extending into the littoral zone, but not yet found 

 anywhere in the lacustric zone. At 10 of the mar- 

 ginal stations, including the principal fish-breeding 

 areas of the lake, it was very abundant and so be- 

 comes an important fish food. This species has more 

 of a tendency to gather in dense swarms than any of 

 the others in the lake, and if the tow -net happens 

 to pass through one of these swarms an exceptionally 

 heavy catch results. It is found in nearly all the 

 small ponds of the lake drainage, and is reported by 

 Marsh (1907) as the most common species in the 

 lakes of Wisconsin and Michigan. It has been found 

 in the stomach contents of the carp sucker and yel- 

 low perch, and is probably eaten by many other 

 small fish. According to Birge and Juday this cope- 

 pod possesses a high percentage of nutritive material 

 and hence should make food of the best quality. 



19. Diaptomus sicilis Forbes. Present in the 

 plankton of all 3 zones, but most abundant in the lit- 

 toral zone and during June and July. It is the largest 

 of the species found in the lake and usually lacks the 

 red oil globules which are so prevalent in D. ashlandi , 

 and this fact aids in separating the two. Its breeding 

 season must have come sometime before the survey 

 began for it was evidently at its maximum by the 

 first of June and afterward its percentages steadily 

 diminished. At the eastern end of the lake in 1928 

 its numbers were less than those of p. ashlandi , espe- 

 cially during July and August. On the contrary dur- 

 ing June 1929, its numbers were much larger than 

 those of p . as hlandi, then dropped so that the two 

 were about equal in July and August, and finally 

 became a Little smaller in September. On con- 

 trasting the surface and bottom tows, the 1928 rec- 

 ords showed very little differences throughout the sea- 

 son at the eastern end of the lake. In June 1929 the 

 bottom percentages were larger than those at the sur- 

 face at about two-thirds of the stations, quite evenly 

 distributed over the entire lake. In July the bottom 

 percentages were somewhat larger at the eastern end, 

 and the surface percentages in the center and at the 

 western end. In August and September there was not 

 sufficient difference between the two to warrant any 

 general statement. Here again, as in the case of 

 D. ashlandi, the vertical migration was much more 

 in evidence during June, diminished through July and 

 August and almost disappeared in September. This 

 species has been found in the stomach contents of 

 the green-backed shiner, the spottailed shiner, the 

 silversides, the troutperch, the yellow perch, the 

 common sucker, and the redhorse sucker. This shows 



that it is freely eaten by different kinds of fish 

 and suggests that future examination will add 

 considerably to the above list. 



20. Epischura lacustris Forbes. Present in the 

 plankton of all 3 zones but most abundant in the 

 littoral zone during June. Although its percentages 

 of the macroplankton obtained in June were the 

 largest for the 4 months covered by the present sur- 

 vey, Birge (1897) found that this copepod reached 

 its maximum in Lake Mendota late in the autumn. 

 The same might be true of Lake Erie, but the num- 

 bers steadily declined through July, August, and 

 September and gave no indication of a later rise. 

 In June the percentages at the surface were con- 

 siderably larger than at the bottom, and there were 

 11 stations at which it did not appear at all in the 

 bottom tow. A similar superiority in numbers at 

 the surface was maintained through July and August, 

 but they grew steadily less until in September the 

 numbers at surface and bottom became practically 

 equal. During this time the species had become 

 more widely distributed and in August was present at 

 every station throughout the entire lake and in both 

 nets. 



In Green Lake, Marsh (1903) found that 81 per- 

 cent of the Epischuia collected in August and 72 

 percent of those collected in September were in 

 the upper 10 meters of water. He also found that 

 during the warm summer nights large numbers came 

 to the surface. He interpreted this to mean that 

 Epischura prefers the warmer water and is controlled 

 in its vertical distribution by conditions of temper- 

 ature and food supply. It is a large species and a 

 powerful swimmer and moves to the surface when 

 the water there is warm and filled with rich food 

 material. 



This species has been identified in the stomachs 

 of the green -backed shiner, the spottailed shiner, 

 and the yellow perch. Its abundance and its wide 

 distribution over the entire lake make it an im- 

 portant fish food. 



21. Limnocalanus macrurus G. O. Sars. 

 Present in the plankton of the littoral and lacustric 

 zones, in Lake Erie but practically absent from the 

 marginal zone. During the first 3 months of the 

 survey it appeared in large numbers in the surface 

 tows, but during September it was virtually confined 

 to the bottom tows. Sars (1918) noted that all the 



165 



