2200 



0200 



0600 1000 



TIME OF DAY 



1400 



1800 



2200 



Figure 2. — Mean sturgeon environmental temperature devia- 

 tion (A^ C) from mean weekly Columbia River temperatures 

 averaged over 4-h periods within the diel cycle, from 19 June 

 through 20 August 1977. 



habitats, we do not believe temperature differ- 

 ences in White Bluffs Pool caused diel sturgeon 

 movements. Rather, diel movements are probably 

 influenced by light. Movement to cool, deep areas 

 occurred prior to sunrise and movement to warm, 

 shallow areas peaked after sunset. Sturgeon were 

 sensitive to light when captured for tagging and 

 remained active until a towel was placed over their 

 eyes. Bait angling for sturgeon was most success- 

 ful during sunset and sunrise, and netting near 

 shore was most productive at night. Evening 

 movements to back eddy and slough areas of 

 White Bluffs Pool, where benthic organisms and 

 smaller fish are more abundant, may be related to 

 feeding. 



Sturgeon are bottom feeders and in freshwater 

 reportedly eat crustaceans (Daphnia and 

 copepods), molluscs (clams and snails), insect lar- 

 vae (chironomids, stone flies, and ephemeropte- 

 rans), and fish (Bajkov 1949; Semakula and Lar- 

 kin 1968). Sturgeon in the mid-Columbia River at 

 Hanford ingest crayfish, fish (including whitefish, 

 suckers, and sculpins), midge and caddis fly lar- 

 vae, snails, and periphyton (Gray et al. footenote 

 3; Page et al. footnote 4). 



Daily environmental temperature records indi- 

 cated sturgeon did not consistently engage in a 

 diel movement pattern. Sturgeon sometimes spent 

 one or more days in cooler, midchannel areas. Oc- 

 casionally, sturgeon remained near inshore areas 

 through midday, although variability within and 

 among fish was common. 



Haynes et al. (1978) suggested long-distance 

 sturgeon movements in the mid-Columbia River 

 were related to temperature. Data collected in 

 1977 confirm these observations. Sturgeon move- 

 ments >2 km up- or downriver began in early 

 summer when river temperature rose to 13° C and 

 ended in fall when temperature dropped below 13° 

 C (Figure 3). 



The possibility that photoperiod plays a role in 

 initiation of long-distance movement and termi- 

 nation of movement appears unlikely. Pacific 

 Northwest drought conditions in 1977 resulted in 

 extremely low Columbia River flows and higher- 

 than-normal water temperatures. River tempera- 

 tures in 1977 did not drop below 13° C until early 

 November, and sturgeon movements continued 

 throughout. In previous years, river temperatures 



JUNE 



JULY AUGUST SEPT 



OCT 



NOV 



Figure 3. — Beginning and end of sturgeon movement ( >2 km 

 up- or downstream from release site) versus weekly average 

 Columbia River temperatures in 1975 and 1976 (derived from 

 Haynes et al. 1978) and 1977. 



369 



