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.
Highlights
The proliferation of invasive tunicates as fouling organisms in the marine environment has led to the development of a suite of approaches to manage infestations
Hydrated lime may improve water quality in estuaries that experience anaerobic events due to excess nutrient levels, which have become common in Prince Edward Island (PEI) over the past few decades (Raymond et al 2002)
Hydrated lime is sometimes added to anaerobic marine environments for odor control, to inhibit sulfate-reducing bacteria, and to precipitate algae, silt and phosphorous to the bottom (Nishimura and Seki 1983; Müezzinoğlu et al 2000)
Summary
The proliferation of invasive tunicates as fouling organisms in the marine environment has led to the development of a suite of approaches to manage infestations. The requirement that treatments that kill tunicates have no long-term environmental consequences is important on aquaculture sites, where the cultured species must remain safe for human consumption and the health and productivity of the environment must not be compromised. To this end, options that have been explored include: chemical treatments with sodium hydroxide, acetic acid, citric acid, formalin, detergents, chlorine bleach, and hydrated lime; and physical treatments using air drying, ultraviolet light, steam, hot water, electricity, smothering, pressure-washing, and puncturing (Carver et al 2003; Coutts and Forrest 2007; LeBlanc et al 2007). Tunicate treatments are commonly perceived by those involved in the industry to have negligible impacts on non-target biota and the environment, but potential impacts in PEI have not been rigorously studied
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