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 it is 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 M is not significantly different between teleosts (n = 105) and elasmobranchs (n = 15). Furthermore, data on 16 teleosts and 2 elasmobranchs suggest that juvenile M 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 tmax, which is shown to be approximately 1-2%. The data utilized here also suggest that if tmax is unknown it can be estimated from growth information by assuming that 99% of the asymptotic maximum length is reached at tmax. Based on these life history parameters, the same indirect M estimators can be utilized for teleosts and elasmobranchs, which may contribute to more reliable assessments of data-poor species