104 



Conservation Department 



increased during the summer, the highest zooplankton production 

 centering in the deeper part of the lake and expanding laterally 

 until as indicated in fig. 25 almost the entire offshore area was 

 included within the 300 cc. curve (300 cc. taken in a five-minute 

 meter net haul). Figures 24 and 25 illustrate the expansion from 

 the deeper area at two times during the season. The highest ])ro- 

 duction of zooplankton, as shown in the following table, was 

 found on the third cruise in the middle of August when the sur- 

 face temperature was highest. Later it declined in volume as the 

 microplankton increased, the latter becoming increasingly abun- 

 dant with the approach of the autumnal maximum at the end of 

 the season. 



Table 7.— Seasonal Variation in Total Volume of Macroplankton 



It has been said that perhaps as the fishes declined their 

 enemies increased and now are destroying most of the eggs and 

 young of commercial species. Enemies of fish have always been 

 present in the lake and are abundant today, but no evidence has 

 been found to indicate that they have increased in number or 

 are more important than in the past. Like the storms this factor 

 has always been present and probably is no more alarming as a 

 destructive agent than it was in the days when the lake abounded 

 with fish. We must look elsewhere for the real cause of the 

 decline. 



Before it was possible to identify the various fry appearing in 

 the nets the early life histories of each species had to be worked 

 out, and this proved an important part of the summer's work. 

 The young upon hatching in no way resemble the adult, or in fact 

 the later stages of the same species. But one character remains 

 reasonably constant, the number of vertebrae of the back bone, 

 and it is upon counts of the body segments that the work had to 

 be based. Of the seventy-five odd species reported from the lake, 

 the young of very few had been described. This year sixty-seven 

 species were taken, identified, and the developmental stages of 

 twenty-five figured and described by Mrs. Fish.* Next year it is 

 planned to continue these studies. In the course of the work the 

 young of two species of sculpins not previously reported from 

 Lake Erie appeared in collections from the bottom of the deep 

 hole. They inhabit the bottom in deep Avater, and it is not sur- 

 prising that they have heretofore escaped notice. 



*See page 70. 



