ABUNDANCE, BIOMASS, VERTICAL MIGRATION AND ESTIMATED DEVELOPMENT RATEOF THE COPEPOD CALANUS-FINMARCHICUS IN THE SOUTHERN GULF OF MAINE DURING LATE SPRING

Abundance, biomass, diel vertical migration and estimated in situ deve lopment in the copepod Calanus finmarchicus were investigated during l ate spring in 1988 and 1989 in the southern Gulf of Maine. This region is an important feeding ground for the planktivorous right whale, Eub alaena glacialis. The 1988 study took place during the declining sprin g bloom, with phytoplankton biomass variable, but relatively high. The 1989 study occurred after seasonal stratification, and phytoplankton biomass was low. During the 1988 cruise the dominant stage in C. finma rchicus shifted from C1-C2 to C4-C5. Stage durations during 1988 (4.0 days for C3 and 6.6 days for C4), estimated from the temporal change i n stage distribution, were similar to maximal values observed int he l aboratory. In contrast, during 1989 stages C4 and C5 were dominant thr oughout the study period and development rate was slow (estimated C4 s tage duration about 24 days). Diel vertical migration patterns changed , from an absence of migration at the first two 1988 stations where yo unger stages predominated (C1-C3), to a very strong diel vertical migr ation at the the later 1988 stations where stages C3-C5 predominated. This was not a simple ontogenetic change in migratory behavior since a ll copepodite stages at each station showed similar patterns. During 1 989 dense aggregations of C. finmarchicus remained in the surface laye r both day and night, and no diel vertical migration was observed. A s mall, nonmigratory population of late-stage C. finmarchicus was found at depth. Individual body size of these copepods was considerably grea ter than those found at the surface. Differences in development rate b etween years reflect differences in the food environment, brought abou t by seasonal hydrographic changes and the development of more intense stratification. Diel vertical migration patterns, however, did not sh ow a simple relation with food availability, and it is suggested that predation may play an important role in regulating the behavior of the copepods.