MASON: FEATURES OF EMERGING COHO SALMON FRY 



Photoresponse tests were conducted in four 

 choice boxes placed in an uncompartmentalized 

 replicate of the emergence channels and located 

 adjacent to them. The choice boxes were con- 

 structed of fine plastic screen on a wire frame- 

 work (Figure 2) and divided equally into two 

 compartments by a vertical partition that al- 

 lowed a passage height of 1.5 cm beneath it. Both 

 hinged top and the partition were made of black 

 polyethylene sheeting. The wooden channel was 

 covered with the same material, except in the 

 areas taken up by the boxes, so that the com- 

 partments not covered by the hinged tops re- 

 ceived most of the illumination in the boxes. Each 

 box presented a choice between sharply contrast- 

 ing light conditions rather than between "light" 

 and "no light," because some light leaked under 

 the partitions. A series of mirrors was mounted 1 

 m above the water surface, allowing observation 

 from a blind. 



Water flow in the channel was 10 liters/min 

 and velocity less than 10 cm/min. Water depth in 

 the choice boxes was 10 cm providing an air space 

 of 3 cm between the water surface and the ceiling 

 of the covered compartment. Average fish density 

 was set so as to allow about twice as much water 

 volume and 2.4 times as much bottom area as in 

 the holding baskets. Temperature of the water 

 supply (stream) ranged from 7.8° to 11.7°C during 



Figure 2. — Light-dark choice box showing the reversible 



opaque lid. 



the experimental period. Light intensities at the 

 exposed water surface ranged from 700 to 4,000 

 ft-candles during photoresponse tests. 



The procedure for a photoresponse test was as 

 follows. The appropriate group of fry was trans- 

 ferred to the test site in a covered pail, 40 fry were 

 netted out and 10 fry put in each of the four choice 

 boxes with the lids in an upright position. The 

 lids were then closed in a common direction, and 

 the remaining fry were returned to their holding 

 basket. In the choice boxes, all fry swam into the 

 dark compartments when the lids were dropped. 

 After 30 min, the number of fry observed in the 

 light compartments were recorded every 10 min 

 for 40 min (5 observations in each of 4 compart- 

 ments = 20 observations). A fish was considered 

 to be in a light compartment when its head was 

 visible. The lids were then reversed and, after 10 

 min, five additional observations were made at 

 10-min intervals. Thus, for each test, 40 counts 

 were recorded on 40 fry, which spent 2 h in the 

 choice boxes per test and about 10 min in the 

 transfer process. The photoresponse tests were 

 initiated 1 day before fry began emerging from 

 the simulated redds and continued until the 22nd 

 day of emergence. Length and weight measure- 

 ments were taken for all fry groups on the follow- 

 ing day. Data were tested for homogeneity using 

 chi-square. There was no significant difference 

 (P<0.01) between the first and second runs of five 

 observations each made in individual choice 

 boxes, x^ values ranging from 0.0 to 2.8 in 132 

 pairs of runs. Similarly, the data from individual 

 choice boxes proved homogeneous within each 

 test in 29 of the 33 tests performed (P<0.01) x^ 

 values ranging from 1.1 to 7.8 with 3 degrees of 

 freedom. The remaining four tests contained 

 heterogeneous data, x^ values ranging from 14.7 

 to 36.5 and were excluded from further analysis. 

 With homogeneity assured within most tests, the 

 data within tests were pooled and processed. 



RESULTS 



Emergence from the Simulated Redds 



Fry began emerging 25 days after hatching and 

 15 days following introduction to the redds. 

 Emergence proceeded for 20-23 days during 

 which 97-98% of the fry emerged. All four redds 

 showed a similar pattern of emergence, peaking 

 at the same time, 74 to 94% of the fry emerging 

 during the median 10 days (Figure 3). 



169 



