Kane: Spatial and temporal abundance patterns of Centropages hamatus 
97 
more, the highest mean abundance of C. hamatus is 
found over the central shoals of GBK where salinity 
usually ranges from 32.2 to 32.7 psu during peak 
abundance, well above the coastal areas where abun- 
dance, on average, is much lower. High salinity off- 
shore may effect C. hamatus production there and 
restrict its distribution, but it is more likely that low 
offshore abundances are caused by low phytoplank- 
ton food stocks that cannot support an overwinter- 
ing population or the generation that produces rest- 
ing eggs after peak abundance is reached. 
There was no strong evidence from survey data 
that predation affects interannual variability or 
causes the seasonal decline of the population in the 
SNE subarea. Ctenophores appear to lower Centro- 
pages hamatus abundance when they are plentiful, 
but this occurred only during one year and in a re- 
stricted area. Chaetognaths and the copepod 
Centropages typicus also appear to have little affect 
on C. hamatus density. Clearly, however, a dedicated 
study analyzing stomach contents and the vertical 
distribution of the predator-prey field is needed to 
define the actual food web. Potential predators such 
as squid, juvenile fish, and populations of plank- 
tiverous adult fish must also be considered in order 
to fully define the role predation has in controlling 
C. hamatus population levels. 
Lindley and Hunt (1989) examined the distribu- 
tion of Centropages hamatus to the north and across 
the Atlantic to the North Sea. They described a life 
cycle similar to the one reported in this paper and 
speculated that the autumn decline in abundance is 
caused by the pressure of competition with Centro- 
pages typicus for food resources. Dagg and Turner 
(1982) studied copepod populations in the SNE and 
GBK subareas during autumn and calculated that 
copepod grazers may consume entire phytoplankton 
stocks. If true, high abundance of C. typicus could 
impact population levels of C. hamatus. However, 
MARMAP survey data indicate that high C. typicus 
abundance does not lead to an early decline of C. 
hamatus in either subarea. For example, in 1985 on 
GBK, median C. typicus abundance was 2-3 orders 
of magnitude above the ten-year average, but C. 
hamatus was also above average and increased in 
late autumn (Kane, 1993). Data presented in this 
report also show that the abundance of the two spe- 
cies are not related in the SNE subarea. Although 
competition pressure between the two species does 
not appear to cause the decline of C. hamatus, labo- 
ratory feeding experiments are needed to measure 
the effect of low food levels on species abundance. 
The copepod Centropages hamatus has evolved a 
unique life history to survive and reproduce within 
the waters of the northwestern Atlantic continental 
shelf. The population has a distinct seasonal cycle 
with peak abundance occurring in shallow areas 
where phytoplankton food stocks are rich and sur- 
face temperature ranges from 12 to 17°C. Predation 
pressure appears minimal, and C. hamatus abun- 
dance peaks between the annual maximum of early 
spring and autumn dominant copepod species 
(Sherman et al., 1983), thus reducing competition 
pressure for food resources. Centropages hamatus 
likely produces resting eggs that hatch and help re- 
populate the ecosystem when environmental condi- 
tions are favorable. Comprehensive laboratory and 
shipboard experiments are needed to distinguish how 
the above biotic and abiotic factors interact to deter- 
mine the annual success of the population. 
Acknowledgments 
The author acknowledges the individuals involved 
with the collection, analysis, and processing of 
MARMAP data. Special thanks go to Jay OReilly for 
his help with the distribution plots and to those who 
critically reviewed early drafts of the manuscript. 
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