Vase tunicate, Ciona intestinalis: Ecology and mitigation strategies

The mussel aquaculture industry on Prince Edward Island (PEI) has grown over the last 20 years to now produce over 17 000 Mt, valued at $107 M to the economy and supplies approximately 80% of the North American market (Department of Fisheries and Oceans, 2006). Recently, several exotic tunicates (Styela clava, Ciona intestinalis, Botrylloides violaceus and Botryllus schlosseri) have been detected throughout PEI which is cause for concern for the industry. Tunicates compete with mussels for space and food, potentially decreasing the growth rates and meat yields of cultured mussels. Ciona intestinalis is presently the dominant tunicate on mussel farms and is considered a serious threat to this aquaculture industry, mainly due to its unmanageable biomass.

Reproductive biology and development of C. intestinalis in a single season was investigated to assist aquaculturalists in developing control strategies and to understand the risk of spread to other aquaculture sites. The study included C. intestinalis gonad development, larval abundance and recruitment, and subsequent development after settlement. Histology showed gonads are ripe from May to December. Larvae were observed in water samples from early August through November, with a distinct peak in early October. Recruitment on experimental collectors, however, occurred from mid June until early December, with a peak in late-August. A rapid increase in biomass was documented in late July, six weeks after the initial recruitment. No substantial increase in C. intestinalis biomass was observed after mid August.

Anecdotal reports from mussel growers suggest that increased stocking density decreases the recruitment of the fouling organisms. To evaluate this hypothesis, 15 mussel socks of low (90 mussels per 30 cm), medium (250 mussels per 30 cm) and high (500 mussels per 30 cm) densities were placed on three longlines in the Brudenell estuary in the fall 2005 and in the spring 2006. Samples were taken in June, August and October to determine the effect of different stocking densities and socking time on mussel productivity and C. intestinalis recruitment and growth. Mussel condition and shell length were not significantly different between the three stocking densities by the end of the field trial in October. In August, the mean length of C. intestinalis was significantly lower on the medium and high density socks and C. intestinalis weight was less on medium and high density socks from the fall socking period (no difference within spring socked densities); however, by the October sampling there was no difference in the weight of C. intestinalis between stocking density treatments. Time of socking did not appear to have any significant effect on the results, although the abundance of C. intestinalis was marginally higher in August and the biomass was higher in October, on the socks deployed in December, 2005. Mussel loss was between 50-60% for all treatments, with no clear pattern being evident.

Ciona intestinalis has proven to be a significant invasive nuisance species to the mussel aquaculture industry. Multiple farm management practices utilizing optimal husbandry practices and knowledge of C. intestinalis reproduction and development are required to mitigate the impacts of this species.