Re-evaluation of Georges Bank Yellowtail Flounder natural mortality based on life history approaches

In this study, we investigated several life-history based approaches to estimating natural mortality (M) for Georges Bank Yellowtail Flounder Limanda ferruginea. In previous Georges Bank Yellowtail Flounder stock assessments, an age and time invariant value of M = 0.2 is assumed based on historical tagging studies and the relationship between total mortality to effort in the late 1950’s. Using both fishery dependent and independent data sources through 2013, Georges Bank Yellowtail Flounder natural mortality was re-evaluated based on maximum age, growth, maturity and fish reproductive potential. Further, we explored a size-dependent approach by relating mean age of Georges Bank Yellowtail Flounder to fish size using an exponential functional form. The size dependent approach was explored as an alternative to the maximum age approach based on the premise of limited sample sizes often encountered using the observed maximum age in the population and the potential for M to be underestimated. Results from our analyses indicated that M may be higher than 0.2 and likely ranges from 0.3 to 0.5. While M appears higher than the current assumption in the stock assessment, we do not believe that the results of this study will change the perception of the stock nor will it resolve the retrospective problems for Georges Bank Yellowtail Flounder. 2014 TRAC Transboundary Resources Assessment Committee Working Paper 2014/06 NMFS (National Marine Fisheries Service) NEFSC (Northeast Fisheries Science Center) Submitted Public Domain 1862