Effect of algae- and silt concentration on clearance- and growth rate of the razor clam Ensis directus, Conrad

Laboratory experiments were carried out with Ensis directus to estimate clearance rate and growth rate as a function of the amount of food and presence of silt. Two food levels were tested: low food availability (6.5 µg chla/l) and high food availability (16.5 µg chla/l) at four silt concentrations (0, 50, 150 and 300 mg/l). Clearance rates of E. directus varied between 0.7 and 5.9 l/h/g DW. At a silt concentration of 300 mg/l clearance rates were significantly lower (16–56%) than rates at 150 mg/l, 50 mg/l and 0 mg/l silt. This indicates that only the highest silt concentration induced a reduction in clearance rate. The tested food levels did not influence the clearance rate of E. directus. Maximum growth rate of E. directus was 0.24 mm increase in shell length and 2% increase in wet weight per day. Long-term (10 weeks) exposure to silt concentrations of 300 mg/l showed significantly higher growth than the 150 mg/l treatment. This indicates that exposure to a high silt concentration did not induce a reduction in growth, but stimulated growth. Long-term (10 weeks) exposure to a food level of 6.5 µg chla per litre reduced the shell growth of E. directus with 40% compared to growth at 16.5 µg chla per litre. The laboratory experiments suggest that E. directus is more sensitive to a reduction in algal concentration than to an increase in silt concentration.