Rapid estimation of cryptic adult abundance and breeding dynamics in a critically endangered elasmobranch from close-kin mark recapture

Abstract Elasmobranchs are one of the most highly-threatened vertebrate taxa. Estimating abundance of spawning adults is often extremely challenging, yet crucial for prioritization of conservation measures. Emblematic of these challenges, the speartooth shark ( Glyphis glyphis , Müller and Henle, 1839) was initially known only from rare specimens collected in a few tropical regions river systems of Australia and Papua New Guinea. Listed as critically endangered in Australia (where only six adults have ever been recorded) and endangered by the IUCN, such scarcity and limited distribution prevent direct assessments of abundance or reproductive biology. We used close-kin mark-recapture to estimate the abundance of mature G. glyphis in a genetically isolated population in the Wenlock River, Queensland, Australia. From 224 juvenile/sub-adults and 2 adult samples taken over three years (2013-2016), 46 half-sibling and 33 full-sibling pairs were found. The adult population was estimated at 897 (80% Credible interval 531 – 1684) with a sex ratio (based on mitochondrial DNA) highly skewed to males (females – 0.09: males – 0.91). Based on juveniles sampled in different years that shared a mother, 71% of females were estimated to skip-spawn. In an average year we estimate 44 breeding females occupy the system. Importantly, these methods constitute a viable and relatively rapid approach to obtain robust estimates of absolute abundance and other key population parameters for similar rare species.