Abstract

A dense breeding population of chaetognaths, Sagitta elegans, was observed on northeastern Georges Bank during autumn, 1978. Four grid surveys conducted on the northeast region of the Bank over the period 17-30 October 1978 revealed that the high concentration "patch" of adult S. elegans. about 56 km on long axis, remained essentially on the eastern part of the grid in weakly stratified water of 67 m average depth. Current meter measurements indicated residual flow to the southeast sufficiently high to transport a passively drifting organism out of the area over the two weeks of study. Discrete vertical sampling showed that the reproductively mature adults underwent extensive migration from surface to near bottom by day in response to the solar cycle. The smaller immature stages did not migrate vertically or only had limited range; they were found throughout the water column, both day and night, and were spread across the shallow well-mixed as well as deeper weakly stratified waters. Using current meter records, transport of the migrating adults and essentially nonmigrating immature stages of the S. elegans population at different sites within the study area was modeled. The simulations revealed that the migration of adults to near bottom for several hours of the day, where the current was assumed to approach zero, resulted in a longer residence time on the Bank than for nonmigrating immatures. More importantly, it is shown how the strong lunar tidal regime characteristic of Georges Bank may be an important mechanism for generating smaller scale variability on the order of <50 km and <2 weeks.

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