Page:Popular Science Monthly Volume 61.djvu/208

202 The diagrams indicate the passage of the fry from the fall run of adults; those from the summer run passed the upper station earlier, some of them being noted at Battle Creek during October and November. Observations in 1900 show that they may begin migrating in September.

The work at Balls Ferry indicates that the greater part of the young salmon passed that portion of the river between the middle of January and the middle of March. Practically all had passed by March 20. Numerous measurements made at frequent intervals show that the daily average size varied but one tenth of an inch during the three and a half months. The average of all measurements is 1.53 inches. This confirms the inference from the Battle Creek work that practically all the fry begin their migration as soon as they are able to swim, otherwise the later ones would have been larger.

Both diagrams show that a big run of fry is not necessarily coincident with a rise in the river. From the Walnut Grove diagrams we note. From the middle of January to the middle of May there were salmon fry in various numbers passing Walnut Grove. The height of the migration was from March 4 to about the 24th, lasting about 20 days. Practically all had passed by April 22. The size of those taken during January was 1.6 inches, during February 1.8 inches, during March 1.7 inches. From March 30 to May 7 they increased from 1.7 inches to 3.0 inches. The field notes show that as many fry were taken during the day as at night. The water of this portion of the river is muddy.

Comparing the two diagrams we note that a large run of fry passed Balls Ferry February 2, and that 34 days later, March 8, there was a large run of fry passing Walnut Grove. Again, a large run passed Balls Ferry February 14, and 34 days later, March 20, there was a large run passing Walnut Grove. The later Balls Ferry runs were caught by the high water the last of March, and their passage was not noticed at Walnut Grove. The high water probably carried them down faster, and they may have passed while the trap was out of the water, March 23 to 26.