AKT. 15 COPEPOD CRUSTACEANS OF CHESAPEAKE BAY WILSON 1 1 



where the salinity was very low. It was probably swept down by the 

 current from the river into the bay. The new species of Rohertsonia 

 was captured in the upper central portion of the bay in water that 

 had a depth of 9 meters over a muddy bottom, where the salinity 

 was only 11.5. The type species of the genus has been reported by 

 Brady and Scott from various localities around the British Isles, and 

 by Sars from the upper part of the Christiania Fiord, but no data 

 were given on salinity. 



In general, therefore, eight of these species confined to single 

 stations compare favorably with those from other regions in depth, 

 salinity, and kind of bottom. In the case of five of the other species 

 the salinity is presumably lower than that of the water from which 

 they have previously been recorded. The remaining species, the 

 fresh-water form, has been explained above. 



SEASONAL DISTRIBUTION 



In Table 1 the seasonal distribution is given for all the species 

 obtained in the bay, the letters denoting the seasons of the year when 

 the species was obtained in greatest abundance at the respective sta- 

 tions. In determining abundance, consideration must be given to 

 both the percentage of the catch and the total number of specimens 

 obtained. Five per cent of a catch totaling 10,000 specimens is 

 manifestly a larger number than 80 per cent of a catch totaling 

 500 specimens, although at first it may appear much smaller. It is 

 freely admitted that the seasonal abundance of any species at a 

 given station may be largely modified by accidental or exceptional 

 conditions, but when the records of all the stations at which the 

 species was obtained are compared the information becomes fairly 

 reliable. The more universally the species is distributed throughout 

 the bay the more trustworthy become the inferences as to its seasonal 

 distribution. 



The two species of Acartia seem to have been equally distributed 

 throughout the year. The total number of specimens was greatly 

 increased during March and slightly during August. Two hauls, 

 each totaling 100,000 specimens, and two of 25,000 each, were made 

 during March, and these are the four highest totals obtained during 

 the entire survey. These large hauls are the result, however, of the 

 breeding season, which immediately preceded them, and in spite of 

 the fact that they are made up almost entirely of the two Acartia 

 species the latter are not to be regarded as spring copepods only. 

 They were present everywhere in the bay throughout the entire year 

 and always in large numbers. Williams reported A. clausii as most 

 abundant in Narragansett Bay during January and February, and 

 Fish listed it as one of the winter species at Woods Hole. These 



