Northern Pikeminnow Predation
The primary goal of the NPSRP is to reduce the effects of predation on salmonids. There are several key points relating to predation that should be considered
1. Northern pikeminnow are a major predator of juvenile salmon
Figure 6. A northern pikeminnow.
- The removal of pikeminnow would potentially reduce salmon mortality rates.
- A better understanding of predation by pikeminnow and other predators would be helpful in determining whether or not the NPSRP is the most effective means of reducing salmon mortality.
- While it has been determined that these invasive species do prey upon salmon, northern pikeminnow are still responsible for around 70% of predation on juvenile salmonids (Poe et al. 1991; Rieman et al. 1991).
2. Adult pikeminnow can become a threat to juvenile salmon upon reaching a length of approximately 300 mm (Chandler et al. 1993; Petersen 2001).
Figure 7. A northern pikeminnow with a mouth full of salmon.
- Smaller pikeminnow more typically will feed upon aquatic insects and other invertebrates (Thompson 1959). Individuals with a total length of greater than 380 mm make up 12 - 59% of the population (Parker et al. 1995). The larger pikeminnow are estimated to consume approximately 16.4 million juvenile salmonids annually in Columbia River Basin (Beamesderfer et al. 1996)
- Predation is at its highest when the juveniles are undergoing their initial migration to the ocean (Petersen 2001).
3. Significant predation occurs along the entire river system with slightly higher levels occurring in close proximity to dams, such as Bonneville on the Columbia (Gadomski et al. 1992).
Figure 8. Bonneville Dam, Oregon, USA.
- Dams act as a major obstacle for juvenile salmon migrating down rivers. The combination of increased stress as well as the disorienting effect caused by passing through a dam would seem to make juvenile salmon more vulnerable to predation (Petersen et al. 1994).
- Several studies have suggested, however, that pikeminnow may not be preying upon more salmon near dams but may be simply taking advantage of the significant number of juvenile salmon casualties that occur at dams (Petersen et al. 1994; Gadomski et al. 1992). Studies have also found that if given the choice, pikeminnow seem to prefer dead salmon to live ones (Gadomski et al. 1992; Peterson et al. 1994).
- Pikeminnow would seem to consume hatchery raised salmon more frequently than wild fish (Fresh et al. 2003).
4. The true extent of predation that actually occurs is still difficult to determine.
Figure 9. A pikeminnow with an unknown number of salmonids in its mouth.
- Stomach content analysis of pikeminnow indicate that the fish are in fact consuming a significant number of juvenile salmon; however, it is very difficult to determine whether or not the salmon were alive or dead at the time that they were consumed (Gadomski et al. 1992).
- ·It can also be easy to underestimate the number of salmon that are consumed (Buchanan et al. 1981).
5. Salmon exhibit a Type III survivorship curve.
Figure 10. Survivorship curves exhibited by various organisms.
- Predation on salmon is known to occur in all life history stages. However, mortality (and predation) is highest for the fish during the earlier stages of life and lower when the fish are adults.
- Many species of fishes produce extremely large numbers of offspring. Most are eaten by predators quickly and the fish that do escape are still vulnerable to predators. This is known as type III survivorship (Hedgecock et al. 2011).
- The NPSRP is designed to reduce the predator mortality in younger fish as more surviving juveniles would then potentially mean more returning adults. A type III survivorship, however, dictates that young salmon will still be highly vulnerable to predation. Predators are believed to be capable of taking up to 85% of young salmon (Peterman et al. 1978).
Other Sources of Salmonid Mortality
Figure 11. A Caspian tern (left), double-crested cormorant (middle), walleye (top right), and smallmouth bass (bottom right).
- Juvenile salmonids experience mortality from a number of predators besides the northern pikeminnow, including both avian (Capsian Terns and double-crested cormorants) and fish predators (walleye and smallmouth bass). However, the total number of juvenile salmonids lost annually differs between these predators.
- In the Columbia River estuary, where the greatest seabird predation occurs, Caspian terns were estimated to consume 5.3 (95% CI: 4.5 – 6.1) million juvenile salmonids in 2010 (Roby and Collis 2011).
- Double-crested cormorants in the same area were estimated to consume 19.2 (95% CI: 14.6 – 23.8) million juvenile salmonids in 2010.
- For walleye and smallmouth bass, Rieman et al. (1991) estimated predation numbers to be 0.35 and 0.24 million respectively, for juvenile salmonids predated upon in the John Day reservoir from 1983 - 1986.
- Other sources of mortality for salmon besides predation can be contributed to dams. Williams et al. (2001) showed that while juvenile migrating salmon losses from dam passageways have improved, they still result in a 31 - 59% survival rate, though this study assumes that mortality rates are mainly from passageway travel, not reservoir, extrapolating the results from Little Goose dam (Muir et al. 1998) to the other dams along the Columbia and Snake Rivers. Since actual predation numbers are known to be a large percentage of the migrating salmon in the lower dams, it is unlikely these results can accurately be extrapolated throughout the Columbia River basin. However, it does highlight a need for further investigation into the partitioning of mortality rates caused by passageway through the dams, either through direct mortality (dam passage travel), or through indirect mortality, e.g. predator susceptibility.
- Predation rates also vary between dams. Ward et al. (1995) showed that predation losses to northern pikeminnow appear to be greatest in the Lower Columbia River reservoirs and the Columbia River downstream from the Bonneville dam. Predation rates were relatively lower in the Middle Columbia and Lower Snake River reservoirs, which is likely due to reservoir size (except in the case of Little Goose) and salmon abundance (Ward et al. 1995).