8097 - Laserfiche WebLink

Home Browse Search

The number of brown trout captured in the river in 1979 was 794; in 1980, 947 were captured. In 1980 only 313 brown trout were collected during the upstream migration; 634 were collected during the downstream migration. Survival of first~year spawners appeared to be very low. In 1980 only 4% of the fin-clipped fish, which first spawned in 1979, and 4% of the tag- ged fish were repeat spawners. Similar results were observed by Jonssen and Stenseth (1977). The apparent high mortality of tagged fish was attribu- ted to old age. The high mortality for fin-clipped brown trout was attribu- ted to low food reserves after spawning. Low food reserves were probably compounded by a reduction in numbers of benthic food items caused by winter drawdowns. McAfee (1976) indicated that the benthic fauna may decrease in irrigation reservoirs as water levels decline. Spawning Habitat In the Blue River, 1.8% (936 m2) of the total study area (50,841 m2) was usable spawning habitat. Much of the spawning habitat was confined to small areas along stream banks. All spawning habitat appeared to be util- ized, including areas that were considered marginal. Spawning substrate consisted of gravel ranging 0.25-76 mm in diameter. The most common size was 16-64 mm (Fig. 5). This is typical of most trout-spawning habitat (Staley 1966, Reiser and Wesche 1977). Small particle sizes (medium and fine sand, < 0.5 mm) averaged 0.7% of all substrate sam- ples collected. This was well below the maximum amount (15%) that restricts gravel flow (McNeil 1964, McNeil and Ahnell 1964). Sixty brown trout nests were located in the study area. Their average width was 31 em, mean length was 41 cm, and averagA depth of depression was 6.1 cm. Mean water velocity and depth were 0.04 m/sec and 20 cm, respec- tively. Average distance to cover was 3.1 m. Harvest Anglers spent fewer hours fishing at Dillon Reservoir in 1980 than in 1979 (Davis 1982). An estimated total of 2063 brown trout was harvested in 1979; in 1980, the estimated total was 1407. Catch per angler hour (CPAH) was 0.011 and 0.009 in 1979 and 1980, respectively. The night creel survey resulted in an estimated 200 brown trout harvested per month in September and October. This suggested that a minimum of 1400 trout were harvested at night in 1980, which doubled the total number of brown trout taken from Dillon Reservoir in that year. Sixty-five percent of the brown trout harvested in 1979 and 1980 ranged 305-405 mm TL, which corresponds to the length range of first-time spawners. DISCUSSION Evaluation of Growth \ One reason for the decline in size of brown trout may be related to food supply, and mainly to forage fish availability. Prior to 1979, the 42