472 



Fishery Bulletin 91(3), 1993 



were both higher than those from the central shoals 

 area (27cc/100m a ), the region where biomass is usu- 

 ally concentrated. 



Centropages typicus dominates the zooplankton popu- 

 lation in early autumn; it makes up on the average 

 41% of total zooplankton abundance. Consequently, 

 variation in its interannual abundance pattern nearly 

 mirrored that of early autumn biomass. The only sub- 

 stantial deviation between patterns occurred in 1985 

 when C. typicus density soared to its ten-year high 

 (Table IE). Biomass was high in early autumn 1985, 

 but not in proportion to this copepod's abundance. 

 Centropages typicus abundance usually declined sub- 

 stantially offshore of the 100-m contour (Fig. 4). How- 

 ever, its abundance ( 10,805/100 m 3 ) in the deep-water 

 depth area in 1980 was substantially higher than the 

 time series median ( 1319/100 m'). Consequently, off- 

 shore biomass in 1980 (30.5cc/100m :i ) was also well 

 above the deep-water 10-year median (7cc/100m 3 ). 

 Though their numbers decline from summer, C. 

 hamatus abundance continues to be a large compo- 

 nent of zooplankton biomass in central shoal waters. 

 Population estimates peaked in 1979 and, like bio- 

 mass, were low in 1986 (Table IE). Early autumn de- 

 partures from average annual or seasonal cycles of 

 biomass and species abundance and distribution could 

 not be related to variations in surface water tempera- 

 ture (Table 2). 



Late autumn The high zooplankton biomass of the 

 late 1970s continued through late autumn (Table IF). 

 The seasonal peak recorded in 1982 was unexpected. 

 Biomass increased from early autumn and pushed 1982 

 measurements above average for the first time. These 

 high years were all significantly different (P<0.05) from 

 the seasons lowest biomass measured in 1983 and 1986, 

 the same years that were low in early autumn. 



Centropages typicus continue to dominate the zoo- 

 plankton as biomass declined towards its winter low. 

 Its abundance was above average in the high biomass 

 years of the late 1970's (Table IF). The high biomass 

 of 1982 was not related to total copepod or zooplank- 

 ton numbers. Median counts of the five dominant cope- 

 pod species and total zooplankton were not significantly 

 different (P>0.05) from any of the other years. Cursory 

 examination of other species abundance indicated that 

 chaetognaths were prevalent and may have increased 

 the biomass. The reverse occurred in 1985 when slightly 

 above average biomass did not correlate to high zoo- 

 plankton abundance. C. typicus and C. hamatus abun- 

 dance in late autumn 1985 were three or more times 

 above the ten-year median (Table IF). 



Warm late autumn temperatures appear to slow the 

 decline of zooplankton biomass and abundance to the 

 annual lows found in winter. High biomass years in 

 late autumn all had above average surface tempera- 



tures (Table 2). The only high biomass levels of early 

 autumn that were not sustained through late autumn 

 were those collected in 1985, when surface tempera- 

 ture had fallen below the ten-year median. Conse- 

 quently, it is not surprising that biomass and domi- 

 nant copepod abundance were highest at stations where 

 surface temperature was warm. The bimodal annual 

 cycle displayed by C. hamatus in the mixed depth zone, 

 that is to say the decline in numbers between early 

 and late autumn from 1977 to 1981 and the increase 

 in subsequent years (Table 1, E-F), cannot be explained 

 by variability in autumn surface temperatures. 



Discussion 



The zooplankton population on Georges Bank exhib- 

 ited considerable interannual and seasonal variability 

 during the period from 1977 to 1986. Overall, biomass 

 was above average from 1977 through 1979 and low 

 from 1982 through 1984. Unique to the late seventies 

 was the high late spring abundance of C. finmarchicus 

 in the central shoal depth zone. The species was only 

 a minor component of the zooplankton community there 

 in later years. Compared with other years, 1977 bio- 

 mass and copepod abundance levels in spring were 

 extraordinary: two to three times above the ten-year 

 median. 



Calanus finmarchicus and P. minutus abundance 

 fluctuations were nearly identical throughout the ten- 

 year time series. This suggests that the annual abun- 

 dance of these two species is regulated by similar pro- 

 cesses and events, despite differences in their life cycles 

 (Davis, 1987). This is in contrast to the results derived 

 by Davis (1984) from model simulations of their sea- 

 sonal cycles on Georges Bank. He concluded that pre- 

 dation pressure alone controls P. minutus population 

 levels and that C. finmarchicus is regulated by both 

 predation and food availability. The data presented 

 here indicates that its unlikely that these species have 

 different factors limiting their annual abundance. Fur- 

 thermore, preliminary studies indicate that abundance 

 estimates of both species on Georges Bank are corre- 

 lated to chlorophyll levels in the water column 1 . Inves- 

 tigations on P. minutus population dynamics should 

 not exclude food supply as a potential limiting factor. 



Centropages hamatus abundance estimates were 

 more variable and temperature sensitive than those 

 for other dominant copepod species. Of special interest 

 is that C. hamatus spring abundance pulses were in- 

 versely related to both C. finmarchicus and P. minutus 

 population estimates. It is unlikely that temperatures 

 which stimulate C. hamatus production would be det- 

 rimental to C. finmarchicus or P. minutus. Laboratory 



'C. Meise, National Marine Fisheries Service, Narragansett, RI 

 002882, unpubl. data. 



