Undersized Acanthopagrus butcheri Caught and Released from Commercial Gill Nets Show High Survival Rates in the Gippsland Lakes, Southeastern Australia

A substantial decline in the commercial catches and catch rates of black bream Acanthopagrus butcheri in the Gippsland Lakes, Australia, signified a depletion of the stock and prompted management to increase the legal minimum length (LML) from 26 cm to 28 cm (total length) in December 2003. The effectiveness of this increase depends on the postrelease survival rate of undersized fish. The present study estimates the rates of release and initial survival (1 h after capture) of undersized A. butcheri during commercial fishing by gill nets in the Gippsland Lakes. Field capture and holding trials were also used to estimate initial and delayed survival (72 h after capture). Almost 6,000 A. butcheri from 347 gill net shots were caught during 2005-2006. The average total length increased by 1 cm from 2005 to 2006, increasing the proportion of the catch that was retained in 2006 (75%) compared with 2005 (50%). The best estimates of initial and delayed survival were 97.2% (SE, 0.3%) and 94.4% (SE, 3.8%), respectively. The total survival rate, which combines the initial survival rate from observer work with the delayed survival rates from the trials, was estimated as 90.8% (SE, 3.8%). The increase in the LML from 26 to 28 cm afforded significant protection to fish between these size limits, but the level of protection will be short term (1 or at most 2 years) for each year-class because of the growth rate of the species. Released, retained, and postrelease survival rates are discussed for various LMLs and with regard to their implications for fishery management.