Development of eggs and the planktonic stages of salmon lice (Lepeophtheirus salmonis) at low temperatures

To verify if and to what extent egg and nauplii development of the salmon l ice take place during winter, the development from egg to the copepodid sta ge at 2,3,4,5 and 10 degrees C was examined. Newly extruded egg strings fro m a winter population of salmon lice were individually placed in 6 mi stagn ant hatching systems. Initially, no significant differences in egg developm ent time were found between these and larger aerated systems, though a tend ency for less synchronised hatching of the total egg string was detected in the stagnant systems (difference < 12 h). In light versus dark conditions the time to hatching was significantly prolonged by darkness (10-15%). The use of small stagnant experimental units was a pronounced simplification of hatching methods used earlier. At low temperature all but one pair of egg strings hatched. Time to first hatching was found to be 45.1+/-0.5 days at 2 degrees C, 35.2+/-0.4 days at 3 degrees C, 27.6+/-0.2 days at 4 degrees C , 21.6+/-0. 1 days at 5 degrees C and 8.7+/-0.1 days at 10 degrees C. The d evelopmental time to hatching correlated to temperature fitted the polynomi al function: Days to first hatching = 0.6638 T-2 - 12.492T + 67.116 (R-2 = 0.99). A high proportion of the nauplii developed to the copepodid (infecti ous stage) stage at 4 degrees C and higher but only a small proportion at 2 and 3 degrees C. Total developmental rime to copepodid ranged from 12.7 da ys at 10 degrees C to 68.5 days at 2 degrees C or to the polynomial functio n Days to first copepodid = 1.0236 T-2 - 19.129 + 101.5 (R-2 = 0.995). The "physiological age" at hatching, i.e. the product of days and the respectiv e temperature, showed nor to be linearly correlated to temperature. but sho wed a maximum at 4 degrees C. Average age at hatching was 90 degrees days a t 2 degrees C and 110 degrees days at 4 degrees C. Thus the egg strings of the winter population appeared to be adapted to very low temperature by red ucing the time for egg development. The present results show that eggs of s almon lice can develop to the infectious stage during winter along the Norw egian west coast.