Conrad's false mussel (Mytilopsis leucophaeata) is an invasive alien bivalve species causing severe biofouling problems in cooling water systems of power and industrial plants. Settlement, seasonal shell size distribution, and growth of this bivalve species were analyzed in relation to relevant environmental factors, using monitoring data from a brackish canal over a period of 4 y. Salinity at the sampling site ranged from3.2 to 9.2,water temperature from 4.5°C to 23.9°C, and chlorophyll a content from 2 to 56 µg l-1. Once a month one short-term exposed panel (1mo exposed) and one long-term exposed panel (exposed from February 1989 to the month of sampling) were collected. Settled spat and attached mussels were collected monthly from these polyvinylchloride panels and from stones in the littoral zone. The short-term exposed panels were used to determine the period of spat fall. The long-term panels were used to study growth, shell length-frequency distribution, and densities of the mussel population after settlement. To study maximum age, M. leucophaeata were kept in cages in the canal. The highest recorded settlement was 204,000 individuals/m2. The largest mussel had a shell length of 24.0 mm. Three cohorts of shell length (1–5 mm, 5–15 mm and 15–24 mm) could be distinguished. Data on survival of individual mussel size groups in cages indicated a maximum age of 4.5 y. Shell growth was very low in winter time (average growth less than 1 µm/day in the period of November to April). The average growth rate on the stones during the summer periods (3–4 mo) was 89 µm/day. On the panels, a rapid shell growth to an average shell length of 4mm was recorded in the period of July until November in their first year, followed by a period of hardly any shell growth from December until May, and a subsequently rapid shell growth to an average shell length of 13mmduring the period of May until August in the second year. Individual mussels grew up to 20.9 mm during a 13-mo period. Growth of shells was significantly correlated with water temperature. The minimum threshold temperature for shell growth stops was 9.1°C (9.1°C–10.1°C) based on monthly measurements. Growth started at a minimum temperature of 9.1°C (9.1°C–14.2°C).
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Vol. 36 • No. 2