SPATIAL VARIABILITY IN ZOOPLANKTON ABUNDANCE NEAR FEEDING RIGHT WHALES IN THE GREAT SOUTH CHANNEL

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 Cha nnel of the Gulf of Maine. The R.V. Marlin deployed and monitored a ra dio tag on one whale, and underway measurements were made near each wh ale from the R.V. Endeavor to investigate the small-scale spatial stru cture of water properties and zooplankton abundance in the upper water column near the whales. These measurements included two CTD tow-yes, zooplankton sampling with a MOCNESS, continuous vertical profiling of currents with a 150-kHz ADCP, and continuous vertical profiling of zoo plankton concentration with a towed acoustic profiler operating at 120 and 200 kHz. The whales were feeding on a relatively homogeneous mixt ure of primarily two stages (copepodite IV and V) of a single copepod species (Calanus finmarchicus), which was most abundant in the upper 1 0-20 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 backscat ter, and light transmission. In particular, a high correlation was fou nd between MOCNESS biomass measurements and certain 200-kHz acoustic b iomass estimates, which enabled the acoustic data to be interpreted so lely in terms of copepod abundance. Acoustic measurements made in a co pepod 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 p eak concentrations of 8.7 x 10(3) and 4.1 x 10(4) copepods m(-3)) in t he 4-10 m depth band, where the whale was probably feeding. With a mea n energy content of 10(-3) kcal copepod(-1), that whale's mean energy intake rate was 3.8 x 10(4) kcal h(-1). The whale was observed to reve rse 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 x 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 f inmarchicus patch in which a bucket sample gave a copepod biomass of 2 56 g m(-3) or 3.3 x 10(5) copepods m(-3). If this one sample approxima ted the mean abundance of the patch, thin the whale had a mean energy intake of 1.4 x 10(5) kcal h(-1). At this rate, it could consume its d aily basal metabolic energy requirement in roughly 9 min, and its annu al requirement in roughly two days (assuming continuous feeding at a m ean speed of 1.2 m s(-1) as determined from the ADCP measurements). Al though physical advection on regional and smaller scales appears to be an important element in the processes that cause such dense patches t o form in this region during late spring, the lack of a clear linkage between the small-scale physical and biological data reported here sug gest that some nonphysical, species-specific animal behavior like swar ming must be partially responsible for creating the very densest copep od patches observed during SCOPEX'89. Copyright (C) 1996 Elsevier Scie nce Ltd.