Estimating predation rates of restocked individuals: The influence of timing-of-release on metapenaeid survival

Abstract The success of aquaculture-based enhancement programs is greatly influenced by the survival of released individuals. Immediate post-release mortality through predation is one of the greatest obstacles to the success of releases, and the choice of a release site or time-of-release can be critical in maximising survival. This paper develops a quantitative method of estimating predation rate to inform release programs, and describes its use in determining whether hatchery-reared Western School Prawns Metapenaeus dalli should be released into the Swan-Canning Estuary in temperate south-western Australia during the night or day. Fish faunal composition was determined during the day and night, both before and after the release of ∼130,000 postlarval M. dalli. Far greater numbers of species and individuals were recorded at night. Stomach contents of 16 abundant teleost species were estimated volumetrically (%V) and any postlarval M. dalli counted. Although diet varied among species, diel phase and size class, crustaceans (including M. dalli) were a key dietary component (>10%V) of 12 species. The data on the abundance of these fish species and the number of M. dalli they consumed were combined and bootstrapped to estimate the total relative number of M. dalli consumed at the time-of-release. The results indicated that while six species consumed M. dalli, two species, Ostorhinchus rueppellii (Apogonidae) and Atherinomorus vaigiensis (Atherinidae), were responsible for ∼99% of the predation, and that the total number of postlarval prawns consumed was 288% higher at night than in the day. These findings suggest that releasing M. dalli during the day will greatly reduce predation and consequently allow a greater survival rate at this release site. The simple methodology developed here could be readily employed to inform release strategies for other species