Blue mussels (Mytilus edulis) that are cultivated in themarine area around Prince Edward Island, Eastern Canada,
are susceptible to the heavy biofouling of their shells by an invasive solitary tunicate, Styela clava, which rapidly
proliferates. To mitigate this issue, mussel farmers periodically lift the longlines on which the mussels are
suspended out of the water to spray a highly alkaline (~12.7 pH units) calcium hydroxide solution onto fouled
individuals. Here, we tested the hypothesis that calcium hydroxide exerts behavioural stress on mussels and
other bivalves. Field surveys revealed that the alkalinity of the seawater in the vicinity of longlines increased
(9.3–11.7 pH units) immediately after treated mussel sleeves were returned into the water column. Thereafter,
pH values declined rapidly, and met federal water quality guidelines (7.0–8.7 pH units) within 3.1 ± 0.5 min
(range 0.3–10.5 min, n = 31 sleeves). Cultivated mussels challenged to both emersion and calcium hydroxide
closed their valves for 14.0±3.3 min (n=18) compared to 6.5±1.6min (n=17) by controlmussels (emersion
only). We subsequently assessed how three benthic bivalve species (M. edulis, Crassostrea virginica (eastern oyster)
and Argopecten irradians (bay scallop)) respond to weak (pH ≤ 9.2) but sustained (3-h daily for 3 days) alkalinity
conditions. All three species consistently responded by completely or partially closing their valves.
However, all behavioural responses were short-lived (0.2–4.7 h), and were generally confined to the treatment
period. In conclusion, spraying calcium hydroxide onto cultivated mussels has limited impact on seawater alkalinity
and the behaviour of nearby bivalves.