SPATIAL AND SEASONAL PATTERNS IN ABUNDANCE AND AGE-COMPOSITION OF CALANUS-FINMARCHICUS IN THE GULF OF MAINE AND ON GEORGES BANK - 1977-1987

The mean seasonal cycle and distribution of various life history stage s of C.finmarchicus throughout the Georges Bank (GB)-Gulf of Maine (GO M) region were characterized based on 5966 MARMAP zooplankton samples collected during 106 surveys over a 10-year period (autumn 1977-autumn 1987). A high degree of seasonal and spatial variability in C. finmar chicus abundance throughout the region was evident in contoured portra yals of data, grouped into standard stations and 2-month ''seasons''. Eight subareas of the Gulf of Maine-Georges Bank region were identifie d through cluster analysis of standard stations having similar seasona l patterns in mean abundance of C. finmarchicus stages C3, C4, C5 and adults. These were the northern Gulf of Maine (Northern GOM); southern Gulf of Maine (Southern GOM); Scotian Shelf-coastal Gulf of Maine (Sc otian-Coastal GOM); Mass Bay; tidally mixed Georges Bank (Mixed GB); t idal front on the Bank separating mixed from seasonally stratified wat er (Tidal Front GB); seasonally stratified water on the Bank (Stratifi ed GB) and the Continental Slope adjacent to Georges Bank (SLOPE). A d istinct seasonal abundance cycle was present in all subareas, but, the magnitude and timing of annual maxima varied greatly among subareas. Peak abundance was reached early (March-April) in Mixed GB, Tidal Fron t GB and Mass Bay, and late (July-August) in Northern GOM and Scotian- Coastal GOM. Remaining subareas had maxima in May-June. Abundance incr eased 10-fold from January-February to March-April and decreased sharp ly from July-August to September-October in all areas except southern GOM and northern GOM. The amplitude of the annual cycle was weakest in northern GOM and southern GOM, where high concentrations of C. finmar chicus persisted year-round, and strongest in the tidally mixed shallo w water on GB, where the Sparsest densities of C. finmarchicus occurre d most of the year. Abundance curves for the various areas converged i n March-April, when C.finmarchicus was ubiquitously very abundant (> 1 0(4)/10 m(2)), and diverged from September to December. C. finmarchicu s stage distribution in the GB-GOM area was highly negatively correlat ed with mean water column temperature during the stratified season. Th is seemed more related to the hydrography of the region, which isolate s warmer well mixed Georges Bank from the Gulf of Maine and the strati fied areas on the Bank, than to temperature, because Calanus abundance s decline on the Bank before water temperatures exceed their preferenc es. A large part of the spatial and seasonal variation in C.finmarchic us abundance and age structure appears to be tightly coupled to major hydrographic regimes and to major circulation patterns in the region. There was a sharp ecotone between well-mixed Georges Bank and the Gulf of Maine as defined by C.finmarchicus abundance patterns and life his tory distributions. The ecotone is present year-round but is most appa rent during the stratified season (May-October), when thermohaline den sity gradients and the near-surface current jet along the northern fla nk are generally strongest. The Gulf of Maine had the highest abundanc es of C. finmarchicus, and lowest spatial and seasonal variation in th e region, while tidally mixed Georges Banks displayed the opposite pat tern. This indication of stable population centers in the Gulf of Main e would make it a major source of Calanus in the region, particularly during March-April. Distributional patterns also suggest a strong Cala nus influence from Scotian Shelf water in northern Gulf of Maine and o n the southern flank of Georges Bank. Published by Elsevier Science Lt d.