I 



W. W. Bentley 

 E. M. Dawley 

 T. W. Newcomb 



Some Effects of 



Excess Dissolved 



Gas on Squawfish, Ptychocheilus 



oregonensis (Richardson) 



ABSTRACT 



In the spring of 1974, large numbers of squawfish were en- 

 countered in the Snake River between Lower Monumental and 

 Little Goose Dams. Squawfish exhibited gas bubble disease 

 symptoms within 1 week after the onset of 125 to 135% satu- 

 ration. A 12-day bioassay in shallow tanks to determine tolerance 

 levels and resistance times at various gas concentrations was 

 conducted. We found squawfish to be similar to juvenile salmon 

 and steelhead trout in their resistance to supersaturated con- 

 centrations of dissolved gas. Feeding response changed after 

 stress to high concentrations of dissolved gas. Average daily 

 food consumption of test groups decreased with increased 

 supersaturation. Squawfish captured in the field during periods 

 of high supersaturation were less abundant and only a small por- 

 tion of them had been feeding compared with survey results 

 taken during lower supersaturation. Nitrogen supersaturation 

 could be an important factor in assessing the effects of predation 

 on juvenile salmonid migrants in the Columbia River system. 



Squawfish have attracted the interest of many 

 investigators in the past, primarily because they 

 have been regarded as an efficient predator. Our 

 attention was focused on them as being one of the 

 possible causes of mortality of seaward migrating 

 juvenile salmon and steelhead trout in the Snake 

 River. 



In the spring of 1974 large numbers of squaw- 

 fish were encountered in the Snake River between 

 Lower Monumental and Little Goose Dams (Fig. 1). 

 At this time, an abnormally high runoff occurred, 

 resulting in high nitrogen gas concentrations of 

 prolonged duration (Table 1). The squawfish ex- 

 hibited gas bubble disease symptoms within 1 week 

 after the onset of high gas saturation. Laboratory 

 experiments and field observation were conducted 

 to determine the tolerance of squawfish to super- 

 saturation of dissolved gas and how it affected their 

 food intake or predation rate. We wish to report on 

 these aspects and discuss problems that may require 

 further clarification. 



Predation and eating habits have been examined 

 by several investigators. Squawfish in the Columbia 

 River were determined to be opportunists in their 

 eating habits and, by and large, the availability of 



prey influences their selectivity of daily food intake 

 (Thompson, 1959). Thompson found that approxi- 

 mately 63% had empty stomachs and only 7.5% 

 showed any evidence of eating juvenile salmon. 

 Hamilton et al. (1970), in the Lake Merwin investi- 

 gation, found that 70% of the stomachs examined 

 were empty, but concluded at the end of his study, 

 that predation precluded the use of the Lake Merwin 

 Reservoir as a rearing area for coho salmon. Brett 

 and McConnell (1950) used an estimated consump- 

 tion rate of 140 salmon fingerlings per squawfish 

 per year, a figure which was accepted as reasonable, 

 to account for calculated losses of sockeye juveniles 

 from Lakelse Lake, British Columbia. If we use these 

 figures for the Snake River during a 45-day juvenile 

 outmigration, each adult squawfish might consume 



FIG. 1 Vicinity map showing location of Lower Monumental 

 and Little Goose Dams. 



Bentley, Dawley, and Newcomb: 

 Service, Seattle, Washington. 



National Marine Fisheries 



41 



