Select Findings from Fisheries Analysis
During our field sampling in summers of 2011-2013, we collected 41 fish species – 22 native and 19 non-native. Of the total of 36,586 individual fish collected, 93% were native species and 7% non-native (Williams, 2014). In mainstem habitat, 97% of the individual fish captured were native and 3% were non-native. A greater proportion of catch in slough habitats was non-native (13%), but native species comprised the majority (87%) of fish captured in the sloughs as well.
Species richness and relative abundance of fish exhibited significant longitudinal patterns (Fig. 1). Higher numbers of fish were collected in the upper river, and higher proportions of those fish were native species. In contrast, non-native species exhibited the opposite pattern, increasing in relative abundance and total number of taxa from the upper river to lower river. The 1-km standard sample reaches in the upper river contained 16-19 native fish species, but similar samples from the lower river contained only 3-10 native fish species.
Figure 1. Number of native fish species captured in BOAT electrofishing samples for the mainstem Willamette River for 2011-2013.
During our three-year monitoring study, we sampled both fish communities, habitat, and water quality throughout the Willamette River mainstem (Fig. 2). The green and red lines and markers in Fig. 2 represent the maximum daily temperature observed at each station in 2012 and 2013, respectively. We do not have modeled estimates of water temperature for 2100, but we projected the consequence of a potential temperature change on the likelihood of capturing representative fish species in our standard sampling protocol. If water temperatures in the Willamette River increase by 2° C (3.6° F, less than the projected air temperature), the longitudinal profile (blue line and markers) would increase to more than 26° C (78.8° F) in the lower river. Such an increase is highly likely in the future during normal flow years. As a frame of reference, the USGS gaging station in Portland recorded temperatures of 26.8° C (80.2° F) in early July 2015, a year that was extremely warm and when flows in the river were much lower than average (Fig. 3).
Figure 2. Longitudinal pattern of river temperature observed in 2012 and 2013 (green and red, respectively) and longitudinal pattern of river temperature in 2100 assuming an increase in river temperatures of 2° C (3.6° F).
Figure 3. Temperature at the USGS gaging station in Portland in summer 2015.
We estimated the likelihood of occurrence of juvenile Chinook salmon in our standard sampling protocol based on the temperatures at which we observed this species in 2011-2013. Projections for the likelihood of occurrence currently decreases from approximately 50% in the upper river to less than 30% in the lower river (Fig. 4). The projected temperature increase by 2100 would lower that likelihood to less than 40% in the upper river and roughly 15% in the lower river. Coastal cutthroat trout are a cold-water salmonid that resides in the Willamette River throughout the year. Cutthroat trout exhibited even greater sensitivity to temperature (based on the locations they were captured). The likelihood of occurrence of cutthroat trout in the lower river would be extremely low under potential future temperatures (Fig. 5). In contrast, the likelihood of occurrence of common carp, a warm-water non-native species, increase longitudinally and would increase to more than 90% in the lower river by 2100 (Fig. 6).
These projections of the likelihood of cold-water fish species occurrence at higher temperatures are likely to be overestimates because the National Marine Fisheries Service (NMFS) does not allow us to sample at temperatures higher than 18° C (64.4° F). We are extrapolating beyond observed temperatures and occurrences. Based on the incipient lethal levels of these species, it is highly unlikely to observe Chinook salmon, cutthroat trout, or other salmonids in water warmer than 26° C (78.8° F).
Figure 4. Longitudinal pattern of the likelihood of capturing juvenile spring Chinook salmon in our standard sampling protocol based on the river temperatures observed in 2012 and 2013 (green and red, respectively) and longitudinal pattern of river temperature in 2100 if river temperatures increase by 2° C (3.6° F).
Figure 5. Longitudinal pattern of the likelihood of capturing resident coastal cutthroat trout in our standard sampling protocol based on the river temperatures observed in 2012 and 2013 (green and red, respectively) and longitudinal pattern of river temperature in 2100 if river temperatures increase by 2°C (3.6°F).
Figure 6. Longitudinal pattern of the likelihood of capturing common carp, a warm-water non-native fish species, in our standard sampling protocol based on the river temperatures observed in 2012 and 2013 (green and red, respectively) and longitudinal pattern of river temperature in 2100 if river temperatures increase by 2° C (3.6° F).
The Willamette River has recovered greatly from past water pollution and river channel modifications, but it faces many threats in the future. Population in the region is expected to continue growing rapidly. Land development continues to see increasing demands for urban and residential lands while agricultural and forest industries are fighting to protect their land base. Much of the new development pressures are in the valley along the mainstem Willamette River and its floodplain. Streams and river temperatures already approach the lethal limits of native cold-water fish species, especially in the lower river near the major urban centers. Many miles of streams in the basin are listed by environmental agencies as water quality impaired because of water temperature. The climate in the basin is projected to warm by 1.0 to 3.4° C (2 to 6° F) by the middle of the century. Our results suggest that the likelihood of occurrence of native cold-water species, such as juvenile Chinook salmon, would decrease substantially if future river temperature increases by 2° C (3.6° F) or more.
One of the greatest challenges is to create a scientifically sound vision of the new river, a river that is changing because of its altered flow regimes and sediment supply, a river that is changing because of social changes in the towns and communities along its banks (Wallick et al., 2013). Water management authorities are facing increasing demands to store water in reservoirs and withdraw more water during low flow seasons when the needs of the aquatic ecosystem also are most acute. Flood control reservoirs already have reduced sediment transport to the mainstem by 60%, and peak flows in the river are reduced roughly 30 to 70%. The momentum of current trends and uncertainty of future changes make it critical for our region to anticipate the future Willamette River.