The effects of incidental trawl capture of juvenile snapper (Pagrus auratus) on yield of a sub-tropical line fishery in Australia: an assessment examining habitat preference and early life history characteristics

Juvenile snapper (Pagrus auratus), predominantly 0+ year old (5-20 cm fork length), were an abundant incidental bycatch in Moreton Bay prawn trawls, with catch rates averaging 100 fish h−1 at some locations. Discard mortality of these juveniles, was high (over 85% mortality after 15 min of air exposure) and under a worst case scenario models predicted that incidental capture of juvenile snapper in prawn trawls from southern Moreton Bay alone could be responsible for the loss of a greater tonnage of snapper than is taken by the total Queensland commercial line fishery. The predicted mean loss was approximately 30 tonnes compared to an annual commercial line catch of 100 tonnes. Juvenile snapper were found associated with several substrates including sand, mud, rubble and soft coral/algal beds in Moreton Bay with population densities not significantly different among the habitat types. Juvenile snapper had not previously been associated with coral habitat. Despite the lack of difference in abundance related to the sampled habitat types, juveniles were more common in the shallower (<8 m deep) southern waters of Moreton Bay. Growth rates of 0+ year snapper (0.38-0.83 mm day−1) in subtropical Moreton Bay were higher than those in temperate areas.