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

• Published in: Canadian Journal of Fisheries and Aquatic Sciences
• Main Authors: Legault, Christopher M., Palmer, Michael C.
• Other Authors: Wilberg, Michael
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2016
• Subjects: atlantic cod; Gadus morhua
• Online Access: http://dx.doi.org/10.1139/cjfas-2015-0232; http://www.nrcresearchpress.com/doi/full-xml/10.1139/cjfas-2015-0232; http://www.nrcresearchpress.com/doi/pdf/10.1139/cjfas-2015-0232

Traditionally, the natural mortality rate (M) in a stock assessment is assumed to be constant. When M increases within an assessment, the question arises how to change the fishing mortality rate target (F Target ). Per recruit considerations lead to an increase in F Target , while limiting total mortality leads to a decrease in F Target . Application of either approach can result in nonsensical results. Short-term gains in yield associated with high F Target values should be considered in light of potential losses in future yield if the high total mortality rate leads to a decrease in recruitment. Examples using yellowtail flounder (Limanda ferruginea) and Atlantic cod (Gadus morhua) are used to demonstrate that F Target can change when M increases within an assessment and to illustrate the consequences of different F Target values. When a change in M within an assessment is contemplated, first consider the amount and strength of empirical evidence to support the change. When the empirical evidence is not strong, we recommend using a constant M. If strong empirical evidence exists, we recommend estimating F Target for a range of stock–recruitment relationships and evaluating the trade-offs between risk of overfishing and forgone yield.