Oral Presentation Australian Society for Fish Biology Conference 2017

Evidence for systemic age underestimation in shark and ray ageing studies (#24)

Alastair V Harry 1 2
  1. College of Marine & Environmental Sciences, James Cook University, Townsville
  2. Department of Fisheries Western Australia, North Beach, WA, Australia

Numerous studies have now demonstrated that the most common method of ageing sharks and rays, counting growth zones on calcified structures, can underestimate true age. I reviewed bomb carbon dating (n=15) and fluorochrome chemical marking (n=44) age validation studies to investigate the frequency and magnitude of this phenomenon. Age was likely to have been underestimated in at least nine of 29 genera and 30% of the 53 populations studied, including 50% of those validated using bomb carbon dating. Length and age were strongly significant predictors of occurrence, with age typically underestimated in larger and older individuals. These characteristics suggest age underestimation is likely a systemic issue associated with the current methods and structures used for ageing. Where detected using bomb carbon dating, growth zones were reliable up to 88% of asymptotic length (L) and 41% of maximum age (AMax). The maximum magnitude of age underestimation, ∆Max, ranged from five to 34 years, averaging 18 years across species. Current perceptions of shark and ray life histories are informed to a large extent by growth studies that assume ageing structures are valid throughout life. The widespread age underestimation documented here shows this assumption is frequently violated, with potentially important consequences for conservation and management. In addition to leading to an underestimation of longevity, the apparent loss of age structure associated with it may unexpectedly bias growth and mortality parameters. Awareness of these biases is essential given shark and ray population assessments often rely exclusively on life history parameters derived from ageing studies.