Bottom Currents Affect Spanner Crab Catch Rates in Southern Queensland, Australia

During daily fishing operations, spanner crab Ranina ranina catch rates can fluctuate substantially, but the environmental drivers responsible for these fluctuations largely remain unresolved. Earlier research suggests that spanner crab catchability increases with strengthening currents, but uncertainties surround the magnitude of the measured current speeds and, consequently, their relationship with catch rates. Here, we explore the effects of bottom currents on spanner crab catch rates in South East Queensland, Australia. Using generalized additive mixed modeling, our results indicated that strengthening current speeds increased catch rates until reaching approximately 0.15 m/s, at which point the catch rates began to gradually decline. Results from a general linear regression model also showed that between fishing periods carried out on the same day, catch rates increased or decreased concurrently with current speeds. We conclude that bottom current speed should be considered in future stock assessment models. Better understanding the processes responsible for changes in bottom current speed will enable more accurate estimates of spanner crab population densities in the Australian fishery and will benefit the economic efficiency of commercial crabbing operations. Furthermore, future studies that investigate the effects of current speed on catch rates for other crab species should consider differences in locomotory characteristics and how they may impact the foraging efficiency of crabs under different flow conditions.