Detecting change in density and biomass of a benthic marine invertebrate following commercial fishing

Established fishery-independent survey designs to measure change in the absolute density and biomass of greenlip abalone (leaded-line; LL) have proved impractical for the assessment of blacklip abalone. Due to the contagious distribution of this species among relatively complex and inaccessible habitat, a stratified survey design and modified radial transect (cross drop; CD) survey method was developed. Surveys were undertaken before and after known levels of commercial fishing to compare the accuracy, precision and efficiency of the LL and CD methods at detecting changes in the legal and sub-legal density and legal biomass of blacklip populations. Differences in length structures indicated substantial declines in the legal-size proportion of the population and emergence of sub-legal-sized blacklip, after commercial fishing. Strong correlations of repeated counts of legal, sub-legal and total abalone along survey transects provided strong evidence of the reliability of survey data among divers. CD and LL surveys typically recorded similar changes in density and biomass of legal and sub-legal sized blacklip abalone following commercial fishing. Although estimates from LLs were more precise, those from CDs provided more consistent measures of declines in legal density and biomass within fished areas and took less time to complete. Thus, CD surveys allow a greater number of primary sampling units to be conducted within a given time, thereby improving the precision of estimates. Power analysis identified that the power of the CD surveys to detect a decline in the legal biomass was >85%. These findings validate the CD survey method as a tool for monitoring blacklip abalone populations through time as a component of a stock assessment program and have subsequently been used to further improve the current survey design in South Australia. They also demonstrate that reliable measures of change in population size and structure can be obtained for species similarly distributed in complex reef systems.