Implications of Retrospective Patterns for Bias in Discard Rates and Unobserved Landings

Previous analyses of retrospective patterns for Georges Bank Yellowtail Flounder have demonstrated that total catch would have to increase fivefold in order to compensate for the retrospective pattern. The increase in catch would require increases in unreported landings, or significant increases in discarding rates on unobserved trips. The implications of a five-fold increase in total catch were examined for three US fleets that constitute the majority of US catch of Georges Bank Yellowtail Flounder during 1989 to 2011. Bias factors required to achieve a five-fold increase in total catch as a function of bias in landings on unobserved trips and bias factors for bias in discard rates on unobserved trips were computed. Results suggest that bias factors greater than five are required to increase the total catch by a factor of five. We conclude that neither increased discarding rates on unobserved vessels nor illegal landings on unobserved vessels seem plausible given the extreme magnitude of change implied by our analyses. Trends in US fishing effort by otter trawls has declined in recent years, indicating that non-observed fishing mortality effects, such as due to injury from passing through meshes, is not a likely cause of the missing catch needed to explain the retrospective pattern. 2014 TRAC Transboundary ResourceAssessment Committee Reference Document 2014/ NMFS (National Marine Fisheries Service) NEFSC (Northeast Fisheries Science Center) Submitted Public Domain 1861