76 



Fishery Bulletin 89(1). 1991 



Variability of catch between sets and 

 sampling periods was higher for yearling 

 chinook salmon than for other salmonids. 

 The origin of marked fish varied sub- 

 stantially among the three purse-seine 

 sampling periods (Table 2). The largest 

 portions of the catch originated in the 

 Willamette, mid-Columbia, and Snake 

 Rivers for the first, second, and third 

 sampling periods, respectively. Stock 

 differences and changes in abundance 

 among stocks during the diel sampling 

 periods may have caused the higher 

 variability in the catch. 



We found reasonable agreement 

 among the diel catch patterns reported 

 here and those from extensive morning 

 sampling (2615 sets) at Jones Beach in 

 May and June 1979-83 (Dawley et al. 

 1986) (Fig. 3). A noteworthy exception 

 was that beach seine catches near sunrise 

 were lower during the diel study because 

 sets were made before sunrise to remove 

 fish which resided in the area overnight. 



It is generally agreed that net avoid- 

 ance is probably greatest in daylight; 

 therefore, decreased net catches at night 

 should represent decreased fish abun- 

 dance in the water sampled. It seems 

 unlikely that decreased catches at Jones 

 Beach during darkness were caused by 

 surface- or midwater-oriented juveniles 

 maintaining their position against cur- 

 rent velocities up to 5 km/hour. Data obtained at Jones 

 Beach by Dawley et al. (1986) showed that marked 

 subyearling chinook salmon released into the shoreline 

 sampling area at night were recaptured at a much 



Table 3 



Measured movement rates of juvenile salmonids and water velocities in a 

 155-km reach of the Columbia River between Bonneville Dam and Jones Beach 

 at two volumes of river flow. 



'Zero to nine marked groups were available for each calculation of average 

 movement rate at these designated river flows (Dawley et al. 1986). 

 'From Blahm (1974). 



higher rate than marked fish released during daylight 

 (30.6 vs. 8.0%). Because midriver-oriented yearling 

 fish do not appear in shoreline areas at Jones Beach 

 during darkness, they probably hold near the bottom, 



