Fiskeribiologiske undersøkelser i Langesundsområdet, 1974-1978
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From 1974 to 1978 R/V "G.M. Dannevig" was used for 7-8 cruises yearly in the Langesund area. Parameters sampled at 7 stations were: temperature, salinity and oxygen in standard depths, phytoplankton studies in the upper 12 m (primary productivlty, chlorophyll a and counting in microscope). Zooplankton and ichthyoplankton were sampled by Bongo net in the upper 50 m. Echo sounder was run continuously and occasionally recordings were sampled by pelagic trawl. The hydrographic results are given in a separate report (Dahl, 1982). The fluctuations in the phytoplankton stock were greatly influenced by the river Skienselva discharge, especially in the inner parts of the fjord system. At the innermost station there was no regular spring bloom but rather a summer maximum. Going outwards gradually the spring bloom of diatoms became more pronounced and the summer bloom of naked monads of less importance. In 1977 an extensive renewal of deep water led to high values of phytoplankton production. Also for zooplankton the highest values were observed this year. Variations in both volume and composition of zooplankton species followed the same pattern at all stations outside Frierfjord. In Frierfjord the supply of zooplankton from outer areas meant a lot, most clearly seen in 1977 after the renewal of deep water. However, also the fresh water outflow from Skienselva influenced the area. Echo soundings showed the yearly presence of sprat in Frierfjord from February to June. Eggs were found in the same period with the highest values from April to June. Outside Frierfjord only small quantities were found in June. Mackerel eggs and larvae were found at the outer stations in June.