Seasonal, Spatial, And Postharvest Variability In The Survival Of Repeatedly Discarded Saucer Scallops In Shark Bay, Western Australia

Saucer scallops, Amusium balloti, are targeted by Shark Bay prawn and scallop trawl fisheries where regulatory discarding can occur during summer and winter months that may adversely impact the recruitment of this resource. Survival of repeatedly discarded saucer scallops was thus estimated for the Shark Bay trawl fisheries using short-term tag recapture experiments under various fishing and environmental conditions. Estimates of apparent survival of discarded scallops was significantly higher during the postspawning winter period (>90%) than during the prespawning summer period (20-90%), but no differences in survival between fishing grounds or between postcapture treatment groups (air exposed vs. those kept in a holding tank) were observed. This suggests that thermal stress from large differences in seasonal temperatures was more critical to scallop survival than differences in scallop reproductive condition. Thus past (pre-2004) management regulations that imposed regulatory discarding during the warmer summer months would have incurred high discard mortalities. Under current management measures, however, regulatory discarding is predominantly during winter months, when scallops exhibit higher resilience to trawl-induced stress. The results support the current management strategy of fishing during the warmer prespawning summer months when the amount of discards is less. Regulatory discarding during the winter spawning period may generate a higher discard rate but the associated discard mortality is at its minimum.