The Role of Physical Structure in the Attachment of Juvenile Green-Lipped Mussels

Abstract

The aquaculture of the endemic green-lipped mussel (Perna canaliculus) is the largest aquaculture industry in New Zealand, with annual production currently valued at well over NZ$300 million a year. The growth of this industry is being constrained by a limited supply of seed mussels, mainly as a result of the overall uncertainty in wild seed sources and high losses of seed mussels once they are placed onto farms. The dominant source of seed mussels (~80%) that are used by this industry are harvested from the wild and as a result the extent of the supply of seed mussels fluctuates year-to-year in relation to environmental conditions. The uncertainty of this wild seed supply is further exacerbated by high losses of seed mussels (typically >60%) once they are transferred into suspended culture systems on mussel farms. The success of the collection of the settling larvae of this mussel in the wild and the subsequent on-growing of seed mussels in an aquaculture situation are largely determined by the settlement and attachment behaviour of the larvae and juveniles of this species. The morphological characteristics, including the surface microstructure, of filamentous substrata have been identified as an important factor influencing the attachment behaviour of larval and juvenile green-lipped mussels. Despite the potential importance of morphological characteristics of filamentous substrata for attachment behaviour no studies have attempted to identify the ideal set of morphological characteristics for promoting the attachment of the early life stages of these mussels. The research presented in this thesis used plastic to fabricate artificial filamentous substrata with varying morphological characteristics that were then experimentally tested for their ability to promote the attachment of juvenile mussels sourced from the wild. Laboratory experiments assessing the role of branch angle, branch distance and degree of branching of filamentous artificial substrata found these morphological characteristics had no effect on the resulting density of attached juvenile green-lipped mussels. In contrast, the branch width of artificial filamentous substrata were found to influence the attachment behaviour of juvenile mussels, with a trend for a higher density of mussels to attach to substrata of narrower branch width. For example, juvenile mussels attached at six times the density to an artificial filamentous substratum with a branch width of 1.6 mm compared to other substrata with wider branch width. Surface microstructure was found to have an inconsistent effect on the attachment behaviour of juvenile mussels. In two out of five experiments the juvenile mussels preferred to attach to the textured surface of the experimental artificial substrata, while an experiment to specifically test a wide range of surface textures produced a complex interactive effect among four geometric variables that could not be further isolated statistically. Overall, the results of this study identify significant new opportunities for improving designs for promoting the attachment of juvenile mussels to the artificial substrata used in mussel aquaculture, such as spat catching and nursery ropes. Doing so has the potential to greatly improve the efficiency of mussel seed collection and reduce the subsequent loss of seed mussels during their culture on mussel farms.