Responses of Mitrella lunata and Caprella spp., potential tunicate micropredators, in Prince Edward Island estuaries to acetic acid anti-fouling treatments

In Prince Edward Island, Canada, acetic acid treatments that are used to control the clubbed tunicate (Styela clava), a fouling pest on mussel lines, may also affect other epifaunal mussel sock species, including potential tunicate predators. We studied the effect of acetic acid treatment on two potential predators of the tunicates, the gastropod Mitrella lunata (lunar dove shell) and the amphipod Caprella spp. (caprellids) in a preliminary lab study and more intensive field study. In the laboratory, caprellids and gastropods were allowed to attach to sections of rope in saltwater. The ropes were lifted and sprayed with 5% acetic acid or saltwater and organisms were monitored for 5–9 days. The acetic acid spray killed all amphipods whereas gastropods were mostly unaffected by the treatment. Gastropods were more affected by the lifting process than amphipods. In the field, gastropod and amphipod populations were compared over the short (5 days) and long-term (5–6 weeks) between mussel socks that were lifted from the water and sprayed with 5% acetic acid (simulating commercially used control methods) and control socks which were lifted from the water but not sprayed, or neither lifted nor sprayed. Gastropod populations were not affected by acid treatment after 5 days, but acid-sprayed populations were significantly lower than untouched control populations 5–6 weeks after treatment. Lifting of the mussel socks without acid spraying did not decrease gastropod populations significantly. Amphipod populations on acid treated mussel socks were lower than those on socks that were simply lifted from the water in both the short-term and the long-term.