Estimating the natural mortality rate of saucer scallops (Ylistrum balloti) on the Queensland east coast from tag-recaptures

Saucer scallops (Ylistrum balloti) were tagged and released on four occasions inside two areas closed to fishing (Hervey Bay A, HBA; and Yeppoon B, YB) on the Queensland (Australia) east coast and their subsequent recaptures over the following months were used to measure the instantaneous rate of natural mortality (M). A total of 13,295 scallops were tagged and 526 recaptured over the 15 month-long experiment (May 2018 to August 2019). Three statistical approaches were applied to the experimental design and analysis of the tagging data, based on 1) the Brownie model, 2) a modified version of the Brownie model, and 3) a binomial logistic regression model of recaptures. Estimates of M based on the Brownie model were much higher for tagged scallops that were at liberty over summer months compared to those at liberty over the winter months, possibly indicating seasonal variation. The logistic model parameter estimates indicated the proportion of recaptures differed significantly with the lunar phase at recapture, scallop size class, the number of days the scallops were at liberty and the interaction between days-at-liberty and closure. All three approaches indicated M was higher in HBA compared to YB. Mean estimates of M for the whole fishery, derived by averaging estimates from both closures, ranged from a minimum of 1.461 year–1 for the logistic model, to 1.501 year–1 for the Brownie model, to 1.548 year–1 (variable recapture rate) and 1.594 year–1 (fixed recapture rate) for the modified Brownie model. Estimates from all three approaches were higher than the previous estimate that was published over 40 years ago and possible reasons for the increase are discussed.