The Use of Acoustics to Resolve Nightly Excursions of Hyperbenthos

Acoustic analysis of die1 vertical migration in the Darnariscotta River estuary, Maine, showed a fairly regular nightly increase in biovolume (mm3 m\u273) of organisms in the water column, presumed to be due to emergence, the entry of hyperbenthic organisms into the water column. Timing of these events was significantly correlated with time of sunset and time of sunrise for more than 50% of the dates where emergence and re-entry could be identified, between June and October 2002. Emergence traps indicate that the mysid shrimp, Neomysis americana, is the predominant migrator. Daily fluctuations in irradiance influence the timing of emergence fiom the hyperbenthos. Local variability in irradiance that may cause populations to emerge before sunset or leave the surface after the beginning of nautical twilight is accommodated in speed of ascent or descent. Emergence before sunset is marked by a slow ascent rate, and leaving the surface after the initiation of nautical twilight is marked by a fast descent rate. This pattern would be expected for populations avoiding visual predators by concealing themselves in dark waters. Mean ascent (f 1 SE) (0.29 f 0.03 cm s-\u27) and descent (-0.26 f 0.02 cm s-\u27) rates show little difference, suggesting that a similar mechanism controls both. Variations in the light regime as both a cue and mechanism for migration under the isolume hypothesis and rate of change hypothesis are discussed

NIGHTLY EXCURSIONS

Acoustic analysis of die1 vertical migration in the Darnariscotta River estuary, Maine, showed a fairly regular nightly increase in biovolume (mm3 m'3) of organisms in the water column, presumed to be due to emergence, the entry of hyperbenthic organisms into the water column. Timing of these events was significantly correlated with time of sunset and time of sunrise for more than 50 % of the dates where emergence and re-entry could be identified, between June and October 2002. Emergence traps indicate that the mysid shrimp, Neomysis americana, is the predominant migrator. Daily fluctuations in irradiance influence the timing of emergence fiom the hyperbenthos. Local variability in irradiance that may cause populations to emerge before sunset or leave the surface after the beginning of nautical twilight is accommodated in speed of ascent or descent. Emergence before sunset is marked by a slow ascent rate, and leaving the surface after the initiation of nautical twilight is marked by a fast descent rate. This pattern would beexpected for populations avoiding visual predators by concealing themselves in dark waters. Mean ascent (f 1 SE) (0.29 f 0.03 cm s-') and descent (-0.26 f 0.02 cm s-') rates show little difference, suggesting that a similar mechanism controls both. Variations in the light regime as both a cue and mechanism for migration under the isolume hypothesi