Ledgerwood et al.: Diel sampling of migratory juvenile salmomds in Columbia River estuary 



73 



night. An insufficient number of fish were captured to 

 allow a more detailed analysis of the diel migration 

 pattern. 



Steelhead 



Over 98% of the juvenile steelhead were caught by 

 purse seine (4673 by purse seine and 74 by beach seine). 

 Purse seine catches were moderate in the four inter- 

 vals after sunrise (average 4% CPS), peaked at the 

 interval from 1416 to 1545 hours (14.6% CPS), decreas- 

 ed at dusk, and remained low throughout the night in- 

 tervals (average 1.7% CPS) (Fig. 2F). The night catch 

 was 8.7% (1.0 SD) of the total catch for a 24-hour 

 period. 



Discussion 



Catch data from our beach and purse seines appear to 

 represent movement and position of juvenile salmonids 

 during their migration through the upper Columbia 

 River estuary. Catches of subyearling chinook salmon 

 at both purse and beach seining sites indicate a substan- 

 tially decreased migration during darkness. Beach 

 (nearshore) and purse (midriver) seine catches of coho 

 salmon indicate a fairly uniform migration throughout 

 the daylight period. Data obtained for yearling chinook 

 salmon, sockeye salmon, and steelhead indicate a 

 midriver orientation with decreased migration during 

 darkness. Other researchers have reported different 

 diel movement patterns, but conditions, equipment, 

 and life stages of the fish sampled are so variable that 

 direct comparison between experiments is difficult 

 (Table 1). 



