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The vertical migration of Calanus finmarchicus and krill (Meganyctiphanes norvegica and Thysanoessa raschii ) was monitored during the summer of 1999 in the Clyde Sea using a combination of acoustic and net sampling methods. A moored 300 kHz acoustic Doppler current profiler (ADCP) identified a sound scattering layer (SSL) that started to ascend to the surface during the last moments of daylight. Net samples showed that the SSL was mostly composed of krill. C. finmarchicus rose to the surface in the late afternoon, causing a small but detectable increase in backscatter that did not vary in time through the summer. The ascent of krill, by contrast, became earlier as day length decreased towards autumn. Net samples showed that the strong downward Doppler velocities following the rise of the SSL was caused by the descent of C. finmarchicus. The fact that this coordinated sinking of C. finmarchicus occurred earlier towards autumn, even though the time of ascent to surface remained constant, implies that the feeding 'window' diminished over the course of the season. Feeding conditions did not become significantly better during this same period, discounting satiation as a likely cause of descent. The close temporal coupling between the arrival of krill and the subsequent descent of C. finmarchicus from the surface suggests that midnight sinking in Calanus is a response to predation. KEY WORDS: Zooplankton · Copepod · Euphausiid · Vertical migration · Clyde Sea · ADCP Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 240: [183][184][185][186][187][188][189][190][191][192][193][194] 2002 methods. This works on the principle that animals found in one depth layer contain food particles that could have only been obtained from another depth layer, proving that they must have migrated vertically at some point.The acoustic Doppler current profiler (ADCP) is a recent technological advance that is able to obtain extra behavioural information that improves our insight into vertical migration behaviour. The instrument was designed to determine the velocity and direction of currents through the water column, but can also measure the vertical distribution and quantity of zooplankton biomass in the water column, as shown by Greenlaw (1979). The advantage of the ADCP over conventional echo sounders is its capability to estimate the velocity of zooplankton in the vertical plane through measuring Doppler shift (Pleuddeman & Pinkel 1989). Analysis of this parameter overcomes some of the problems of traditional methods because it measures the average swimming behaviour of individuals rather than the bulk movements of populations.This study uses a 300 kHz ADCP over a continuous period in the summer of 1999 in the Clyde Sea to investigate the vertical migration behaviour of zooplankton species, in particular the copepod Calanus finmarchicus and the krill species Meganyctiphanes norvegica and Thysanoessa raschii. C. finmarchicus is particularly widespread throughout most of the Nort...

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