Impacts of the invasive ascidian Didemnum vexillum on green-lipped mussel Perna canaliculus aquaculture in New Zealand

Biofouling can pose a significant threat to shellfish aquaculture, as fouling organisms are often strong spatial competitors that are able to reach high densities or biomass in relatively short time frames. This study investigated the potential impacts of the colonial ascidian Didemnum vexillum on cultured New Zealand green-lipped mussels Perna canaliculus at one farm in the Marlborough Sounds region. Three size classes of mussels were examined: small (20 to 40 mm shell length at deployment), medium (40 to 60 mm), and large (60 to 70 mm). Replicate 4 m mussel lines were assigned to 1 of 3 treatments: (1) ambient fouling, (2) fouling enhanced by D. vexillum fragment inoculation (in addition to ambient fouling), or (3) control lines that were kept free of D. vexillum. After 15 mo, subsections of lines (0.5 m length) were processed to determine the effects of fouling cover on mussel density within lines, as well as on individual mussel size and condition. A highly significant negative relationship was shown between D. vexillum biomass and mussel density for small mussels, and to a lesser extent for medium mussels. Values of mussel condition indices were similar across size classes and across fouling treatments within each size class. These results indicate that mussels may only be vulnerable to direct D. vexillum fouling impacts at early stages of production, and that impacts may be restricted to displacement of mussels as opposed to reduced size and condition. This information will assist in the implementation of management procedures through increased understanding of D. vexillum effects at different stages of mussel production.