Transport and retention of vertically migrating adult mysid and decapod shrimp in the tidal front on Georges Bank

Abstract

Vertical profiles of the adult epibenthic shrimp Neomysis americana and Crangon septemspinosus obtained during June 1985 were used to simulate possible rates of ascent from bottom (40 to 50 m) to near surface at night and return by day, and the consequence of these rates on their horizontal distribution. Numerical particles were released at the sampling site using archived model current fields with specified vertical rates (from no swim behavior to 20 mm s −1 ) and tracked for up to 30 d. The best match between observed and modeled vertical profiles was with a vertical swimming speed of 10 mm s −1 for N. americana and 2 mm s −1 for C. septemspinosus. Whereas N. americana rapidly swims towards the surface at dusk and descends to bottom by dawn, C. septemspinosus tends to only swim up to the middle of the water column at night. After 16 d, the simulation with 10 mm s −1 swim speed showed most particles were concentrated in an area centered around the 60 m isobath, where the tidal front was located. At 2 mm s −1 swim speed particles were concentrated more shoalward onto the western end of Georges Bank. N. americana are expected to be more closely associated with the tidal front, since they spend more time near the front surface convergence, but are more likely to be transported off the bank due to the south- westward-flowing surface tidal jet, whereas C. septemspinosus would be retained primarily on the bank, since they are found deeper in the water column during both day and night.