| The sea louse (Lepeophtheirus salmonis), whose larvae are planktonic and disseminated in the water column, is an economically important parasite of Atlantic salmon (Salmo Salar). The effect of temperature (5, 10, 15°C), diet (larvae alone, larvae plus phytoplankton), and bivalve size (small, medium, large) on the amount of L. salmonis larvae ingested by various species of filter-feeding bivalves (Pacific oysters, Pacific scallops, blue/Gallo's mussel hybrids, basket cockles) was examined in a series of laboratory experiments. Four separate temperature/diet experiments were conducted (one for each species) in which large bivalves were individually placed in 2-L containers holding 750 ml of aerated, filtered seawater and fed one of three treatment diets: (1) phytoplankton: ∼7.1 x 104 cells ml-1 of Isochrysis sp. (Tahitian strain, TISO); (2) sea lice larvae: ∼431 larvae (mostly nauplii); and (3) phytoplankton and larvae (at the levels mentioned above). There was also a control treatment of phytoplankton and larvae, but no bivalve. After feeding for 1 h, the bivalve soft tissues were excised and preserved, the digestive system was dissected, and sea lice larvae were removed and counted to provide direct evidence of ingestion. The larvae remaining free swimming in the container were preserved and counted. The proportion missing from the container was used to estimate ingested larvae in statistical analyses. Two additional experiments investigating the effect of bivalve size (small, medium, large) on the ingestion of sea lice larvae were conducted with Pacific oysters and Pacific scallops. The heights for oysters (anterior-posterior axes) were 19.2, 44.2, and 84.0 mm, and scallops (dorsal hinges to ventral margins) were 40.3, 64.1, 102.7 mm. The methodology for the size experiments was as previously described for the temperature/diet experiments with the following changes: (1) the diet of larvae alone was not used; (2) the mean number of larvae in each container was ∼498; (3) the mean concentration of TISO added to each container was ∼7.8 x104 cells ml-1, and (4) the mean water temperature was 10.4°C. The data for the four temperature/diet experiments indicate that all four bivalve species ingested sea lice larvae, whether their diet included phytoplankton or not, and that temperature had no significant effect. The data for the two size experiments indicated that all three sizes of oysters and scallops ingested sea lice larvae and that there was a significant size effect. Large shellfish consumed a significantly greater proportion of the sea lice larvae than the small shellfish. Bivalves grown at salmon net pens as part of an IMTA (Integrated Multi-Trophic Aquaculture) system may be able to reduce the number of sea lice larvae as well as being an additional crop of market value. Future research, conducted at a commercial scale at a salmon farm, is warranted in order to determine if bivalves can serve in this role. |
