Depredation of spanner crabs (Ranina ranina) by endangered batoids off the east coast of Australia

Cryptic mortality in fisheries relates to the unobserved or unrecorded mortality in a target ecosystem and is an important aspect of harvest management for fishery managers. Depredation is a key, observable form of cryptic mortality that relates to predators consuming a targeted species being caught within a fishing industry. This is of particular interest to the Queensland spanner crab fishing industry, where additional unobserved or unrecorded mortality through depredation of catch could be restricting current efforts to rebuild the population. High-resolution cameras were deployed on 178 baited tangle-nets (dillies) to investigate cryptic mortality, species interactions, and depredation within the spanner crab Ranina ranina fishery in Queensland, Australia. Physical parameters including current speed, temperature, depth, and time of soak were recorded. Depredation events were observed in the fishery by two species of endangered batoid species, the bowmouth guitarfish Rhina ancylostoma and wedgefish Rhynchobatus spp. However, rates of depredation in the fishery were low, with only 3.82% of crabs depredated. Fishing losses were calculated by comparing the total crabs on retrieval of a dilly, against total crabs observed while still soaking (MaxN) and at the beginning of retrieval. Overall, there was a loss of 37% in potentially harvested crabs through a combination of cryptic mortality and inefficient fishing practices. However, 27% of the losses could be reduced through shorter deployment times. We identified a significant correlation between the rate of depredation and current speed (similar to 0.6-0.8 knots), soak time and depth (<35 m). We also report spanner crab shell damage caused by mantis shrimp interactions, that likely contribute to an increase in spanner crabs discards due to unmarketable product. By identifying the species and drivers involved in spanner crab depredation, this study provides insights into ways that depredation events can be mitigated and managed.