VERTICAL MIGRATION OF COREGONID LARVAE IN THE 1ST 2 MONTHS OF DEVELOPMENT

Using an acoustic system in a 9 m deep enclosure, the daily vertical d istribution patterns of white-fish larvae were studied from hatching t o the age of two months. Four patterns of daily vertical distribution and migration were observed in accordance to different developmental s tages: 1) Newly hatched larvae dispersed in the enclosure during the d ay and congregated at the water surface at night. 2) After 2 weeks of age larvae stayed close to the water surface during day and night. 3) At 15 to 18 mm length, larvae showed distinct vertical migration at da wn and dusk, they stayed close to the surface during the day and dispe rsed at night. 4) At 28 mm length the depth pattern was reversed. Thes e fish frequented greater depths during the day than at night. No rela tion was found between larval depth distribution or vertical migration and food availability or water temperature. Low levels of solar radia tion triggered the dawn and dusk vertical migration behaviour of the m ore developed larvae. Possible reasons for the different behaviour pat terns occurring over time are discussed.