Discussion leader: C. Miller
Rapporteur: J. Runge
C. Miller opened the discussion with an assessment of the current status of knowledge on target species and relevant physical process in the Gulf of Maine/Georges Bank system.
C. finmarchicus passes through two generations in the Gulf of Maine/Georges Bank system. Overwintering females are found in December-February, the first generation in March-May and the second generation in May-July. Life history studies of C. finmarchicus have recently been completed at locations in the NE Atlantic, the Gulf of St. Lawrence and the Scotian Shelf, providing a database for comparison of life cycles among regions throughout its range.
There is interesting new knowledge of abundance trends and interactions with climate variation. Analysis of CPR data (e.g. Planque and coworkers) show a negative correlation between Cfin abundance and the NAO index. Cfin in presently at an all time low in the E. North Atlantic. CPR data from the w. Atlantic (Sameoto) indicate a minimum in Cfin abundance in the 1990’s along the eastern Scotian Shelf.
Process studies have shown that Cfin growth and reproduction is often food limited on Georges Bank. D. Gifford has shown that the diet of developmental stages comprises primarily dinoflagellates and microheterotrophs, not diatoms. Food concentrations in terms of µgC l-1 are frequently less than the estimated critical concentration for maximum growth and reproduction. Durbin, Campbell, Runge and coworkers have shown that growth, development rates and reproduction are often on the order of 1/3 to 2/3 maximum for a given temperature, as estimated from laboratory experiments under optimal conditions. On occasions, such as April, 1995, on the southern flank, food resources may be severely limiting.
Campbell et al. have measured growth and development times in the laboratory as a function of food concentration and temperature. The results are necessary for the biological-physical modelling and provide a standard for estimating the extent of food limitation on the bank.
Runge and coworkers have measured egg production on Georges Bank, from which data there is now knowledge of the seasonal, spatial and interannual variation in egg output and hatching success. There is now a good dataset on the E. Atlantic as well (due to the TASC project), although data are still lacking from the Irminger Sea.
The first generation of biological-physical models describing the population dynamics of Cfin in the 3D flow of the Gulf of Maine/Georges Bank have been achieved (Lynch, Gentleman, Lewis, Miller). What we don’t have yet in the model is a component describing predation.
Spatial and seasonal variation in mortality rates of Cfin have been measured in 1995 and 1996 broadscale surveys (Ohman and coworkers). When completed, this analysis will extend through 1999.
It is generally acknowledged that we haven’t got there yet. We don’t understand the nature of diapause well enough to predict timing of entry and arousal.
In general, we are unable to detect genetic differences in the western Atlantic region (Georges Bank to Gulf of St. Lawrence). There is, however, a clear difference in genetic compostion between the eastern and western sides of the Atlantic.
1. Identification
It is very difficult to morphologically distinguish P. moultoni from P. newmani. A. Bucklin, however, has pioneered molecular techniques to make identification of species in preserved samples possible.
2. Distribution patterns
Ann Bucklin has shown that P. moultoni is more a resident on the crest of Georges Bank and P. newmani is more abundant on the northeast peak and southern flank, suggesting advective transport from offshore.
3. Life History and timing, fecundity and mortality rates
Knowledge lags behind C. finmarchicus. Durbin, Runge, Ohman and Campbell have collected data and a better understanding of these characterstics will be forthcoming by the end of the program.
We know reasonably well:
1. Timing of spawning, from groundfish surveys.
2. Transport of larvae occurs generally along the southern flank.
3. Growth rates of cod and haddock larvae have been measured by Lough, Buckley, and colleagues. Werner et. al. have constructed a 3-D, coupled physical-trophodynamic model of growth and survival. Short term growth rates are generally temperature-limited up to 7°C, above which they tend to be food-limited.
4. Mortality rates have been measured directly by cohort analysis during broadscale surveys.
5. Recruitment prediction. The trophodynamic model of Werner et al. is a tool for synthesizing understanding of the various influences on larval fish growth and survival on Georges Bank. Annual recruitment prediction is less important to fisheries management than prediction of climate interactions affecting longer term variability and trends.
Check marks for good progress were made for many of the physical components of the program. For example, we have a good idea of the flow (where does the water go?) from the 3D finite element models of Lynch and coworkers, the forces driving the flow and the sources of water on Georges Bank. However, during the course of later discussion the uncertainties in knowledge of physical processes were pointed out and discussed at more length on Wednesday. A remarkable feat this past year was the development of a real-time data assimilation and flow prediction model, which was used during several cruises this spring to predict water movement on Georges Bank.
Data on nutrient supply and chl. a on later cruises have been collected by Townsend and colleagues.
An open discussion followed this brief general review. In the report below, the discussion is organized by subject, in approximate sequence of occurrence.
C. finmarchicus in diapause
Diapause in C. finmarchicus is a complicated behavior and may comprise more than overwintering at depth and absence of feeding. There is evidence that overwintering CV take snacks in the fall and winter. They can wake up, go to the frig and eat diatom sandwiches, then go back to sleep. Diapause is not necessarily total; there is residual active behavior, including alertness to attacking predators.
The control mechanisms for diapause appear to be different across the Atlantic. Whereas arousal from diapause may be in January-February in the Gulf of Maine, it is not until March-April in the Irminger Sea.
The life cycle of C. finmarchicus on Georges Bank: number of generations.
The overwintering G1 generation occurs between January-March. Its offspring, the G2, mature starting in March and continuing through May. The third generation (G3) generally starts appearing in May. There may be large populations of C. finmarchicus in May-June. From broadscale surveys, it is evident that there is considerable spatial and temporal variation in cohort structure on Georges Bank.
There was a question about the extent of food limitation, especially in G1. The difference in reproductive behavior between the NE and NW Atlantic Ocean was noted. In the NE Atlantic, substantial spawning is observed prior to the spring bloom. However, in the NW Atlantic (e.g. the St. Lawrence Estuary), the spawning cycle commences with the onset of the phytoplankton bloom even if the bloom does not occur until mid-June. Runge and coworkers have observed that the egg production rate of C. finmarchicus on Georges Bank fits an Ivlev function, in which the threshold concentration is 15-20 mg. chl. a m-2 and the critical concentration (at which the maximum e.p. is attained) occurs at about 150 mg chl a m-2. Chl. a may serve as a proxy for food availability to Calanus females, as process studies indicate that their principal prey is microzooplankton, not diatoms.
Vertical distribution of C. finmarchicus in the Gulf of Maine
Fall surveys in the Gulf of Maine indicate that C. finmarchicus is not piling up near the bottom. The modal distribution is highest in the water column (90-125 m in Jordan and Georges basin, data presented by Wiebe). There was a question whether deep-feeding siphonophores or krill might be influencing the vertical distribution pattern, perhaps by removing the deepest dwelling individuals or by forcing them upward. Results from the Norwegian Sea and fjords indicate that Calanus overwinters below acoustic scattering layers. The hypothesis is that Calanus adjusts its vertical overwintering distribution to actively avoid predators; it appears to react more strongly to fish than to invertebrates.
It was recognized that the Gulf of Maine is vastly undersampled. Early research (e.g. Fish) suggests that reproduction occurs in August and September in the Gulf, and broadscale surveys indicate that a fall generation is possible. However, the sample and data base in the Gulf of Maine are inadequate to address this question. There is insufficient knowledge of the late summer and winter vertical and horizontal distributions.
Sources of supply to the Gulf of Maine
There is a buildup of C. finmarchicus in the Gulf of Maine in November-December. The question arose as to the relative contribution of slope water to the Gulf of Maine supply of Calanus. There are clearly two kinds of slope water with different origins: the Labrador Current slope water is characterized by low S and low T, whereas the warm slope water has high S and high T. The knowledge of the depth distribution of C. finmarchicus is inadequate for the proper understanding of upstream sources; nevertheless, there are interesting observations about differences in Calanus concentrations between the two slope waters. The abundance of C. finmarchicus in Labrador SW is generally low (<1000 m-2 in a 1500 m water column). Nevertheless, Calanusare present and a slug of Calanus arrives from the Labrador Current from its spring production and may enter into the central part of the Scotian Shelf in the upper 50 m. Physically, the only way to get Calanus up on the shelf is by transport in the upper part of the water column. Transport off Cape Sable is largest in winter. There may be injection of Calanus into the Gulf of Maine as they descend in summer. Calanus are found in slope water in April., May and June, both in the warm SW and in the colder Labrador SW.
An identified gap in understanding of sources of supply is the connection of the life history characteristics, including vertical distribution, to the seasonal three-dimensional circulation. Modelling should be expanded to the NE of the Gulf of Maine in order to understand the relative contribution of the upstream sources. In addition to depth distribution data, there is a need for information on what time of year fluxes of C. finmarchicus onto the Scotian Shelf and into the Gulf of Maine occur.
Trends in Calanus abundance in the Gulf of Maine
Abundance levels during the recent surveys are approximately an order of magnitude less than concentrations observed in basins on the Scotian Shelf (Sameoto). There is a remarkable correlation between variation in zooplankton displacement volume and the salinity anomaly in the 0-30 m layer, from 1970-present.
CPR (Continuous Plankton Recorder) records of May-June abundance in the Gulf of Maine indicate that abundance was relatively low in the 1960’s compared to the 1980’s. During the former period, the NAO (North Atlantic Oscillation) index was negative, whereas the NAO index was positive during the 1980’s to mid-1990’s. In 1995-96, the NAO index dropped dramatically, from positive to negative. Calanus numbers have also dropped off, and there is the question whether it is related to the decline in the NAO index.
In the 1960’s, during the low NAO index, there was more Labrador SW around the corner from the Gulf of Maine. This was also the case in 1995-96 (P. Smith). In the normal situation, the Gulf Stream is situated further north, and there is less intrusion of Labrador SW offshore of the NE Channel. A pulse of 100-500 m Labrador SW formed in 1996 was tracked. It appeared off SW Newfoundland in 1997, in Emerald Basin in February 1998, the NE Channel in June 98, Jordan Basin in August 9198, and off NY Bight in March 1999.
The possibility of a connection between this large scale climate variation and trends in C. finmarchicus abundance in the Gulf of Maine was discussed. Periods of low Calanus abundance may result from filling of the Gulf of Maine with colder, relatively Calanus poor Labrador SW. It was pointed out, however, that the high Calanus abundance in the Gulf of Maine in the mid 80’s may have been the consequence of the absence of predation pressure from herring stocks, which were exploited to very low levels during the same period. An outstanding question is whether Calanus abundance in the Labrador SW is indeed much lower than in the warm SW or the long term Gulf of Maine average.
The zooplankton displacement volume time series occurs during a period when the NAO index was always positive. The CPR time series is longer, covering the transition from negative to positive. This must be taken into account when interpreting the two time series together.
This discussion highlighted the importance of maintaining time series for identification and understanding of climate variation and its ecosystem impacts.
Other copepod species
Centropages species are very important on Georges Bank. They are found in the stomachs of predators more commonly than Calanus. However, the trophic link to cod larvae seems to be primarily through Pseudocalanus species, at least on the southern flank. Oithona naupliar stages are also common in the diet of Georges Bank fish larvae. Pump samples in March on NE Peak show that C. finmarchicus and Oithona dominate the abundance of copepod nauplii. However, Pseudocalanus species are abundant on the southern flank. It was generally acknowledged that the diet of cod and haddock larvae on the bank should reflect the composition of prey (copepod nauplii) in the water column.