Approaches for estimating natural mortality in tuna stock assessments: application to Indian Ocean yellowfin tuna

The values used for natural mortality (M) are very influential in stock assessment models, affecting model outcomes and management advice. There is often limited information about the true levels.
This paper summarises the evidence used to estimate natural mortality at age for the four main stocks of yellowfin tuna, identifying some problems and information gaps. It also describes the history of parameter values used in stock assessments by each tuna RFMO.
Through time, Indian Ocean yellowfin tuna (IO-YFT) stock assessments have assumed a variety of values for M. The values used in the most recent assessment were intermediate between the higher levels assumed in assessments for the Eastern Pacific and the Western and Central Pacific, and the lower levels assumed in the Atlantic.
In June 2021 an online meeting was held by the Center for the Advancement of Population Assessment Methodology (CAPAM), to provide advice and guidance on practices for modeling natural mortality in fishery assessments. Based on presentations and discussions at the meeting, this paper provides IO-YFT natural mortality options for prior distributions derived from maximum observed age and suggests approaches for using these priors in stock assessments. It also recommends future research needed to develop improved estimates of natural mortality.