Diel changes in small-scale vertical distribution of hyperbenthic mysids

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Abstract
Erythrops serrata (G.O. SARS), Boreomysis megalops G.O. SARS, Leptomysis gracilis (G.O. SARS) and Lophogaster typicus M. SARS were studied at 150 m depth in Fanafjorden, western Norway in April, June, September, and December 1979. Sampling was performed with hyperbenthic sledges in intervals of 0-80 cm and 30-80 cm above the sediment. During daylight all species were concentrated in immediate vicinity of the sediment. During darkness E. serrata performed increased swimming activity near the bottom. A part of the population of B. megalops always undertook vertical migration at night. Distribution of the remaining hyperbenthic part was less stratified than during daylight. A substantial portion of vertically migrating individuals returned to depth before sunrise. Almost the entire population of L. gracilis and L. typicus left the hyperbenthic region prior to sunset, and returned after sunrise. Approaching sunset distributional changes first occurred for L. typicus (1-2 hours prior to sunset), followed by L. gracilis and later by B. megalops and E. serrata. At twilight conditions, occasionally a transition state between daylight- and night-distribution was found, most prominent in L. typicus in the afternoon.