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 lice take place during winter, the development from egg to the copepodid stage at 2,3,4,5 and 10°C was examined. Newly extruded egg strings from a winter population of salmon lice were individually placed in 6 ml stagnant hatching systems. Initially, no significant differences in egg development time were found between these and larger aerated systems, though a tendency 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°C, 35.2±0.4 days at 3°C, 27.6±0.2 days at 4°C, 21.6±0.1 days at 5°C and 8.7±0.1 days at 10°C. The developmental time to hatching correlated to temperature fitted the polynomial function: Days to first hatching=0.6638 T² – 12.492T + 67.116 (R²=0.99). A high proportion of the nauplii developed to the copepodid (infectious stage) stage at 4°C and higher but only a small proportion at 2 and 3°C. Total developmental time to copepodid ranged from 12.7 days at 10°C to 68.5 days at 2°C or to the polynomial function Days to first copepodid = 1.0236 Tˉ² – 19.129 T + 101.5 (R²=0.995). The «physiological age» at hatching, i.e. the product of days and the respective temperature, showed not to be linearly correlated to temperature, but showed a maximum at 4°C. Average age at hatching was 90°days at 2°C and 110° days at 4°C. Thus the egg strings of the winter population appeared to be adapted to very low temperature by reducing the time for egg development. The present results show that eggs of salmon lice can develop to the infectious stage during winter along the Norwegian west coast.