Tidal modulation of nocturnal vertical migration from the benthos: a high resolution acoustic analysis

Emergence-trap sampling has shown that several species living primarily on or in the sediment emerge nightly into the water column. A TRACOR acoustic profiler, TAPS, was mounted on the sea bottom, looking upwards in approximately 10 m of water in the Damariscotta River Estuary, Walpole, Maine. TAPS emits high-frequency (265-3,000 kHz) sonar pulses and each minute measures backscatter from 12.5 cm range bins in the water column. TAPS data showed a distinct increase in backscatter in the water column most evenings, beginning near or after dusk and terminating before dawn. Emergence-trap samples suggest that the acoustic signal was dominated by Neomysis americana and Crangon septemspinosa, a possum and decapod shrimp, respectively. In addition, there appeared to be a second emergence event per night. A higher-density emergence event occurred shortly after the first peak in tidal speed after dark, as the tidal currents were decelerating. Although "emergence" of hyperbenthic organisms has only been documented as occurring at night, a similar increase in backscatter was observed during the day, however, was significantly smaller than at night. Our acoustic results suggest that emergence in the Damariscotta River Estuary is tidally modulated. They also suggest that emergence in an environment with strong tidal velocities may be more temporally complex than previous studies by non-acoustic means have been able to resolve.