Planktonic and parasitic stages of sea lice (Lepeophtheirus salmonis and Caligus clemensi) at a commercial Atlantic salmon (Salmo salar) farm in British Columbia, Canada

Planktonic and parasitic sea lice (Lepeophtheirus salmonis and Caligus clemensi) were examined at a commercial Atlantic salmon (Salmo salar) farm near the Broughton Archipelago, British Columbia (BC) from December 2012 through March 2014. Parasitic sea lice were counted on a minimum of 20 fish in three pens per month. Surface seawater salinity ranged from 19 to 35PSU. Parasite abundance was highest in winter, peaking in January 2013 at 6.5 ind fish−1 (motiles were 14.3% L. salmonis, 85.7% C. clemensi) and February 2014 at 3.3 ind fish−1 (motiles were 96.5% L. salmonis, 3.5% C. clemensi). SLICE® (emamectin benzoate) administered in both winters rapidly reduced parasitic sea lice numbers. Monthly parasite abundance was significantly correlated with the density of sea lice larvae (nauplius and copepodid stages) (r2=0.65, p=0.01) suspended in the water column of the salmon pens. Larval density was calculated monthly via triplicate vertical plankton hauls within six pens, as well as at a reference site approximately 150m away. Larval density at the farm peaked in January 2013 (mean±SE: 1.3±0.6indm−3) and January 2014 (1.0±0.3indm−3). Sea lice nauplii were found in all samples at the reference site in densities similar to those observed within pens. Overall, the majority (87.8%) of sea lice in the plankton samples were nauplii, with copepodids comprising 5.2% and motile stages 1.8%. Surprisingly, the remaining 5.2% of planktonic sea lice were chalimus stages of C. clemensi, and were found both before and after SLICE® administration. For comparison, L. salmonis nauplius densities were estimated from established relationships between water temperature and salinity, the number of female lice present, and the approximate number of fish on site. These estimated densities were significantly (t=3.41, p=0.009) higher than actual nauplius densities observed at the farm, suggesting that sea lice larvae quickly dispersed from the farm after hatching. This study provides information on all sea lice stages present throughout an Atlantic salmon production cycle in BC (through two SLICE® treatments) and the first report of planktonic chalimus stages of C. clemensi, a commercially-relevant though relatively under-studied sea louse species.