Assessment of chemical treatments for controlling Didemnum vexillum, other biofouling, and predatory sea stars in Pacific oyster aquaculture

Abstract

Aquatic invasive species in biofouling communities continue to cause ecological and economic impacts globally. Didemnum vexillum, an aggressive invasive tunicate, can have detrimental impacts on the shellfish aquaculture industry and a reliable method of control is required. In this study, cultured Pacific oysters (Crassostrea gigas) fouled with D. vexillum were treated with various concentrations of brine, freshwater, hydrated lime, and acetic acid at four exposure times (0.5, 1, 5, and 10min). Results demonstrated that only lime and acetic acid significantly reduced total biofouling and D. vexillum coverage. Lime concentrations of 1 and 2% at 5 and 10min exposures and 4% at 5min exposure successfully removed up to 92.3% of D. vexillum fouling while maintaining high (≥80%) oyster survival. This led to the testing of 1, 2, 3, and 4% lime at 1 and 5min exposures on C. gigas in a laboratory setting to determine potential impacts of these treatments on oyster survival, growth, and condition. The same treatments were also repeated on the mottled sea star, Evasterias troschellii, a major predator of Pacific oysters, to ascertain effects on sea star mortality. Combined, the results of field and laboratory experiments demonstrated that exposure to 3 and 4% hydrated lime for 5min was effective in removing total biofouling, D. vexillum fouling, and predatory sea stars without causing significant adverse effects on oyster survival, growth, or condition. This study provides insights to assist the shellfish aquaculture industry with possible solutions to control invasive tunicates and predatory sea stars.