Unified natural mortality estimation for teleosts and elasmobranchs

Natural mortality, M, is a key parameter for the assessment and management of living resources but is difficult to observe directly. Therefore, M is often estimated indirectly from life history traits and typically assumed to be invariant over size, age, and time. Such indirect estimators are particularly relevant for data-poor species including many elasmobranchs (sharks, skates and rays). However, as commonly used estimators were developed largely with teleost (bony fish) data, their performance for elasmobranchs is currently unknown. Here we show that the relationship between observed maximum age, tmax, and mean adult natural mortality rate is not significantly different between teleosts (n = 105) and elasmobranchs (n = 15). Furthermore, data on 16 teleosts and 2 elasmobranchs suggest that juvenile natural mortality can be estimated from adult M when juvenile M is inversely proportional to body length and when a reference length can be provided. We introduce this reference length as the length at the age after which M is assumed to be constant and demonstrate how it can be estimated using the von Bertalanffy growth function and the proportion surviving to maximum age, which is shown to be approximately 1-2%. The data utilized here also suggests that if tmax is unknown it can be estimated from growth information by assuming that 99% of the asymptotic length is reached at maximum age. Based on this moderate life history information the same indirect natural mortality estimators can be utilized for bony fish and elasmobranchs and thus may contribute to more reliable assessments of data-poor species.

Journal article

Marine Ecology Progress Series 667, 113–129