Lethality of microalgae to farmed Atlantic salmon (Salmo salar)

Phytoplankton blooms in the Bay of Fundy have been implicated in the deaths of farmed Atlantic salmon (Salmo salar). To establish whether or not elevated concentrations of some phytoplankton species can cause harmful effects or mortality, monocultures of three species, Alexandrium fundyense, Ditylum brightwellii, and Chaetoceros socialis were grown in large quantities. Atlantic salmon smolts were exposed to a range of concentrations of each of these cultures for 24 h and, when mortality occurred, an LC50 was calculated (cells DEL id="del2" orig="
• "; L−1). Eucampia zodiacus was collected by plankton tow and used in bioassays as well. Exposure of Atlantic salmon to D. brightwellii, C. socialis or E. zodiacus at concentrations as high as 1.0 × 106 cells L−1, 4.0 × 106 cells L−1 or 9.0 × 105 cells L−1, respectively resulted in no apparent deleterious effect. These concentrations are equal to or greater than concentrations of these species observed in the field. A. fundyense was the only species of the 4 examined that was shown to be lethal to Atlantic salmon smolts. The 6 h LC50 was estimated as 7.24 × 105 cells L−1 and the 24 h LC50 was estimated as 6.14 × 105 cells L−1 indicating effects occur quickly and longer exposures do not change the LC50 estimates significantly. The LC50 concentrations represent cell counts similar to those observed in the field. The no observable (lethal) effect level for A. fundyense and Atlantic salmon is estimated as approximately 2.0 × 105 cells L−1 over 6 h and 1.0 × 105 cells L−1 over 24 h. Cell counts do not reflect the toxicity of individual cultures but may be valuable for aquaculture site management. Chemical analyses of the cultured A. fundyense cells in this study show a mean concentration of 33 pg saxitoxin equivalents ⋅ cell−1.