1. The number of returning adults is a function of survival of juvenile salmon and steelhead during migration in fresh water.
Figure 12. A regression with the number of returning adults as a function of the number of outmigrating juvenile salmonids. Reproduced from Beamesderfer et al. 1996.
- This hypothesis assumes that increasing the number of out migrating juveniles will increase the number of retrurning adults. There is some evidence for this, as data from 1970 to 1984 show a positive correlation between returning adults and the number of out-migrating juveniles passing their first dams in the Columbia River basin (Raymond 1988).
- However, while the trend is significant (p-value = 0.047), the R^2 value (0.27) indicates that only 27% of the variation is explained by juveniles passing the first dam and adults returning. This low percentage suggests that there are likely other factors affecting this relationship. It is also possible that juvenile survival may not directly impact adult returns.
- In 2002, Koslow and associates found a significant relationship between juvenile salmonid survival and meteorological/oceanographic conditions. Ocean conditions are known to vary from year to year, which suggests that so will juvenile salmonid survival. Assuming a constant level of mortality in the ocean may be unrealistic, as this mortality would vary with the annual changes in ocean conditions.
2. Northern pikeminnow in the main-stem Columbia and Snake River reservoirs consume significant numbers of juvenile salmon and steelhead that would otherwise have survived migration.
Table 1. Consumption estimates for juvenile salmonid predators.
- This hypothesis assumes that juvenile salmon and steelhead predated upon by northern pikeminnow would not have experienced other mortality on their route to the ocean.
- Juvenile salmonids experience mortality from a number of predators besides the northern pikeminnow, including both avian (Caspian Terns and double-crested cormorants) and fish predators (walleye and smallmouth bass). The total number of juvenile salmonids lost annually differs between these predators.
- Note, the Caspian Tern estimate is the annual average for the past 10 years. Additionally, the most recent double-crested cormorant estimate for 2010 (19.2 million) is double that of the previous year.
3. Large, old northern pikeminnow are the most important predators of salmonids.
- The population of northern pikeminnow is heavily weighted in the older and larger age classes (fish over 250mm and up to 16 years of age) (Beamesderfer et al. 1996). For these individuals, salmonids compose a large and important part of their diet (Poe et al. 1991, Vigg et al. 1991).
- The number of juvenile salmonids consumed increases exponentially with increasing pikeminnow size, up to around a fork length of 500 mm (Rieman et al. 1991, Vigg et al. 1991).
- Due to the majority of salmonid consumption occurring in the older age classes and the northern pikeminnow population age structure, older individuals have the highest predation rates and are therefore the greatest threat to juvenile salmonids, consistent with Beamesderfer and associates (1996) hypothesis.
4. The cumulative effect of a 10 - 20% annual exploitation rate reduces the predation rate from northern pikeminnow by 50% by reducing the population of older piscivorous individuals.
Figure 13. System wide exploitation rates of northern pikeminnow ≥ 250 mm fork length for the northern pikeminnow sport reward program 1991 – 2010. Error bars denote the 95% confidence interval. Confidence intervals were not available for 1991 – 1992 (graph on left). Maximum (A), median (B), and minimum (C) estimates of predation reduction by northern pikeminnow on juvenile salmonids relative to predation prior to implementation of the northern pikeminnow management program. Estimates of predicted predation after 2010 are based on average values for post-program model inputs for the most recent seven years (graph on right). Figures reproduced from Porter (2010).
- The model created by Rieman and Beamesderfer (1990) predicts that annual removal of 10 to 20% of northern pikeminnow (≥ 250 mm fork length) will reduce predation levels by around 50%.
- Target exploitation rates have been set between 10 and 20% because of uncertainties in pikeminnow population dynamics. If the population is density independent a10% exploitation is needed to reach the 50% reduction in predation target. If it is density dependent, a 20% exploitation rate is needed (Beamesderfer et al. 1996, Rieman and Beamesderfer 1990).
- Harvest levels from the program have achieved the 10 to 20% removal goal (fig. 8 on left) and there appears to be a shift towards younger individuals (Porter 2010).
- 2010 predation models show(calculated using pikeminnow exploitation rates, figure 8) there has been a 35% reduction in pikeminnow predation rates since the beginning of the program.
5. Target exploitation rates can be achieved and sustained with a combination of removal methods.
- Between 1990 and 1995 over 900,000 northern pikeminnow (fork length ≥ 250 mm) were removed from the Snake and Columbia Rivers using reward fishery, dam angling, site-selective fishery, and evaluation sampling (Beamesderfer et al. 1996).
- Multiple removal methods are being implemented, the sport reward fishery continues to be the most effective means to achieve the target pikeminnow removal rate.
6. Northern pikeminnow population dynamics (i.e., factors regulating population size) or the composition of the resident fish community do not compensate for removals.
- This hypothesis assumes that the northern pikeminnow populations do not respond to increased exploitation rates by increasing predation, growth, or reproduction rates (Rieman and Beamesderfer 1990; Zimmerman 1995). It also assumes that other predators do not respond to decreased northern pikeminnow predation rates by increasing their own predation rates.
- In 2010, it was found that only 8% of northern pikeminnow stomachs sampled contained juvenile salmonids (Porter 2010) . Previously, pikeminnow predation rates were estimated to be 78% (Rieman et al. 1991).
- The current report (Porter 2010) does not evaluate population growth and reproduction levels. Lack of historical data on age class structure of pikeminnow adds uncertainty to the way in which pikeminnow populations have been impacted.
- There is no conclusive evidence to show compensatory predation; however, an increase in smallmouth bass predation has been observed (Porter 2010).
- Continued monitoring of compensatory predation by other aquatic predators is needed as fish communities are still adjusting to changes from the removal efforts.