Solving the problem of poor spat retention on New Zealand's Greenshell mussel (Perna canaliculus) farms

Abstract

Green-lipped, or Greenshell™ mussels (Perna canaliculus) are the single most important species for New Zealand’s aquaculture sector in terms of volume and value of production. High losses of recently settled seed mussels, or “spat” during the early stages of Greenshell™ aquaculture is a major problem for the industry due to the resulting inefficient production and the creation of a shortage of spat supply which constrains the growth of the industry. Despite the severity of the problem, few studies have documented the timing and magnitude of spat losses on Greenshell™ farms, and little is known about the causes. Therefore, the research presented in this thesis, aimed to quantify the timing and magnitude of spat losses on Greenshell™ farms in relation to a range of possible causes, and several possible solutions to the problem. The magnitude of spat losses were estimated across the Greenshell™ industry as a whole, and the resulting estimate was confirmed at the farm scale in a subsequent series of field experiments (i.e., losses of > 99%). The timing and magnitude of spat losses were compared among farm sites throughout the Coromandel mussel growing area at multiple times throughout the year using spat from two sources (i.e., Kaitaia spat and line-caught spat). The impact of seeding density (11,000, 17,000 and 59,000 spat-1 m-1) and the physical structure of grow ropes on spat losses were also examined, and co-seeding of Kaitaia spat with a red macroalga (Pterocladia lucida), and the nursery rearing of spat in floating upwelling systems (FLUPSYs) were investigated for their potential to improve spat retention. Most spat losses were found to occur early on in production, shortly after seeding out, and were consistent among farm sites and spat sources. Secondary settlement behaviour was identified as a likely predominant cause of spat losses, along with the process of seeding out with large quantities of macroalgae that comprises Kaitaia spat material, which appears to prevent spat from successfully migrating onto the structure of the grow rope. The use of high seeding densities resulted in avoidable spat losses. Modifying the structure of grow ropes had no effect on spat retention. The co-seeding of Kaitaia spat with the macroalgae Pterocladia lucida improved spat retention by 33% relative to controls, by encouraging a greater proportion of spat to migrate onto the grow rope, most likely through the production of natural chemical cues. Finally, the introduction of a FLUPSY-based nursery system into the Greenshell™ production cycle was shown to reduce spat losses compared to spat grown directly in adjacent coastal waters (i.e., 78% vs 99.8%). Overall, the results of this research increases the understanding of the timing and magnitude of spat losses on Greenshell™ farms, identifies some possible causes of the problem, and presents some practical solutions. These results will facilitate further, more targeted research into the problem, and provide a starting point for further research into developing more effective solutions.