Natural mortality rates in tropical tunas: size really does matter

Important size-related variability in natural mortality rates has been revealed in populations of skipjack tuna (Katsuwonus pelamis), yellowfin tuna (Thunnus albacares), and bigeye tuna (Thunnus obesus) in the western tropical Pacific Ocean using a new, size-structured tagging model. The tagging data were classified into release groups by size at release, and a growth model was used to predict size at each time at liberty interval. Natural and fishing mortality rates were categorized by broad size-classes and estimated by maximum likelihood. The method incorporates a correction for incomplete mixing of new releases. For each population, the natural mortality rate declined by about an order of magnitude over the initial size-classes. For skipjack and yellowfin tuna, there was also evidence of elevated natural mortality for some of the larger size-classes. Size- or age-related variability in natural mortality is likely to have important implications for tropical tuna stock assessment in the western Pacific.