Abstract:
The green-lipped mussel (Perna canaliculus) is the most valuable aquaculture product exported from New Zealand and a quarter of the aquaculture production of these mussels comes from the Hauraki Gulf. The dramatic increase in fouling by the triangle barnacle, Balanus trigonus, on mussel farms in the Hauraki Gulf in the summer 2009-2010 caused significant problems for this industry, with lost in export revenue of around NZ$4 million per year. The presence of this barnacle on mussel shells disrupted the normal functioning of the automated mussel opening machinery. Consequently, there was a need to better understand the biology and ecology of this barnacle species in relation to mussel aquaculture operations in the Hauraki Gulf and in an effort to begin to identify potentially effective management actions for controlling this barnacle. Therefore, in this thesis, after an extensive review on the current knowledge about B. trigonus, some aspects of the larval, juvenile and adult life history were examined in laboratory experiments, followed by field studies in green-lipped mussel farms, where the pattern of settlement of this species was examined and innovative control mechanisms tested. The analysis of the historical weather data revealed that a likely cause of the outbreak of this species in 2009 was an El Niño event that promoted a diatom bloom which, as a consequence, positively affected the reproductive success of B. trigonus in that period. Laboratory experiments using reared larvae revealed that diatoms are an ideal food source for this barnacle species and that larval survival and the progression of larval development are both affected by culture temperature, with temperature of 20-22 °C being the most suitable for rearing this barnacle. Field studies showed that the settlement season of B. trigonus in the Hauraki Gulf is between December and March, reaching a maxima in January. The settlement of this barnacle was greater in deeper waters (9-13 m) compared to shallow waters (1 m) and appeared to be related to diatom abundance and rising seawater temperature, with an optimum around 19-20 °C. The adult extract (AE) of B. trigonus promoted the settlement of conspecific larvae both in laboratory and in field experiments, and applying it to plates deployed nearby group of mussels, reduced the number of barnacles settling per 1 cm2 on the mussel shells and increased the number of mussels free from settled barnacles by 32.6%. Overall the results of the research presented in this thesis provide useful information about the life history of B. trigonus that can help mussel farmers in the Hauraki Gulf to begin to tackle this fouling barnacle in their farms. Furthermore, this research highlighted the possibility of using seawater temperature and diatom abundance as tools for predicting further outbreaks of this species and also revealed the possibility of using sacrificial settlement surfaces treated with AE to reduce the impact of B. trigonus in mussel farms.