Copepod Patches and Right Whales in the Great South Channel off New England

For about a month every spring, most of the northwest Atlantic population of right whales (Eubalaena glacialis) is found in the Great South Channel off New England. We hypothesized that the whales aggregate in this region during the spring because of the increased abundance or aggregation of their major food item, the copepod Calanus finmarchicus. During a 4-day cruise in May 1986 to the Great South Channel, we located a group of surface-feeding right whales spatially associated with a large, dense, nearly monospecific concentration of copepods, primarily C. finmarchicus. A 200 kHz acoustic system showed that these copepods formed an extensive, nearly continuous surface layer that may have occupied an area of over 2,500 km². The copepod layer began about 20 km north of a front and extended northward at the surface in the region where water depth was deeper than 100 m. In MOCNESS plankton tows, a maximum copepod biomass of 23.6 g wet wt·m⁻³ and abundance of 41,600 copepods·m⁻³ were found, with highest abundances in the upper 20 m. The dominant developmental stage was the copepodite IV, but developmental stages from nauplius II to adult males and females were also present. The patch was a regional abundance maximum for the older developmental stages, but relative abundances of different stages changed with depth. The range of diel vertical migration of Calanus in the patch was small, and maximum abundances occurred near the surface day and night. The lower and side borders of the patch could be distinct and abrupt. Calanus abundance inside the patch was about 83 times higher than immediately below the patch and 311 times higher than immediately adjacent to the patch. Several biological and physical mechanisms, interacting with copepod behavior, may be important in the formation of the patch and the maintenance of its edges.