Complexity affects habitat preference and predation mortality in postlarval Penaeus plebejus: implications for stock enhancement

Global attempts to offset declines in fishery populations through stock enhancement have had varied levels of success clue to the absence of preliminary Studies to determine which habitats best Support release species and the mechanisms controlling their distribution. Habitat preference was examined as a possible mechanism driving distribution of postlarval Penaeus plebejus, a current candidate prawn for stock enhancement in Australia. Occupancy of complex (artificial macrophyte) and simple (bare sand and mud) habitats by postlarvae was compared in the presence and absence of a choice between the habitats. Predation mortality was also compared amongst these habitats. P. plebejus settled into the different habitats randomly during the night, but actively selected macrophyte over the simple habitats during the day. Mortality caused by the predatory fishes Centropogan australis and Acanthopagrus australis was higher in simple habitats than in complex habitats, but was similar across habitats when large penaeid prawns, Metapenaeus macleayi (which are tactile rather than visual feeders), were used as predators. Postlarvae may select macrophyte habitats during the day to lower predation risk, but because nighttime foraging efficiency is reduced in their predators, which are primarily visual hunters, this may preclude the need of postlarvae to obtain shelter in macrophyte habitats at night. Predation mortality of stocked P. plebejus may be minimized by releasing postlarvae directly into macrophyte habitats. Studies such as these must precede all stock enhancement attempts because they identify optimal release strategies and allow ecological and financial costs of enhancement to be weighed against projected benefits, and thereby assess the practicality of enhancement as a management option.