Strategies to mitigate the impact of Ciona intestinalis (L.) biofouling on shellfish production

A sudden increase in the population of the solitary ascidian Ciona intestinalis (L.) is causing serious biofouling problems for shellfish growers on the Atlantic coast of Nova Scotia, Canada. The objective of the present study was to document the growth, spawning, and recruitment patterns of this species, and to develop strategies to minimize its impact on the culture of European oysters at two locations in Lunenburg Bay, Nova Scotia. Profiles of condition index, which may be indicative of spawning activity, suggested that the C. intestinalis population at the Bayport site spawned from mid-May through June, whereas the population at Mason's Beach spawned from mid-July to mid-August. Histological assessment of reproductive status indicated a period of gameto-genesis in March-April (>3°C) followed by spawning from mid-May to mid-August (>8°C). Although mature eggs were observed in the ovary in July-August, spawning trials suggested a decline in the fecundity of the Bayport population during this period. Two main recruitment events were observed at Mason's Beach (June and August), but only one at Bayport (June). From the data on fecundity and settlement rates, it was estimated that a 100-mm long C. intestinalis (0.6 g dry weight) may produce 12,000 eggs in a season and that recruitment intensity may reach 3,000 individuals m-2. Laboratory predation trials indicated that rock crabs (Cancer irroratus) consumed significantly more C. intestinalis than did green crabs (Carcinus maenas). A maximum predation rate of 11 individuals per day per rock crab (80 mm carapace width) was recorded at peak water temperatures of 18°C. In a series of chemical width eradication trials, exposure to 5% acetic acid was found to be a more effective strategy for eliminating C. intestinalis than hydrated lime, saturated brine, or hypochlorite solution. Total mortality was observed following exposure to 5% acetic acid for 15 to 30 s, with no corresponding mortality in the control mussels or oysters. Initial field trials indicated that spraying with acetic acid might prove to be an effective means of eliminating C. intestinalis under commercial conditions.