Exploring biases in estimating survival probability of marine resources from mark-recapture analyses : a case study with the southern rock lobster Jasus edwardsii
Creators: name: Kordjazi, Z
Publisher: name: University Of Tasmania
Year: 2023

description: Capture-mark-recapture (CMR) modelling is commonly used for direct estimation of demographic parameters in the wild. Whilst it is well known that the precision of estimated parameters in CMR studies increases with sample size, duration of study and the number of recapture events there are few studies that explore the design of CMR projects to achieve optimal outcomes. Research funding and time allocation are two constraints that place restrictions on the research design. This study addresses this challenge and seeks to demonstrate issues that need to be addressed and ways to design CMR that minimise costs while still providing the precision and accuracy required for assessment of marine resources. A twelve year tagging data set for Jasus edwardsii at a non-fishing site (Crayfish Point Scientific Reserve, Tasmania, Australia) was used in the study. Cormack-Jolly-Seber (CJS) modelling was used to compare estimates of survival probability with increasing numbers of surveys from 3 to 12 for both equal (annual) and unequal survey intervals. The study found that more surveys were required to obtain precise estimates of survival probability in females than in males, and the number of surveys required increased when intervals between surveys were unequal. For annual surveys, stable estimates of survival probability generally required at least five years, which is often beyond the three-year duration commonly used in scientific studies. Five annual surveys were used to determine the effect of sample size and fishing effort (the number of traps set per day and days fished) on the precision of estimated survival probability. Larger sample sizes (&gt;=500) and more sampling effort were required to determine sex specific survival probability estimates. Tag induced mortality (TIM) and tag loss reduce the number of tagged lobsters in the population and result in an under-estimation of survival probability. TIM is often investigated in aquaria studies with small sample sizes and over short time periods. In this study, a model based on three annual-surveys was used to provide in-situ estimates of TIM proportion. Estimates of TIM proportion were high and varied being on average between 25% and 40%. Capturing sufficient lobsters during the third survey that were tagged in the first survey and also seen in the second survey was a major limitation in using this method. Tag loss is usually estimated by double tagging in the wild. This study investigated the fate of new tags and existing ( old) tags applied to lobsters. Estimated tag loss rate was different between males and females and the annual rate of tag loss was similar between years during the two year study indicating that tag loss is chronic and needs to be accounted for throughout the entire period of mark-recapture programs. In general, older tags were lost at a slightly higher rate than new tags. The design of mark-recapture programs will be dependent on the species under study and although these results are specific for J edwardsii, the issues considered and methods used in this thesis should assist researchers in the design of tagging surveys. Importantly, both tag loss and tag induced mortality can be of a magnitude that impacts estimates from mark recapture studies and the thesis demonstrates issues to be considered in designing a tagging program that meets the objectives of the study and also enables estimation of issues such as tag loss rate and TIM proportion which can bias survival probability and population estimates.