Studies on the biology and ecology of the free swimming larval stages of Lepeophtheirus Salmonis (Kroyer, 1838) and Caligus Elongatus Nordmann, 1832 (Copepoda: Caligidae)

Abstract

The study investigated biological and ecological parameters controlling and influencing the production and distribution of the free-swimming larval stages of Lepeophtheirus salmonis (Kroyer, 1838), and to a lesser extent Caligus elongatus Nordmann 1832, in the natural environment The reproductive output of L. salmonis was influenced by seasonal effects. The number of eggs produced per brood showed an inverse relationship with increasing temperature. The number of eggs per brood was also influenced by adult female body size (cephalothorax length), which in itself exhibited an inverse relationship with increasing temperature. Photoperiod had no significant effect upon the number of eggs produced or on adult female size. Mean egg size of L. salmonis varied significantly over the year; larger eggs were produced during the summer months and smaller eggs over the winter. However, factors controlling the size of the eggs were not elucidated. The proportion of viable eggs per L. salmonis ovisac remained constant throughout the year. Large variations in egg number per egg string were found in both L. salmonis and C elongatus populations sampled at one point in time. These were attributed in part to phenotypic variation in adult female size and also the number of broods individual females had produced. Egg viability was not correlated with brood size, but mean egg size was related to the number of eggs per brood. Experimental studies indicated that hatching and development of L. salmonis was highly variable. The percentage of eggs hatched and the time period over which hatching occurred varied markedly, even when held under constant and optimal environmental conditions. Temperature did not affect hatching success or viability of the nauplius I stage, although at higher temperatures the period over which hatching occurred was reduced. Low and medium salinities caused a significant decrease in both hatching success and nauplius viability. Photoperiod had no effect on initiation of hatching. Hatching occurred in a manner similar to that observed in free-living copepods. The nauplii were enclosed by two egg membranes, the outer one bursting within the ovisac, the inner one after the ovisac membrane has split. Swelling of the egg and its subsequent hatching was attributed to osmotic effects, with water being taken up from the external environment. Development was also highly dependent upon both temperature and salinity. At 5'C, nauplius 11 stages failed to enter the moult to the copepodid stage. At 7.5'C, although moulting was initiated, in a large proportion of cases it was not successfully completed. At I O'C, development to the copepodid stage was successful. Nauplii only developed successfully to the copepodid stage at salinities of 25%o or greater. Copepodids raised under optimal conditions then exposed to a range of salinities had a greater salinity tolerance than nauplii. Biochemical analysis of the eggs of L. salmonis revealed that lipids constituted a large proportion of their dry weight. Naupliar stages contained a discrete area containing lipid which decreased in size over time, suggesting that the free-swimming larval stages utilised this as an energy reserve. Rate of depletion was faster in nauplii held at higher temperatures. Longevity, activity and infectivity of the infective stage decreased with age. However, both spontaneous and stimulus dependent activity ceased many hours before death and both activity and longevity were affected by temperature. Infectivity of I day old L. salmonis copepodids was higher than 7 day old larvae, and was considered to be related to the size of the energy reserves. The settlement and distribution pattern of copepodids did not change with age of copepodid, the majority being recorded from the fins. All three L. salmonis free-swimming larval stages demonstrated a "hop and sink" swimming pattern. The velocity and duration of both passive sinking and active swimming was recorded for both nauplii and copepodids. Although greater periods of time were spent passively sinking, the speeds obtained during both upward spontaneous and stimulated swimming meant that a net upward movement of larvae in the water column occurred. At higher temperatures spontaneous swimming activity increased, whilst low salinities caused a cessation of such ability. L. salmonis larvae were positively phototactic and negatively geotactic. As well as their positive responses to light intensity, the nauplius 11 and copepodid stages reacted positively to blue-green spectral wavelengths. Moulting times were relatively short, although the larvae were not able to swim during such periods. No relationship was found between the level of lipid reserves and the overall buoyancy of the larvae. Naupliar stages of both L. salmonis and C. elongalus were obtained from the water column as a result of a plankton sampling programme at a commercial Atlantic salmon farm. No copepodid stages of either species were found. There was no difference in the vertical distribution of the two L. salmonis naupliar stages. Live larvae tended to aggregate between 0 and 5m in depth, with no diurnal vertical migration. Dead nauplii, and those with low lipid reserves, were found deeper in the water column. Naupliar stages, and in particular the first larval stage, were concentrated in number within cages indicating that the cages have a retentive characteristic. A novel control method in the form of a commercially available light lure was tested. Though increasing the numbers of free-living copepods captured, it had no effect on the numbers of L. salmonis naupliar or copepodid stages obtained in plankton samples. The present study has therefore provided valuable data concerning the biology and ecology of the free-swimming larval stages of sea lice, in what was a comparatively poorly understood area.