Persistent spatial and temporal abundance patterns for the late-stage copepodites of Centropages typicus (Copepoda: Calanoida) in the US Northeast continental shelf ecosystem

Abstract

The annual cycle of abundance and the monthly distributions of the copepod <it>Centropages typicus</it> are described for US Northeast Continental Shelf waters from samples collected on broadscale plankton surveys during 1977-87. High numbers of the copepod were captured throughout the region during the autumn months in weak south-north and onshore-offshore abundance gradients. The highest individual station densities were found near the mouth of the major estuaries and the heaviest broad-scale concentrations were usually located where bottom depth ranged from 20 to 39 m. Numbers declined throughout the ecosystem after winter arrived, less so in the southern half of the region where<it>C.typicus</it> abundance remained high year round in nearshore and midshelf waters between New York City and Chesapeake Bay. The copepod's abundance fell to much lower levels further north in the Georges Bank and Gulf of Maine subareas, and disappeared entirely from shelf waters in the northernmost offshore region until summer. Interannual abundance variability was substantial, but no long-term trend was detected. Analyses of samples collected from 1988 to 1996 on Georges Bank during early autumn indicate that abundance levels of <it>C.typicus</it> have been high here in the 1990s, completely recovered from low density values measured there in 1986 and 1987. Temperature and food availability were found to be the key factors that determine the copepod's distribution and annual abundance cycle. Mean abundance was high throughout the ecosystem where surface temperature was &gt;9<IMG SRC="/math/deg.gif">C and in regions where annual mean chlorophyll levels exceeded 1 mg m^-3. The copepod's abundance appeared to be independent from variation in water column salinity.