Horizontal Distribution — Swarms. 373 



Two similar swarms of the same species were seen in 1896 on 

 October 3rd, and on November 3rd; both days when the lake 

 was perfectly calm. On the first occasion there was a fog on 

 the water; on the second occasion the sky was clear. 

 These swarms were nearer the shore and were much more exten- 

 sive. On the first occasion the Daphnias occurred in patches of 

 irregular extent and shape — perhaps 10 meters by 50 meters, 

 and these patches extended in a long belt parallel to the shore. 

 The surface water was crowded by the Daphnias, and an im- 

 mense number of perch were feeding upon them. The swarm 

 was watched for more than an hour, during which the fog passed 

 away, and the water could be seen disturbed by the perch along 

 the shore as far as the eye could reach as one was standing in 

 a boat. After a time a light breeze sprang up and, of course, 

 prevented further observation. On this occasion the number 

 was determined to be 1,170,000 per cu. m. in the densest part 

 of the swarm. On November 3rd a similar swarm was seen, 

 and water was again dipped up from the denser part of the 

 swarm. The Crustacea were crowded into an extremely thin 

 layer, not more than 2-3 cm. thick. The surface water only 

 was allowed to fall into the vessel and the number determined 

 in 6 catches made by straining 10 liters of water, was from 

 800,000 to 1,492,000 Daphnias per cubic meter, about 99 per 

 cent, adult. In addition there were present about 1,000 Cyclops 

 per cubic meter, but nothing else was found. On this occasion 

 one ephippial female was present, the only one that I have ever 

 seen in this species; the ephippium was fairly developed, but 

 no eggs had been deposited in it. No males were in these swarms. 



The highest number is found nearly ten times the maximum 

 number of this species per cubic meter, as derived from the 

 three-meter hauls. It is also nearly fifty per cent, more than 

 the maximum catch of this species as obtained from a depth of 

 18 meters, and nearly five times as great as the average for 

 November 1-15. On November 3d, catches were made below 

 the swarm from 0.3m. to 3.3m. The average of two gave per 

 cubic meter: 



Diaptomus 4, 900 



Cyclops 26,600 



D. hyalina 18,200 



Chydorus 15, 700 



