Estimates of net volume available for fish shoals during commercial mackerel (Scomber scombrus) purse seining

Slipping, the release of fish from the net while it is still in the water is used to adjust catch size or increase catch value in many purse seine fisheries. High fish crowding densities prior to slipping have been shown to result in high mortality of the slipped fish. There are currently no available estimates of fish densities inside the net during purse seining or any practical methods for monitoring the catches or the net during commercial fishing, making the regulation of slipping difficult. In this study, a method for monitoring net volume and shoal size during commercial mackerel (Scomber scombrus) purse seining is developed. The net volume for eight separate purse seine sets was monitored during hauling (5 to 80% hauled net) using an omnidirectional fisheries sonar and an acoustic positioning system with transponders mounted in the net wall. The sonar was also used to monitor shoal size and position within the net in one purse seine set. The average volume available to individual fish in the monitored sets was estimated to be above 0.07 m3 per fish when 80% of the net was hauled. Shoal density was estimated to range between 6 and 31 fish m−3 during the first 60% of hauling. The results indicate that the net volume alone is not directly forcing the mackerel into lethally high densities during the first 80% of hauling, but the estimates are subject to some uncertainty and data is not available for the last and most critical phase of the set.