-structure and fisheries catch data to simultaneously estimate time-varying natural and fishing mortality. Here we test that hypothesis by simulating the ability to accurate estimate natural and fishing mortality from size structure and catch data and compare performance among four alternative estimation models. We show that it is possible to estimate time-varying natural mortality in a size-based model, even when fishing mortality, recruitment, and selectivity are changing over time. Finally, we apply the model to North Sea sprat, and show that estimates of recruitment and natural mortality are similar to estimates from an alternative multispecies population model fitted to additional data sources. We recommend considering diagnostic tools such as ours within forage fish stock assessment to explore potential trends in natural mortality.