

o 



o 

 o 



o 

 o 



100 no 120 



PERCENT SATURATION|(T.D.G.) 



FIC. 3 Feeding inhibition oi northern squawfish on a weight 

 of food basis. 



in the reservoir of Lower Monumental Dam on the 

 Snake River the dissolved gas level remained near 

 140% for an extended period of time this year and if 

 the predatory squawfish had maintained an average 

 daily depth of 2.76 m (9 ft), the effective saturation 

 would have been 110% — low enough to allow the 

 squawfish safe sojourn with no effective curtailing 

 of predatory capacities. During experiments done 

 earlier at the Prescott Field Station (Blahm, 1974), 

 as well as those reported here, the squawfish 

 tended to reside on the bottom of the tanks. How- 

 ever, this behavior may be an artifact due to the 

 unnatural conditions of the laboratory test tanks. 

 Nevertheless, the behavior of remaining on the 

 bottom of the tanks changed the effective satura- 

 tion value of the Prescott 1 m depth test from 119.7 

 to 109.1% T.G.P. and the 2.5 m test from 119.8 to 

 95.8% T.G.P. saturation. As a result, no mortality 

 was reported in tests at Prescott, but gas bubble 

 disease signs were apparent on all fish from the 

 1 m test after 35 days. 



Seattle and Prescott data suggest that knowl- 

 edge of the depth distribution is needed on the 

 squawfish before its role as a juvenile salmonid 

 predator can be correctly defined. 



100 ►- 



>- 

 ■< 



LIVE STEELHEAD 



100 110 120 



PERCENT SATURATION (T.D.G.) 



FIC. 4 Selective feeding inhibition of northern squawfish — % of 

 test days when ration is completely consumed. 



fish held at 120% saturation ate live steelhead only 

 on the first test day. 



Lethargy exhibited by the highly stressed fish 

 may be of significance in regard to squawfish preda- 

 tion on juvenile salmonids migrating down the 

 Columbia and Snake Rivers. However, the average 

 water depth inhabited by squawfish may compen- 

 sate for the effects of supersaturation. For example, 



44 Bentley, Dawley, Newcomb 



Merwin Trap - Purse Seine 



Fish taken in our Merwin trap located in the 

 Palouse River arm from January 1 to August 12, 

 1974, numbered 80,060 (Table 3). Squawfish totaled 

 16,626, with the majority taken in April, May and 

 June, when dissolved gas saturation, temperatures 

 and water flows were increasing (Table 1). Large 

 numbers of squawfish may have concentrated in 

 the Palouse River arm to escape the high dissolved 

 gases in the Snake River. The Palouse River arm 

 was sampled on July 2, 1974, and showed 106.8% 

 nitrogen saturation at 21.7° C (71.1° F). Recaptures 

 of marked fish released in the vicinity of the trap 

 were 1,590, indicating that a high percentage 

 remained in the area. Purse seining the navigation 

 locks and spill area at Little Goose Dam captured 

 2,101 squawfish which were tagged and released. 

 Fifty-five of those marked at the purse seine were 

 subsequently recaptured in the seine at Little Goose 

 Dam; and 97 that had been marked at the Merwin 

 trap, over 11 miles downriver, were also taken in 

 the seine at Little Goose Dam. Thirteen that were 

 tagged at the dam appeared in the trap. One 370 mm 

 squawfish marked at the trap made a round trip to 

 the dam and back to the trap. Approximately 300 of 

 the 1,590 recaptures at the trap were multiple 

 recaptures. One 359 mm squawfish appeared in the 

 trap 10 times. 



Tagged recoveries showing movements between 

 Lyons Ferry and Little Goose Dam indicate that high 

 nitrogen values in surface waters appear to be 



