Capture depth related mortality of discarded snapper (Pagrus auratus) and implications for management

Variables affecting the short-term survival of snapper (Pagrus auratus) captured using commercial fish traps and subsequently released were investigated by holding the fish in cages. A logistic regression model showed that capture depth had the greatest affect on short-term survival of snapper, with no mortalities observed from depths of less than 21 m and 2% from depths of less than 30 m. Mortality of snapper increased rapidly after 30 m and was 39% between capture depths of 30 and 44 m and 55% between capture depths of 45 and 59 m. Survival was also effected by fish length, with smaller fish being more likely to die. The rate of ascent of captured snapper and the density of fish in cages were kept reasonably constant and did not appear to affect survival. The number of snapper swimming upside-down prior to being returned to the sea floor in cages was not a good predictor of mortality. Future studies that use cages to assess discard mortality rates would benefit from underwater video observations of fish behaviour. The results demonstrate that the discard mortality of snapper should be considered when managing the fishery in New South Wales, Australia.