MEPS 178:29-38 (1999)  -  doi:10.3354/meps178029

ABSTRACT: The vertical migration of the sardine Sardina pilchardus W. was investigated in terms of a dynamic movement in the wild. The hourly position of the centre of sardine density (the weighted mean depth of sardines in the water column) was estimated by means of a dual beam echosounder during the daytime. The luminance profile of the water column was measured hourly over an extended network of 154 stations. The diel patterns of the vertical movements of the sardines were examined in relation to surface light intensity, bottom depth and ambient light intensity changes at the depth they occupied. Light intensity and bottom depth were found to affect the sardine movements. The centre of sardine density tended to level out as the light intensity increased and a wide range of 'preferred' light intensities was observed. An abrupt descending and ascending took place at dawn and dusk, respectively, whereas in the intermediate hours the depth of the sardines was rather constant. The importance of background light intensity seemed to be greater than measured light intensity. The field measurements were fitted to a cosine model to describe the pattern of diel movements.

KEY WORDS: Sardina pilchardus · Vertical migrations · Light intensity