Preliminary evaluation of effects of invasive tunicate management with acetic acid and calcium hydroxide on non-target marine organisms in Prince Edward Island, Canada

Abstract

The proliferation of invasive tunicates in Prince Edward Island (PEI) estuaries has necessitated the development of approaches for managing tunicates that foul aquaculture structures, especially Styela clava and Ciona intestinalis. Spraying or immersion with a saturated solution of hydrated lime (calcium hydroxide) or 5% acetic acid are effective against these tunicates, but are also biocidal to a variety of non-target organisms as demonstrated by bioassays with the bacterium Vibrio fischeri, sand shrimp Crangon septemspinosa and threespine stickleback Gasterosteus aculeatus. Both chemicals have the potential to alter estuarine pH, which should remain within the limits 7.0-8.7 mandated by federal water quality guidelines. Acetic acid is no longer used as a commercial treatment in PEI and currently poses no risk to the environment. The pH of saturated hydrated lime solutions used for immersing mussel socks reached 12.6, but the “pH footprint” in the estuarine water column was limited to a radius of 5 tonnes/ha) of quicklime (calcium oxide) pellets mainly converted to hydrated lime within minutes, but chemical conversion of hydrated lime to harmless calcium carbonate took up to 18 days depending on temperature, pellet size, and the amount of quicklime applied. Conversion of hydrated lime to calcium carbonate should be more rapid at the much lower daily application rates (<0.007 tonnes/ha) of hydrated lime in powdered form, which represented the maximum amount likely to be used in mussel aquaculture for tunicate management in PEI. In most PEI estuaries, dilution by tidal mixing alone is probably sufficient to return the pH to normal values within a tidal cycle at these relatively low inputs of hydrated lime, even without taking into account the chemical conversion which would be occurring simultaneously. Decisions about whether to use chemical treatments must balance economic and potential environmental costs of treatment against the known economic consequences of unmanaged tunicate biofouling. A recent shift to use of pressure washing will reduce the potential for impacts associated with chemical treatments, but may not be feasible in estuaries infested with Styela clava.