SCOPEX, a Multidisciplinary Study of Right Whale - Ecosystem Interactions

by The SCOPEX Group

The right whale, Eubalaena glacialis, is an endangered species with only a few hundred individuals remaining in the northwestern Atlantic. For at least the past ten years, a large proportion of the population of these zooplankton-feeding baleen whales has been present for about two months during spring in a small area just north of the Great South Channel (GSC) in the southern Gulf of Maine (GOM)(Fig. 1). SCOPEX, the South Channel Ocean Productivity Experiment, was a multidisciplinary program to investigate springtime aggregations of right whales in the southern GOM, the interactions between these whales and their main prey, the copepod Calanus finmarchicus, and physical processes influencing these biological interactions. This program was carried out between 1988 and 1991 with field sampling during late spring in 1988 and 1989. Support for a pilot study in 1986 was provided by ONR with ship time provided by NMFS/NOAA. The main study described below was supported by NSF and the Minerals Management Service.

The basic hypothesis being investigated was that the whales concentrated in this limited area each spring because of dense concentrations of their copepod food. Three (not mutually exclusive) sub-hypotheses concerning causes of possible copepod aggregations in the GSC were developed as follows:

Physical oceanographic surveys in late April 1988 and early June 1989 indicated substantial differences in the extent of a freshwater plume which occurs east of Cape Cod in spring. This plume is derived from spring river runoff into the western and northern GOM. In April 1988, the surface plume was just beginning to form along the outer coast of Cape Cod, while six weeks later, in 1989, the plume was fresher and extended further eastward towards Georges Bank. The difference in the amount of freshening between the two surveys was primarily due to the later sampling and increased river runoff in 1989. The offshore spreading of the low-salinity plume was driven by the deep circulation and upwelling favorable winds. Hydrographic data and satellite SST images showed relatively permanent continuous fronts along the eastern flank of Nantucket Shoals, across the northern shallower region of the GSC, and along the northern flank of Georges Bank.

Satellite-tracked drifters were deployed to determine flow patterns in the GSC region (Fig. 2). Near-surface (5m) flow in the western GSC was southward out of the GOM along paths of nearly constant bathymetry. In the central GSC, especially near and east of 69W, near-surface water flowed east and then northeast along the northern flank of Georges Bank, passing north of the 20 m crests of Cultivator Bank. Subsurface (50 m) flow was generally slower than the near-surface currents, and often opposite in direction. The 50 m circulation in the GSC was generally southeastward west of 69W and northeastward east of 69W. The mean Lagrangian time scale was 57 24 days for near-surface cyclonic recirculation in the Wilkinson Basin and 48 8 days for anticyclonic recirculation over Georges Bank.

Seasonal hydrographic changes also have a major effect on the biological processes in the southern GOM. The spring phytoplankton bloom begins in February and early March in shallow areas of the western GOM and extends into deeper areas as spring progresses. In early May 1988, some regions of the study area still had high concentrations of large diatoms, while in other areas the spring bloom had terminated. By late May and early June of 1989, the spring bloom had ended and the dominant phytoplankton were small (<7 m). In the well-mixed shallower areas on Nantucket Shoals, in the shallow portion of the northern GSC, and over Georges Bank, phytoplankton concentrations were somewhat higher. However, these are outside the areas frequented by right whales where the dense aggregations of Calanus finmarchicus occur.

Calanus finmarchicus dominated the zooplankton of the GSC in both years. They were aggregated in patches which were acoustically detectable (200 kHz). The locations of highest Calanus abundances differed in the two years, as did the locations of feeding whales. Younger life stages of Calanus were most abundant in the northwestern part of the study region each year, whereas older stages of Calanus tended to be found in the eastern part of the study area, particularly near the front of the freshwater plume, where a large-scale convergence was present (Fig. 3). These stage-distribution and abundance patterns result from the advection of maturing copepods by the regional circulation.

Maximum Calanus abundances and biomass of older stages were significantly higher in 1989 than in 1988, both in the region as a whole and at locations where whales were feeding. This difference might reflect the later sampling in 1989 rather than a real interannual difference in Calanus abundance. High concentrations of younger Calanus stages in the northwestern part of the study region in 1988 suggest that they are advected into the region from the north.

Calanus populations in both years were actively growing (and not diapausing), as evidenced by the temporal progession of life stages during the several week cruises. Stage durations estimated from plankton collections indicated development rates in 1988 that were similar to maximal rates observed in the laboratory, whereas development rates in 1989 were greatly reduced. These results were consistent with observations of intermediate to high and very low in situ feeding rates in 1988 and 1989, respectively.

Calanus diel vertical migration patterns differed between years. Both strongly migrating and non-migrating populations were present in 1988. In 1989, there was little migration, with dense aggregations being found at the surface both day and night. Reasons for these differences in vertical migration are not clear; there was no obvious relation between migration behavior and stage of development, food abundance, or feeding rates. The possibility that predation intensity is responsible for these behavioral differences is being explored using acoustical data collected during the study.

Within the broad-scale distributions described above, there was considerable small-scale horizontal and vertical variability in Calanus abundance. This was most apparent in 1989 when the non-migrating population was observed at the surface. Abundance varied by up to 1000X in horizontal tows spanning about 0.5 - 1.5 km. Maximum concentration from the MOCNESS tows in 1989 was 30,800 m-3. However, a number of visibly red surface patches were observed in 1989 at scales too small to be sampled effectively with a MOCNESS; one of these sampled with a bucket yielded a concentration of 331,000 m-3.

Whales were observed feeding in the regions with older (larger) Calanus. In 1988, whales were found primarily in the western side of the study area, while in 1989 most of them were seen on the eastern side. Whale diving patterns differed, and appeared to be directed by the distribution of the copepods, in the two years. In 1988, when the copepods spent the day near the bottom, right whale dives were much longer in the day. Conversely, whale daytime dives in 1989, when the copepods stayed near the surface, were both shorter and shallower. Tagging studies indicate that the whales were diving only to 5-20 m where copepods were most abundant.

The SCOPEX study has improved our understanding of this area where Calanus finmarchicus, a preferred food of the right whales, is concentrated during late spring. The higher regional concentration of Calanus may be related to recruitment processes occurring earlier in the spring in coastal waters in the western GOM, which is outside the SCOPEX study area. Earlier, upstream recruitment may lead to the development of large populations of later-stage Calanus as the waters are advected into the GSC. Furthermore, the low salinity surface plume with its associated fronts and convergences creates considerable small-scale patchiness of Calanus within the study area. It is the presence of these-small scale patches with very high concentrations of older stages of Calanus in them which offers a favorable feeding habitat for the right whales.

As a footnote, the high concentrations of Calanus observed in this region in 1988 and 1989, as well as during the pilot study in 1986, did not occur in 1991 and 1992. In both of these years whales either did not stay long in the region or were absent. This interannual variability is especially pertinent to U.S. GLOBEC studies of Calanus in the NW Atlantic; drifter information from the SCOPEX study demonstrates a close link between the southern GOM and Georges Bank populations of C. finmarchicus.

The SCOPEX group: R. Beardsley and R. Limeburner (Woods Hole Oceanographic Institution); P. Cornillon, A. Durbin, E. Durbin, R. Kenney, H. Winn, and K. Wishner (University of Rhode Island); M. Macaulay (University of Washington).

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