The first shovelnose was collected on April 22 in 1976; large concen- 

 trations appeared on May 7 (figure 13). Sampling continued until July 27, 

 but the last sturgeon was taken July 15. The first ripe male was collected 

 on June 2 and the first ripe female on June 7. The last spent female was 

 caught on June 29, but other females were taken after this date that had 

 neither spawned nor were ready to spawn. 



Spawning occurs in the Tongue River from early June until mid-July. 

 In the Red Cedar River, Wisconsin, Christenson (1975) reported that shovel- 

 nose sturgeon spawned from the last week of May through the first week of 

 June. Mississippi River sturgeon spawned from May 30 through June 14 

 (Helms 1974). Rehwinkel et al. (1976) reported the first 1976 sturgeon in 

 the Powder River was taken on April 13 with the farthest upstream migrant 

 taken on May 13. No ripe sturgeon were collected on the Powder River. 



HABITAT PREFERENCE 



Depth 



The gill net used in sampling was marked off in three-meter intervals; 

 the number of fish taken in each segment was recorded. One end of the net 

 was always kept close to the shore. In a selected reach with a uniform 

 bed, 85 shovelnose were taken throughout the season. The catch rate per 

 net section was plotted against the bed profile (obtained from WSP) to 

 determine depth preference (figure 14). The majority (61.2 percent) of the 

 fish were taken from the more gently sloping edge of the stream 15 to 21 m 

 (49to 69ft) from the shore. The mode of the sturgeon catch occurred at 

 the thalweg. Depth preferred by the fish ranged from 0.43 m (1.4 ft) to 

 0.90 m (3.0 ft), with an average of 0.67 ±0.17 m (2.2 ±0.6 ft). These 

 depths correspond to those found by Bovee (1976) for shovelnose sturgeon. 



Discharge 



Daily sturgeon catches were totaled for each week of the sampling season. 

 To evaluate the catch rates in relation to discharge, two catch-per-effort 

 rates were calculated. 



1) The total catch for each week was divided by the length of river 

 sampled during that week, resulting in the number of fish taken 

 per km. 



2) The week's catch was divided by the total number of sampling efforts 

 made that week, producing the number of fish caught per trip. 



Both catch rates were averaged over the week to avoid sectional bias, 

 since one to three sections were sampled during any week. Optimum flow con- 

 ditions were evaluated in terms of the catch rates; as Zakharyan (1972) 

 showed, the scale of spawning depends on the number of fish taking part. 



37 



