Understanding the ecology of the Western School Prawn to maximize restocking success

Historically the Western School Prawn (Metapenaeus dalli) was an important and iconic catch for commercial and recreational fishers in the Swan-Canning Estuary. However, the abundance of this penaeid declined dramatically since the 1960s, resulting in the closure of the commercial fishery and essentially the cessation of the recreational prawning. In 2012, a restocking and associated research program was initiated to try and increase the abundance of this popular species. Monthly sampling, for two years, of the fish and crustacean faunas of the shallow and deeper waters of this estuary has shown that; 1) the abundance of M. dalli changes markedly throughout the year, particularly in the shallow waters. 2) M. dalli moves into the shallows between November and March to spawn, but that that timing changes slightly among years according to environmental conditions. 3) The distribution of adult and juvenile prawns changes temporally and can be used to inform broodstock collection and release sites. In addition, dietary analysis was performed on 11 abundant fish species in the estuary to determine those species that predate on post-larval M. dalli (direct predators) and those that also target other small crustaceans (potential predators). Key predators included the Gobbleguts Ostorhinchus rueppellii (all sizes) and the Black Bream Acanthopagrus butcheri (<100 mm in length). These dietary results together, with monthly prawn and fish abundance data for 36 sites across the estuary have been used to design an effective release strategy and highlight the value in restocking programs being supported by robust quantitative faunal surveys.