Cladocera

marine fish, breeding populations of which have become established in various bodies of fresh water, including Lake Cayuga, New York, and the Great Lakes. With self-perpetuating populations of alewives are within about 40 kilometers of the present coastline, and each is drained directly, by a small stream or river, or indirectly, through the estuaries of the larger Connecticut or Thames rivers, into Long Island Sound. As such streams and rivers are normally ascended by marine alewives, it is assumed that the establishment of these self-sustaining populations in the lakes is natural. Although they did not at the time examine the food of the alewives in these Connecticut lakes, studies in other lakes suggested that planktonic copepods and Cladocera are the primary food. An opportunity to test this hypothesis was provided when changes in Crystal Lake plankton were caused by the introduction into a lake in northern Connecticut of Alosa aestivalis "glut herring," a species closely related, and very similar, to Alosa pseudoharengus. The plankton of Crystal Lake had been quantitatively sampled by the researchers in 1942 before Alosa was introduced. Resampling 10 years after Alosa had become abundant revealed plankton similar in composition to that common in the lakes with natural populations of Alosa pseudoharengus and unlike that characteristic of Crystal Lake before Alosa became abundant. The plankton of four lakes with natural populations of Alosa was sampled for…