Abstract
The ectoparasites were collected from sprat caught by beach seining, pelagic trawling, and commercial purse seining in 1977–1983. The material is presented in tables which also show the length distribution of sprat. Small sprat in their first and second year, which shoal inshore in the inner fjord (N = 10 000), were more heavily infected by L. sprattae (mean 3.5 %) than those from the outer fjord (N = 5000, mean 0.3 %). Fish from the inner fjord caught by beach seine were smaller, mostly O-group of sprat, and were more heavily infected by L. sprattae (mean 4.3 % and range 0.2-18.3 %), than those collected with other gear. Sprat are parasitized by L. sprattae very early in their life history as they shoal inshore in the inner Oslofjord. The smallest sprat carrying an adult L. sprattae was only 3.1 cm long. Data on L. sprattae and L. encrasicoli infection of sprat in different studies are presented and discussed. Sprat is the principal host for L. sprattae in northern European waters, whereas pilchard is an additional host further south. L. sprattae is an obligate eye parasite with a very strong preference for the upper rear quadrant of the sprat's eye.
L. encrasicoli is not as host-specific and infects several clupeid fishes. The mean rate of L. encrasicoli infection on sprat was only 0.4 % (N = 12 000). This species was found more often in the outer fjord than at the inner stations, but only 42 infected sprat were found, while 8 times as many (368) were infected by L. sprattae. The prevalence of L. sprattae infection found here is similar to that reported from other inshore localities, and confirms that both species, and especially L. encrasicoli, are consistently rare parasites. The migratory pattern of sprat and the prevalence of Lernaeenicus infection is discussed. The sprat samples also included varying proportions of juvenile herring which were never found carrying Lernaeenicus parasites.