Larval settlement and epidemiology of Lepeophtheirus salmonis Kroyer, 1837 (Copepoda; Caligidae)

Abstract

This study has been carried out to investigate the biological and environmental parameters influencing the settlement and post-settlement survival of the infective stages of Lepeophtheirus salmonis Kroyer 1837. The abiotic factors investigated were temperature and salinity. Temperature was found to have a significant effect on the settlement success of the copepodids with an inverse relationship between temperature and settlement. Survival of the louse at 10 days post infection showed a decrease at the reduced temperature. Temperature was also shown to have a direct relationship on lice development; higher seawater temperatures resulted in faster development. Regression analysis of temperature and settlement shows a significant correlation. A constant reduced salinity, 24%, resulted in a reduced ability of the copepodid to infect its host compared with 34%. Post-settlement survival in 24%, at approximately 13°C resulted in 5.8% survival of lice to day 10 post-infection compared to 79% in 34% salinity. When this experiment was repeated but with elevated seawater temperatures of up to 18°C, survival at the reduced salinity was found to be 75.3%, higher than the ambient control group. The developmental rate at day 10 post-infection of L.salmonis larva at 24% was shown to be slower than development at 34%o. Distribution of the L.salmonis copepodid on its host showed the highest settlement on the gills and on the fins, particularly the pectoral and dorsal fins. Examination of L.salmonis survival at day 10 post-infection indicated the highest losses on the gills and the pelvic, caudal and dorsal fins. Settlement on the pectoral fins showed the highest settlement and the greatest survival. The infective copepodid has a reduced ability to infect its host after 7 days following the moult from nauplius 2, compared to copepodids aged 1 and 3 days following the nauplius 2 moult. For copepodids of all ages, once settlement had been achieved, survival at 10 days post-infection was approximately 50% in all groups. Copepodids of all ages did not show any difference in the development rate at 10 days post infection. Highest settlement was found to be on the gills and pectoral and dorsal fins. The effects of varying dose rates of copepodids, has shown that a finite percentage of lice settle and survive the first five days post-infection. Settlement distribution was found to be highest on the body, gills and pectoral and dorsal fins. In serial infections of fish there was a reduced settlement count with second infections, possibly through intraspecific competition. Experiments using different host stocking densities showed that with an increased number of hosts the intensity of the infection of individual fish was reduced. Smaller fish appear more susceptible to settlement of L.salmonis than larger fish, and this is associated with the relatively greater fin area of those fish compared to larger fish. L.salmonis exhibits a preference for the fins as an area of settlement in all sizes of fish. Comparison of copepodid settlement on salmon and sea trout showed that in single populations of fish salmon had the highest intensities of infection whilst in mixed populations of fish sea trout had a higher intensity. Settlement distribution of L.salmonis on salmon showed greatest settlement on the body, pectoral and dorsal fins, whilst on sea trout settlement was highest on the body, pectoral, pelvic, caudal and dorsal fins. The comparative development of L.salmonis between the two species of host fish showed an increased rate of development on salmon. The calculated energy levels for L.salmonis larval stages show a decrease in available energy within each developmental stage. After 6 days from the nauplius 2 moult the copepodid starts to show a sharp decline in energy levels which coincides with the reduced ability of the copepodid to infect the host. Post-settlement energy levels remain constant even though the copepodid is actively feeding, as seen by SEM examination at 2 days postinfection. The principal lipid class found within L.salmonis larval stages as energy reserve is triacylgylcerol (37.6% of the total lipid). A preliminary epidemiological model for sea lice population dynamics is proposed. This is a differential equation compartmental model that has been designed to examine the flow of L.salmonis developmental stages on the host. The model was able to predict the timing of the maximum number of pre-adult 1 lice stages to within one day. The difference between the observed data and the model output is probably due to the considerable variability in the parameters used in the model construction.