Chris Legault¹ and Michael Palmer^1
1Northeast Fisheries Science Center
January 27, 2015

What direction should the fishing mortality target change when natural mortality increases within an assessment?

Traditionally, the natural mortality rate (M) in a stock assessment has been assumed to be constant over years and ages. When M increases within an assessment, as has occurred in a number of Canadian cod stocks, the United States Gulf of Maine cod and Atlantic herring stocks, the question arises how to change the fishing mortality rate target (Ftarget). Yield per recruit considerations lead to an increase in the Ftarget, while maximum sustainable yield considerations often lead to a decrease in the Ftarget. Both approaches are examined using results from recent assessments of Gulf of Maine cod and Georges Bank yellowtail flounder. Problems are found with both the yield per recruit and maximum sustainable yield approaches, leading us to recommend either not allowing M to change within an assessment model or if strong empirical evidence supports a changing M to base the Ftarget on the natural mortality rate considered most appropriate based on the life history traits of the species of interest.