The construction of dams and other hydroelectric infrastructures can act as barriers to the migrations of fishes. Fishways built at the dam might provide a way through for most of the fishes. However, previous studies have shown that the fishways are not as efficient as expected, and the fish migration is still disrupted (Williams et al., 2005). Therefore, monitoring these fishways is important for fish conservation.
Two populations of Sockeye Salmon have been studied during two years during their migration to the Seton-Anderson watershed. This study aims to quantify the mortality rate along the migratory route and test the efficiency and assess the impact of the fishery on migration success. Previous studies were conducted in the area and suggest that mortality may be relatively high in this section of the migration route. These two populations have suffered a significant decline in the last years. The IUCN is considering listing this specie as threatened or endangered.
Methods and Materials
Eighty-seven Gates Creek Sockeye Salmon were captured and implanted with transmitters. They have been released either down or upstream of the dam. Using Telemetry the fishes were followed during spawning season for 2 years.
Fishes were tested for physiological condition (stress or maturation levels) measured from non-lethal biopsy to obtain a small blood sample. Environmental conditions were measured (e.g. discharge, temperatures) and related to fishes’ fate.
Results and Discussion
Physiological measurement shows similar values to previous studies in the literature. There was no significant difference in physiological condition between successful fish and failed fish, suggesting that sockeye released downstream of the dam were in good condition to re-ascend the fishway and that failure was probably not related to physiological conditions.
Of the total number of fish released downstream of the dam, 52% died between the release sites of downstream of the dam and spawning grounds.
Females released downstream of the dam suffered a higher mortality rate than male, 71% of 17 males and 40% of 38 females reached spawning grounds. These results have serious implications for conservation, since female participate largely in spawning success.
Five fishes were found in the powerhouse tailrace on the Fraser River, suggesting that the tailrace may attract and delay sockeye even under the current Seton River dilution guidelines. The cause of this attraction is unknown, but it may involve attraction to home stream water, seeking of alternate route by fallbacks or utilization of a thermal refuge.
The efficiency of the fishway is 80%. This failure rate is considered to be a conservative estimate because the subjects had prior experience in entering the fishway. The failure of fishway passage was related to failure of entrance location and not ascencion of the fishway itself.
Mortality in the lake upstream of the dam was greater in fish released downstream of the dam (33% for fish released at the powerhouse tailrace and 19% for fish released in the lower Seton) compared to fish released upstream of the dam (7%). The cause of this in-lake mortality is not clear, but it is unlikely to be associated with stress or energetic costs caused by re-ascending the fishway. It is possible that the fishery is responsible for that loss; at least one tagged sockeye was captured by fisheries, therefore other fishes that disappeared might have been also harvested.
A small number of tagged sockeye was observed falling back downstream when the upstream exit of the fishway was blocked during sampling. These fishes generally did not re-ascend. Temporary blockage or obstruction of the fishway could have serious consequences for populations of migrating adult sockeye.
Future Action Recommendations
The authors recommend to monitor and maintain the fishway frequently (daily) during the migration season so that blockages are cleared immediately. Moreover, they suggest that any new modifications to the fishway, such as fish enumeration devices, should be carefully evaluated in terms of their effects on passage.
More research needs to be done to quantify delay, and ramifications of delay, when sockeye initially encounter the powerhouse tailrace on the Fraser River. This should result in a re-assessment of the ‘dilution level’ of the tailrace of the dam, by studying tailrace attraction and examining the role of the tailrace as a thermal refuge. At this stage no suggestion for management action aiming at reduced tailrace attraction can be made.
The authors suggest that when possible, managers should strive to minimize relatively high discharge levels (≥ 60 m3/s) in the Seton River in order to facilitate sockeye passage.
Conclusion of Research
The conclusion of this study, based on the combined results from 2005 and 2007, is that failure to ascend the dam was primarily associated with locating the fishway entrance and not with the passage of the fishway itself.
The conditions near the dam vary largely with discharge changes and it affect the fishes orientation cues.
Studies should examine if there is a threshold level of delay that causes salmon to fallback or seek alternate routes, reducing the probability of successful migration.
The estimate of passage failure at the fishway should be considered an underestimate and future studies should sample fish that are “fishway-naïve” by catching sockeye in the lower Seton River. Due to low sampling the experiment attempted in 2007 using tangle and dip nets wasn’t concluant and it is expected that in the near future this sampling problem will persist. For this reason they recommend using a fish weir with trap boxes in the lower Seton River, to be installed and operated during sockeye spawning migrations.
- Source Scott Hinch David Roscoe. Fishway passage, water diversion and warming temperatures: Factors limiting successful spawning migration of Seton-Anderson watershed sockeye salmon. Prepared by Pacific Salmon Ecology and Conservation Laboratory and UBC Forestry. Available at : http://nsfisheries.synthasite.com/resources/BC%20Hydro%20Lillooet%20presentation%20Sept%202008.pdf [Accessed 03/2021].