Midnight sinking behaviour in Calanus finmarchicus: a response to satiation or krill predation?

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.