Seasonal and Areal Distribution of Zooplankton in 

 Coastal Waters of the Gulf of Maine, 1967 and 1968 



By 



KENNETH SHERMAN, Fishery Biologist 



Bureau of Commercial Fisheries Biological Laboratory 

 W. Boothbay Harbor, Maine 04575 



ABSTRACT 



The abundance, composition, and seasonal variations in the distribution of zoo- 

 plankton are described. Thirteen taxa were among the more abundant zooplankters 

 in the samples: 6 were holoplanktonic, and 7 were meroplanktonic. Copepods were 

 the dominant zooplankters during all seasons in both years. Zooplankton volumes 

 were highest in the western area (Cape Ann to Cape Elizabeth), intermediate in the 

 central area (Cape Elizabeth to Mt. Desert Island), and lowest in the eastern sector 

 (Mt. Desert Island to Machias Bay). The differences in zooplankton abundance among 

 areas and between years were caused by variations in the timing of vernal warming 

 and coastal differences in water column stability and circulation. 



INTRODUCTION 



In 1963 the Bureau of Commercial Fisheries 

 Biological Laboratory, Boothbay Harbor, 

 Maine, began studying zooplankton in coastal 

 waters of the Gulf of Maine. Sampling was 

 designed to monitor seasonal changes in the 

 composition, distribution, and abundance of 

 large zooplankton, particularly calanoid cope- 

 pods, the predominant food of juvenile Atlantic 

 herring, Clupea harengus harengus L. The 

 studies are part of an investigation of how the 

 environment affects the abundance of herring 

 and their availability to the commercial fishery. 

 This is the fourth in a series of reports on the 

 coastal zooplankton assemblage in the Gulf of 

 Maine (see also Sherman 1965, 1966, and 1968). 



METHODS 



As in previous years, four stations in each 

 of three Gulf of Maine coastal areas--western 

 (Cape Ann to Cape Elizabeth), central (Cape 

 Elizabeth to Mt. Desert), and eastern (Mt. 

 Desert to Machias Bay)--were sampled sea- 

 sonally on cruises of the research vessel 

 Rorqual (see fig. 1 for station locations and 

 cruise dates). In each of the seasons in 1967, 

 and in the winter, spring, and summer of 

 1968, collections were made with a Gulf III 

 sampler, fitted with a 20-cm. nose cone and 

 nnetal netting (aperture width 0.366 mm.). 

 Step-oblique tows of 30 minutes-- 10 minutes 



each at 20 m., 10 m., and the surface--were 

 made during daylight. Recent experimentation 

 has shown, however, that the Gulf III under- 

 samples the smaller copepods when towed at 

 6 knots (Sherman and Honey, in press). To 

 reduce this sampling bias, a bongo-type sam- 

 pler (Posgay, Marak, and Hennemuth, 1968) 

 was substituted for the Gulf III in autumn 

 1968. The towing method, mesh apertures, and 

 mouth openings of the bongo net were identical 

 to those of the Gulf III, except that nylon was 

 substituted for the metal netting used in the 

 Gulf in. The volumes of zooplankton and counts 

 of copepods obtained fronn the bongo tows were 

 adjusted by reducing them to Gulf III values, 

 using data obtained from comparisons of the 

 bongo and Gulf III samplers made in autumn 

 1967 (Sherman and Honey, 1968).^ A calibrated 

 flowmeter was mounted in the tail section of 

 the Gulf III and in the mouth opening of one of 

 the bongos. Each tow covered about 5.6 km. and 

 filtered about 200 m.^ of water. Towing speed 

 was 6 knots (308 cm. per second). 



Zooplankton volumes were measured in the 

 laboratory by the mercury imnnersion method 

 of Yentsch and Hebard (1957). Ctenophores, 

 large coelenterates, other large organisms 



'The correction factor used was the ratio of the average 

 difference between volumes obtained with the Gulf III and 

 with the bongos and between smaller copepod species — 

 Centropages hamatus , Pseudocalanus minutus , Temora 

 longicornls , Acartia longiremis, and Acartia clausii — 

 obtained by both samplers in 10 simultaneous tows. 



