An assessment of the use of macroalgae to improve the retention of Greenshell™ mussel (Perna canaliculus) spat in longline culture

Large-scale losses of spat are commonplace during the early stages of mussel aquaculture costing the industry millions in lost production. The dominant cause of this poor spat retention is thought to be secondary settlement behavior, whereby spat detach from farming substrata in an effort to relocate to another site. One approach that could be used to reduce these spat losses is to use natural substrates, such as macroalgae, to which wild mussel spat commonly attach. These natural substrates are often associated with chemical and morphological settlement cues. In New Zealand, the majority of mussel spat used by the aquaculture industry are harvested from the wild while attached to a range of species of macroalgae and other general debris (referred to as Kaitaia spat material). The filamentous morphology and chemicals released by some of these macroalgal species, including the rhodophyte Pterocladia lucida, have been shown to improve mussel settlement. Therefore, in this study, the spat of the green-lipped mussel, Perna canaliculus, were seeded onto aquaculture grow ropes with and without P. lucida to determine whether the presence of the macroalgae would improve the retention of the mussel spat. The presence of P. lucida increased spat retention by 33% relative to a control over 12 weeks and also increased the proportion of spat (by 16%) that successfully migrated away from biodegradable seeding substrata onto the permanent structure of the grow rope. The timing of spat losses relative to the breakdown of seeding substrata is consistent with the macroalga producing chemical cues that encourage spat settlement. In contrast, no spat attached directly to the P. lucida suggesting morphology of the macroalga was not playing a role in the observed increase in spat retention. These results suggest that there may be chemical cues present in P. lucida that may have the potential to be used to help resolve the problem of poor spat retention in green-lipped mussel aquaculture.