Spatial variability in zooplankton abundance near feeding right whales in the Great South Channel

Abstract On 3 June 1989, during SCOPEX'89, two right whales were observed to be feeding close to the surface at separate sites in the Great South Channel of the Gulf of Maine. The R.V. Marlin deployed and monitored a radio tag on one whale, and underway measurements were made near each whale from the R.V. Endeavor to investigate the small-scale spatial structure of water properties and zooplankton abundance in the upper water column near the whales. These measurements included two CTD tow-yos, zooplankton sampling with a MOCNESS, continuous vertical profiling of currents with a 150-kHz ADCP, and continuous vertical profiling of zooplankton concentration with a towed acoustic profiler operating at 120 and 200 kHz. The whales were feeding on a relatively homogeneous mixture of primarily two stages (copepodite IV and V) of a single copepod species ( Calanus finmarchicus ), which was most abundant in the upper 10–20 m of the water column above the seasonal pycnocline. Descriptions of the spatial structure of copepod abundance in patches traversed by the whales were developed based on MOCNESS samples, acoustic backscatter, and light transmission. In particular, a high correlation was found between MOCNESS biomass measurements and certain 200-kHz acoustic biomass estimates, which enabled the acoustic data to be interpreted solely in terms of copepod abundance. Acoustic measurements made in a copepod patch while closely following one whale indicated mean and peak copepod biomasses of 6.0 and 28.4 g m −3 (corresponding to mean and peak concentrations of 8.7 × 10 3 and 4.1 × 10 4 ) copepods m −3 in the 4–10 m depth band, where the whale was probably feeding. With a mean energy content of 10 −3 kcal copepod −1 , that whale's mean energy intake rate was 3.8 × 10 4 kcal h −1 . The whale was observed to reverse course and turn back into the patch when it swam into a region of lower copepod abundance, with biomass less than roughly 1–3 g m −3 or 1.5–4.5 × 10 3 copepods m −3 . This concentration is consistent with independent estimates of the minimum prey concentration required for a right whale to regain the energy it expends when it feeds. The next morning, one of the whales was found to be skim-feeding on a Calanus finmarchicus patch in which a bucket sample gave a copepod biomass of 256 g m −3 or 3.3 × 10 5 copepods m −3 . If this one sample approximated the mean abundance of the patch, then the whale had a mean energy intake of 1.4 × 10 5 kcal h −1 . At this rate, it could consume its daily basal metabolic energy requirement in roughly 9 min, and its annual requirement in roughly two days (assuming continuous feeding at a mean speed of 1.2 m s −1 as determined from the ADCP measurements). Although physical advection on regional and smaller scales appears to be an important element in the processes that cause such dense patches to form in this region during late spring, the lack of a clear linkage between the small-scale physical and biological data reported here suggest that some nonphysical, species-specific animal behavior like swarming must be partially responsible for creating the very densest copepod patches observed during SCOPEX'89.