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



71 



downstream and back to shore. The effective fishing 

 depth of the net was 2-3 m in water up to 6 m deep. 

 The net was pulled onto the beach which crowded the 

 fish into the bunt for capture. Completion of each set 

 required about 25 minutes; sets were made at 45-min- 

 ute intervals. 



Catch data from the first two or three seine sets on 

 the first morning of each sampling period were not 

 used for analysis of diel movement because some 

 salmonids probably resided in the sampling area over- 

 night, and the initial sets were used to clear the sam- 

 pling site of those residents. Of the beach seine sets 

 in each 24-hour period, 34% (11 of 32) were made dur- 

 ing darkness and 66% (21 of 32) during daylight. 



Purse seining 



The purse seine was 206 m long by 11m deep with 

 1-2 cm (stretch) webbing (Durkin and Park 1967). A 

 depressor weight was used during the pursing opera- 

 tion to increase the effective fishing depth to about 6 m. 

 The vessels used were a 10-m pontoon barge powered 

 by outboard engines and an outboard- motored seine 

 skiff; lights were mounted on the barge for night opera- 

 tion. A depth finder, a compass, and channel markers 

 were used to locate the sampling site. 



The seine was set near midriver in water 9- 14 m 

 deep, and towed upstream at constant power in a "U" 

 configuration (Dawley et al. 1985). After 5 minutes, the 

 ends of the net were brought together and the net bot- 

 tom was closed (pursed) and hauled aboard the barge 

 with a boom and hydraulic capstan. Then the cork line 

 and webbing were retrieved and the catch was placed 

 in 75-L containers supplied with circulating river water. 

 Completion of each set required about 40 minutes; sets 

 were made at 90-minute intervals. About 31% (5 of 16) 

 of the purse seine sets were made during darkness and 

 69% (11 of 16) during daylight. 



Data analysis 



Each set represented one time interval within the 

 24-hour sampling period. Twice as many beach seine 

 sets were made in each 24-hour period as purse seine 

 sets; consequently, time intervals are one-half those for 

 the purse seine. The catch per set (CPS) interval was 

 calculated in terms of the percentage of the total 

 24-hour catch by species and stock. An overall percent 

 CPS was calculated for each seine type by averaging 

 interval values from the three appropriate sampling 

 dates. 



Diel catch data for each species were compared 

 graphically and with linear regression to corresponding 

 tidal heights at Jones Beach. 



Results 



During 14-15 May 1980, the only period we sampled 

 with both beach and purse seines, the beach seine 

 accounted for 79% of the total catch of subyearling 

 chinook salmon (predominately fall race) (Van Hyning 

 1973), while the purse seine produced the largest 

 catches of yearling fish: 92% of the yearling chinook 

 salmon (predominately spring race); 82% of the coho 

 salmon; 100% of the sockeye salmon O. nerka; and 97% 

 of the steelhead. Daylight sampling in previous years 

 produced similar beach seine to purse seine catch ratios 

 (Dawley et al. 1986). 



Examination of catch data indicated there was no ap- 

 parent relationship to tidal variations for any species 

 during any sampling period; correlation coefficients 

 ranged from -0.51 to 0.14. Catch/tidal data are avail- 

 able upon request. Dawley et al. (1986) also observed 

 a lack of correlation between tidal cycles and beach 

 seine catches of subyearling chinook salmon from the 

 Columbia River estuary. 



Subyearling chinook salmon 



Beach seine catches of subyearling chinook salmon 

 (13,513 fish) peaked during the interval about 1.5 hours 

 after sunrise (6.9% CPS) followed by steady catches 

 during the daylight intervals, each near 4.0% CPS. 

 About 1.5 hours before sunset, a second, smaller peak 

 was observed in two intervals (CPS of 5.2% each), 

 followed by a sharp and continued decrease with dark- 

 ness through the night intervals (average CPS = 0.9%). 

 The night catch was 10.2% (3.8 SD) of the total catch 

 for a 24-hour period. Catches increased again about 45 

 minutes before sunrise (Fig. 2A). 



Purse seine catches of subyearling chinook salmon 

 (1461 fish) increased just before sunrise and decreased 

 throughout the day (Fig. 2B). Again, only 10% (1.7 SD) 

 of the total purse seine catch was at night. 



Coho salmon 



About 21% of the yearling coho salmon captured were 

 from beach seining (1092 by beach seine and 3990 by 

 purse seine). The June 1978 sampling period produced 

 only 17 fish and was not included in the assessment 

 of movement behavior. Beach seine catches in daylight 

 were low until about 1000 hours then generally in- 

 creased, with large fluctuations between intervals, to 

 a peak at about 1430 hours (10.7% CPS) (Fig. 2C). In 

 the late afternoon and evening, catches generally 

 decreased with large fluctuations between intervals. 

 The CPS dropped at dusk to 2.5% followed by lower 

 catches during darkness. The night catch averaged 



