During the months of September and October 1939=UO, plankton samples 

 were taken at regular intervals from the two lakes. A standard- tjipe 

 plankton net >d.th a 3-3/Ii-inch orifice, a brass collection bucketj and 

 body of No, 20 silk bolting cloth was used. In Lake Osoyoos, l6 vertical 

 plankton hauls cf 86-foot average depth produced an average of O.Ii50 c.c. 

 of plankton material. In Lake Wenatchee, lU vertical hauls of lU3-'foot 

 average depth collected 0,191 c.c, of material. Siirface tows pro-vdded an 

 average of 0.67 c.c, of plankton per 100 yards in Lake Osoyoos to C,l8 c,c. 

 in Lake Wenatchee, 



A superiority of more than 2 to 1 in plankton for Lake Osoyoos deiiKin- 

 strates the desirability of that lake as a rearing place for blueback 

 salmon. This was also borne out by observations. Landlocked blueback 

 (kokanee) in Lake Wenatchee are dwarfed^ averaging 5 inches in length,; 

 whereas in Lake Osoyoos they range from 8 to lU inches. This could in part 

 be a result of population pressure, but it could well be a direct result of 

 difference in plankton production. The shallox'7 marginal area of Lake OsoyooSj, 

 the warm climate,, and the surface drainage of large lal<es upstream all lend 

 themselves to good food production.. Steep slopes and colder temperatures of 

 Lake Wenatchee are not as favorable for this purpose. 



Owing to the productivity of the Lake Osoyoos region, the Okanogan 

 blueback runs should be preserved if at all possible. This is especially 

 so in light of the possibility that Lake Wenatchee^ already taxed by popu- 

 lation pressue and having limited soiorces of food, could not support these 

 additional blueback stocks as satisfactorily as the Okanogan region. 



Such programs as the Okanogan flood-control project raise a question 

 about the wisdom of having the spawning area for bluebacks remain exclusive- 

 ly within the two areas now utilized. Any temporary or permanent obstruc- 

 tion occurring in the stream can virtually wipe out a migration into that 

 particular area. Obstructions of this type have occurred on several occa- 

 sions. For example, during low-water years there are dry stretchesB/ in 

 the Wenatchee River which have delayed and jeopardized the riins. 



Another example of how a temporary or permanent obstruction can affect 

 the runs was provided at the time of construction of McNary Dam located on 

 the main Columbia River between Bonneville and Rock Island Dams. Two of 

 the fishways provided there were put out of operation in 19$1 during the 

 spring freshet. Tne migration was delayed and the bluebacks were subjected 

 to high-velocity flows. Some salmon were hoisted over the structure by 

 means of a large dip-net. Considerable numbers eventually found their way 

 through the navigation locks. Fish counters at Rock Island Dam reported 

 that approximately 25 percent of the bluebacks bore injuries. This per- 

 centage was never so high in previous years. On the Wenatchee River at 

 Dryden Dam approximately ten percent of the passing bluebacks had deep head 

 and back injuries and hatchery personnel who took spawn in the Wenatchee 

 area reported eight percent of the females had damaged ovaries. This affected 

 ripening of all or part of the eggs. 



^ Power dams located at Dryden and Tumwater during low water years such as 

 experienced in 19U4'-ll5 may draw virtually all of the stream flow, preventing 

 or delaying the salmon migrations (Fish and Hanavan 19ti-8, pp. ii7 and 55) • 



26 



