Orientation and Swimming Speed of Plaice Migrating by Selective Tidal Stream Transport

Abstract

We observed selective tidal stream transport in 10 large female plaice tracked in the southern North Sea and eastern English Channel during the winters of 1994 to 1999. We tagged the fish with 300 kHz transponding acoustic tags and followed them using sector-scanning sonar. We determined the position of the research vessel by differential GPS and measured the speed and direction of the tidal stream using an Acoustic Doppler Current Profiler (ADCP). We are in the process of estimating the swimming speed and direction of the fish by vector subtraction, using data from 53 midwater excursions. We present here the data for one representative track, which agree with theoretical predictions that migrating plaice should head downtide in midwater and swim at a speed close to their optimum. Mean headings for the selected fish were mostly within ± 35° of the mean direction of the tidal stream, all were within ± 60°, and the average down-tide swimming speed was approximately 0.6 fish lengths s−1. Summary data for the remaining nine fish do not differ significantly from these values. We discuss our results in relation to the reconstruction of the geographical tracks of fish from pressure records obtained from data storage (archival) tags and the sensory mechanisms involved in the migration of demersal fish in tidal seas. We also compare them with the results of an earlier analysis based on whole-tide ground tracks and average tidal stream vectors derived from navigation charts.Key wordsplaiceselective tidal stream transportmidwater orientationswimming speed and directionacoustic tagsADCP