The swarm dynamics of northern krill (Meganyctiphanes norvegica) and pteropods (Cavolinia inflexa) during vertical migration in the Ligurian Sea observed by an acoustic Doppler current profiler

A ship-mounted 153 kHz narrow-band ADCP and 1 m(2) MOCNESS were deployed be tween 16 and 24 Sept. 1997 in the Ligurian central zone (similar to 43 degr ees 20'N 7 degrees 48'E). Results from both instruments showed that the zoo plankton community performed vertical migrations that conformed to the clas sical pattern of ascent at dusk ( similar to 18:30 h) and descent at dawn ( similar to 06:30 h). Depth-discrete net samples between 0 and 500 m showed that the community was dominated by two species, the euphausiid meganyctip hanes norvegica (Northern krill) and the pteropod Cavolinia inflexa, which migrated in separate discrete bands that were detectable by the ADCP. Infor mation from the ADCP was used to estimate vertical migration speed in two w ays: (ij from the trajectory of the back-scattering bands over time and iii ) from the Doppler-shift vertical velocity measured within depth zones at t he corresponding time and depth of these bands, Estimates of the migration speed of C. inflexa were between 2 and 7 cm s(-1) upwards and between 4 and 7 cm s(-1) downwards, M. norvegica was estimated to migrate at speeds betw een 7 and Gems-l upwards and over 11 cm s(-1) downwards. The consistently l ower migration speeds estimated from Doppler measurements as compared with estimates obtained from measuring trajectories of back-scattering bands ove r time was believed to result from a methodological artefact. The Doppler m easurements were nevertheless useful in a relative sense in revealing the r elative speed of individuals within swarms. It was shown that individuals a t the front of the upwardly migrating band of M. norvegica moved more slowl y than those at the rear. These results illustrate the extra biological inf ormation that can be obtained by ADCPs compared with conventional echo-soun ders. (C) 2001 Elsevier Science Ltd. Ail rights reserved.