Using experimental ecology to understand stock enhancement: Comparisons of habitat-related predation on wild and hatchery-reared Penaeus plebejus Hess

Marine stock enhancement is often characterized by poor survival of hatchery-reared individuals due to deficiencies in their fitness, such as a diminished capacity to avoid predators. Field experiments were used to examine predation on Penaeus plebejus, a current candidate for stock enhancement in Australia. We compared overall survival of, and rates of predation on, wild P. plebejus juveniles, naïve hatchery-reared juveniles (which represented the state of individuals intended for stock enhancement) and experienced hatchery-reared juveniles (which had been exposed to natural predatory stimuli). Predation was examined in the presence of an ambush predator (Centropogon australis White, 1790) and an active-pursuit predator (Metapenaeus macleayi Haswell) within both complex (artificial macrophyte) and simple (bare sand and mud) habitats. Overall survival was lower and rates of predation were higher in simple habitats compared to complex habitats in the presence of C. australis. However, the three categories of juveniles survived at similar proportions and suffered similar rates of predation within each individual habitat. No differences in survival and rates of predation were detected among habitats or the categories of juveniles when M. macleayi was used as a predator. These results indicate that wild and hatchery-reared P. plebejus juveniles are equally capable of avoiding predators. Furthermore, exposure of hatchery-reared juveniles to wild conditions does not increase their ability to avoid predators, suggesting an innate rather than learned anti-predator response. The lower predation by C. australis in complex habitats was attributed to a reduction in this ambush predator's foraging efficiency due to the presence of structure. Ecological experiments comparing wild and hatchery-reared individuals should precede all stock enhancement programs because they may identify deficits in hatchery-reared animals that could be mitigated to optimize survival. Such studies can also identify weaknesses in wild animals, relative to hatchery-reared individuals, that may lead to the loss of resident populations.