The effect of mitigation strategies for the clubbed tunicate Styela clava on mussel collectors

The culture of the blue mussel Mytilus edulis in Prince Edward Island (PEI) began in the 1970's. Production has since increased, such that PEI produces 80% of Canada's cultured mussels and in 2004 accounted for over $36 million CND. Recently, an increase in fouling organisms has lead to a change in the culture methods used by mussel growers across PEI. The focus of this study is on two fouling organisms that have become pests in several mussel growing areas; the invasive solitary clubbed tunicate Styela clava and several algae species collectively referred to as macroalgae (identified as Cladophora, and Enteromorpha ). These are specifically devastating for the seed collection stage, and a primary objective of the mussel industry is to eradicate both with the application of one treatment.The goal of our first study was to examine the effects of two chemical treatments on S. clava, macroalgae, and mussel seed at the seed collection stage. The treatments used were 5% acetic acid and 4% hydrated lime. The former is used to mitigate S. clava on mussel socks, while the latter is used to eradicate starfish from collectors and socks. Results strongly support 4% hydrated lime as the treatment of choice for controlling tunicate and macroalgae fouling at the seed collection stage. Collectors receiving this treatment had the highest mussel abundance and the lowest tunicate abundance, and had lower macroalgae weights than untreated control collectors. In addition, we examined when the treatments should be applied to the collectors. Results showed that treating twice with 4% hydrated lime, first in early August and again in late August, was successful at mitigating fouling by S. clava and promoting a set of mussel seed. These treatment times coincide with the main settling period of both starfish and S. clava, which occur in conjunction with water temperatures that promote spawning by these species. The use of acetic acid as a mitigating treatment was effective at controlling macroalgae and S. clava fouling, however it would not be recommended on mussel collectors as it led to high seed mortality. A laboratory trial to determine the effect of 5% acetic acid on juvenile mussels (