ABUNDANCE, DISTRIBUTION AND POPULATION-STRUCTURE OF THE COPEPOD CALANUS-FINMARCHICUS IN A SPRINGTIME RIGHT-WHALE FEEDING AREA IN THE SOUTHWESTERN GULF OF MAINE

Springtime aggregations of the planktivorous right whale (Eubalaena gl acialis) occur in the northern Great South Channel region of the south western Gulf of Maine, where they feed upon dense concentrations of th e copepod Calanus finmarchicus. This association was studied during th e multidisciplinary South Channel Ocean Productivity Experiment (SCOPE X) in 1988 and 1989. The spatial and temporal variability of the abund ance, geographic distribution, and population structure of these copep ods were analyzed using data from 99 vertically-stratified or horizont ally-sequenced MOCNESS plankton tows. Higher water column abundances a nd higher relative proportions of older copepod lifestages occurred ne ar feeding whales compared to sites without whales, but total water co lumn copepod biomass and Calanus abundance did not always differ betwe en these types of locations. This suggests that the whales seek out ag gregations of older copepod lifestages rather than simply the most den se aggregations. Other factors (and perhaps an element of chance) may influence which specific patches, among all patches potentially suitab le in terms of copepod abundance and age composition, the whales utili zes at a particular time. The times and locations of the highest Calan us water column abundances varied between years, as did the presence o f feeding whales, probably because of year-to-year differences in the springtime temperature cycle and current strength. A temporal progress ion of lifestages occurred within the region in both years during the roughly 3-week duration of each survey, indicative of a growing rather than a diapausing population, at least up to the copepodite 4 (C4) st age. Due in part to a delay in the springtime warming in 1989 compared to 1988, the copepod development cycle, which is largely driven by in situ temperature, was delayed about 1-2 weeks in 1989. Peak abundance s of younger Calanus were found in the northwestern part of the region each year, whereas peak abundances of older Calanus were found in the southwestern and northeastern part. This was probably due to the adve ction of maturing copepods by the regional circulation, especially the near-surface current associated with the movement of the low-salinity surface plume which forms each spring off Cape Cod. The copepod devel opment cycle occurs within a moving frame of reference (i.e. the water itself); thus, peak abundances of the older copepods (those fed on by the whales) occurred later in the spring and further downstream in 19 89 (when there were colder springtime temperatures and faster currents ) than in 1988 (when the springtime temperatures were warmer and curre nts slower). Maximum Calanus abundances and biomass and water-column a bundances of older copepodite stages were significantly higher (about double) in 1989 than in 1988, both in the regions as a whole and at si tes where whales were feeding. Maximum concentrations from the MOCNESS tows were 13,300 m(-3) in 1988 and 30,800 m(-3) in 1989; however, a t hin, visibly-red surface patch of Calanus, sampled in 1989 by a bucket , had a concentration of 331,000 m(-3). Copepods were also more aggreg ated in the vertical (i.e. more highly concentrated at the depth of ma ximum abundance) in 1989 than in 1988, and samples from whale-feeding areas were more homogeneous in composition (higher proportion of Calan us relative to all zooplankton) in 1989. At smaller spatial and tempor al scales, abundances varied by a factor of 1-890X in samples from hor izontal tows spanning about 0.5-1.5 km and by a factor of 1-50X over 2 4 h in the same geographic location in whale-feeding areas. Some of th is variability was probably due to advection by the semidiurnal tidal currents. Near feeding whales, the copepod spatial distribution was pa tchy on small scales (with an estimated mean patch ''size'' of about 5 00 m), but the patchiness varied in texture interannually. Copepod abu ndances were much lower in early spring (March 1988) than in later spr ing (May 1988), with the March population structure dominated by adult females and the May population dominated by copepodite 4 and 5 stages (C4 and C5).