Abstract
Pelagic Lernaeenicus larvae are regu1arly present in the innermost part of Oslofjorden during autuinn.They probably undertake diurnal vertical migrations and are most frequently recorded in neuston tows at 0–90 cm depth, taken at night, total 1183 specimens. The neuston catches are always dominated by copepodids (the first infective stage),whereas there are few adult males and nauplii II. In laboratory experiments at 15°C and 30%S, emergence from the egg sacs of L. sprattae and hatching to nauplii took place within 4-5 hours. Nauplius I changed to Nauplius II after 23–27 hours and after another 24 hours these moulted to copepodids,which lived for 5–6 days. Most nauplii (60–80%) survived to the copepodid stage in experiments at 20-30% Sand 10-20°C.The nauplii did not survive at salinities <15% which may be the decisive factor which excludes Lernaeenicus from the Baltic Sea. Copepodids swam actively toward the surface in low light-levels or in darkness. This behaviour and reversal of the direction of swimming seem to agree with observations under natural conditions, where both sprat and copepodids preferred the upper water masses during darkness. Spatial proximity between the pelagic stages of the parasites and the host increases the chance of parasite-host contact. Both prevalence and intensity of cope pod infection on sprat were significantly higher in the dark than in light experiments.