Summer Riverscape Patterns of Fish, Habitat, and Temperature in Sub Basins of the Chehalis River, 2013-2016

Habitat, temperature, and fish distributions in the Chehalis River tributaries were identified as data gaps by the Aquatic Species Enhancement Plan Technical Committee of the Chehalis Basin Strategy (Aquatic Species Enhancement Plan Technical Committee, 2014). This information is needed to address questions pertaining to the development of flood reduction strategies, including a proposed dam, and to anticipate fish responses to the combined effects of future restoration actions and climate change, both of which may further influence fish habitat through changes in the hydrological, physical, and thermal environment.

The primary goal of this study was to identify habitat and temperature characteristics associated with summer rearing distribution of juvenile salmon and steelhead in the Chehalis River. Our objectives were to compare and contrast habitat and temperature characteristics associated with fish distributions 1) across six main stem survey areas within four major sub basins of the Chehalis River, and 2) across four years in one sub basin (Upper Chehalis River). While the focus of this study was on juvenile salmon and steelhead, we collected information on all native and non-native fish species observed, providing a broader perspective on fish occupancies and distribution patterns during summer months. Our survey areas were six main stem sections in four major sub basins of the Chehalis River watershed . the Upper Chehalis River main stem, South Fork and North Fork Newaukum River, East Fork and West Fork Satsop River, and West Fork Humptulips River. Spatially continuous 'riverscape' surveys conducted in each survey area provided information to describe longitudinal (upstream-downstream) fish and habitat patterns. Surveys were 24 km to 77.3 km in length, began at approximately 250 m above sea level, and were conducted during the summer months (late July - early September) of 2013 . 2016. Snorkelers counted fish by species, age class, and origin (wild or hatchery), and surveyors collected habitat measures for 200 m segments within the study areas. Summer stream temperatures were quantified at fixed monitoring sites spaced an average of 4.3 km apart within each survey area.

Habitat

Pool-riffle was the dominant channel type in the survey areas (>82% in all survey areas). Most survey areas had comparable wetted widths except the South and North Fork Newaukum which were narrower on average than the other survey areas. Large woody debris densities were variable among survey areas and highest in the East Fork Satsop and West Fork Humptulips and lowest in the Upper Chehalis. In general, a longitudinal habitat pattern included upstream segments being characterized by higher pool densities and coarser substrate relative to downstream segments; this longitudinal pattern was observed in all survey areas except the East Fork Satsop. The upstream segments of the East Fork Satsop were characterized as spring-fed headwaters, a habitat type not observed elsewhere in our survey areas.

Temperature

August temperature patterns varied among and within survey areas. Among survey areas, mean daily temperatures during August were on average warmest in the Upper Chehalis and West Fork Satsop (18.9 and 18.5° respectively) and coolest in West Fork Humptulips and East Fork Satsop (15.6 and 14.3°, respectively). The South and North Fork Newaukum were intermediate in terms of mean daily August temperature compared to all other survey areas at any elevation (17.4 and 16.9°, respectively). At a given elevation, mean daily August temperatures differed by up to 7.1°C among survey areas. The East Fork Satsop, with its spring-fed headwaters, was uniquely cool among the survey areas. Temperatures in the East Fork Satsop at elevations less than 100 m were comparable to temperatures in the West Fork Satsop and North Fork Newaukum at elevations above 200 m and colder than temperatures in the Upper Chehalis above 200 m. All survey areas were characterized by a longitudinal pattern of colder temperatures in upstream, higher elevations and warmer temperatures in downstream, lower elevations. The difference in mean daily August temperatures between the upper and lower extent of each survey area ranged between 2.7°C and 7.6°C.

Fish

In combining all surveys, we collected roughly 850,000 individual fish observations. Fish observations included 13 species and 27 species-life stage-origin combinations. Juvenile coho salmon (Oncorhynchus kisutch) and steelhead (O. mykiss) were the most commonly observed of all species and life stages among the survey areas. Juvenile Chinook (O. tshawytscha) were rarely observed, likely due to the late summer timing of our surveys. Cyprinid species, including redside shiner (Richardsonius balteatus), dace (Rhinichthys cataractae, R. osculus), and northern pikeminnow (Ptychocheilus oregonensis) were commonly observed across all survey areas except the East Fork Satsop and the West Fork Humptulips. Resident trout were observed across survey areas with the most observations in the East Fork Satsop. Wild adult spring Chinook salmon were observed in the Upper Chehalis and the South and North Fork Newaukum survey areas. Wild adult steelhead were rare across survey areas but observed in the Upper Chehalis, East and West Fork Satsop, and West Fork Humptulips. We also observed adult sockeye (O. nerka) and adult bull trout (Salvelinus confluentus), both in the West Fork Humptulips. Mountain whitefish (Prosopium williamsoni) adults were observed in all survey areas, however observations of juveniles were rare. Adult largescale suckers (Catostomus macrocheilus) were observed in all survey areas except the West Fork Humptulips and juvenile suckers were observed in all survey areas except the East Fork Satsop. Threespine stickleback (Gasterosteus aculeatus) were observed in the South and North Fork Newaukum and East and West Fork Satsop.

Hatchery origin salmonids were observed in the sub basins where hatchery fish are released, including the South Fork Newaukum (juvenile steelhead), East Fork Satsop (juvenile coho and steelhead, resident trout, and adult steelhead), West Fork Satsop (adult steelhead), and West Fork Humptulips (adult steelhead). Non-native fish, including smallmouth and largemouth bass and bluegill were observed in the main stem Chehalis River downstream of Rainbow Falls and in downstream segments of South and North Fork Newaukum survey areas.

Within survey areas, longitudinal patterns were observed in terms of the amount of habitat occupied (e.g. occupancy) and densities of some species and age classes. Occupancies and the densities of juvenile salmon and steelhead, resident trout, and adult mountain whitefish were generally higher in upstream than downstream segments of the survey areas. In contrast, occupancies and densities of cyprinid species and juvenile largescale suckers were generally higher in downstream than upstream segments of the survey areas. Other species and age classes either had low occupancy overall (i.e. unable to examine longitudinal patterns) or no longitudinal pattern was observed.

Among four survey years in the Upper Chehalis survey area, variability in occupancy for juvenile coho salmon and steelhead (age-0 and age-1) was similar to variability observed among survey areas. Occupancy of juvenile coho salmon and steelhead age-1 class varied less than one and a half-fold and juvenile steelhead age-0 age class was consistently >97% among years. In contrast, densities of juvenile coho varied sixteen-fold and juvenile steelhead varied three (age-0) to eight-fold (age-1) among years in the Upper Chehalis. The longitudinal patterns observed for salmonid and cyprinid species were relatively consistent, in that juvenile steelhead occupied more habitat and were observed in greater density upstream compared to downstream and the opposite pattern was observed for cyprinid species.

Synthesis

Juvenile salmonids and cyprinids represented roughly 97.5% of the total fish observed in our surveys. Therefore, for the purpose of analysis, the fish assemblage of each 200m segment was categorized as 'low' (0-24.9%), 'medium' (25-75%), or 'high' (?75.1%) according to the proportion of juvenile salmon and steelhead in the total counts observed in each segment. Segments classified as 'high' salmonid would also reflect 'low' cyprinid and vice versa.

Within each of the six survey areas, the fish assemblages were more consistently associated with summer (August) temperature than habitat characteristics. The East Fork Satsop and West Fork Humptulips had the coolest temperatures and very few (<1%) 'low' salmonid segments. This contrasted with the Upper Chehalis, South and North Fork Newaukum, and West Fork Satsop survey areas which had warmer temperatures and a higher proportion of 'low' salmonid segments (12.1-35.0%). Within each survey area, the summer temperatures associated with 'high' salmonid segments were consistently cooler than those associated with 'low' salmonid segments. However, the absolute summer temperatures associated with 'high' salmonid segments differed among survey areas. Specifically, the mean daily August temperatures associated with 'high' salmonid segments in the Upper Chehalis and West Fork Satsop were up to 3.9° warmer than those observed for 'high' salmonid segments in the other survey areas.

Among years in the Upper Chehalis, segments in upstream locations were consistently dominated by juvenile salmonids ('high' salmonid) and segments in downstream locations were consistently dominated by cyprinids ('low' salmonid). Among years, 'high' salmonid segments were consistently cooler than 'medium' and 'low' salmonid segments. The majority (80-92%) of 'high' salmonid segments were observed upstream of the proposed dam site among years. Of the three fish assemblage categories, the 'medium' salmonid segments were most variable in terms of location among years; the mean river kilometers of 'medium' salmonid segments were 3 to 5 km further upstream in 2014 and 2015 than in 2013 and 2016. However, August temperatures associated with each fish assemblage category did not vary among years therefore, stream temperatures earlier in the summer may be important in shaping the structure of fish distributions observed in August.

Conclusions

Our results demonstrate that despite the extensive amount of aquatic habitat available in the 6,889 km² of the Chehalis River watershed, a very limited portion of the watershed has physical characteristics suitable for summer rearing of juvenile salmon and steelhead. A combination of temperature and habitat characteristics are associated with the summer rearing distributions of juvenile salmon and steelhead and their distributions are likely to be further influenced by interactions with the native cyprinid species. Upstream locations within each of the surveyed sub basins are currently valuable summer rearing habitat for juvenile salmon and steelhead and for resident trout. In the Upper Chehalis sub basin, we surveyed summer distribution of fishes over 77 km of the main stem Chehalis River from the confluence of the east and west forks to the confluence with the Newaukum River. Within this survey area, the majority of summer rearing by juvenile salmon and steelhead occurred in close proximity and upstream of the proposed dam location.

A combination of physiological tolerance to stream temperatures and temperature-mediated competition with native cyprinid species likely influences the lower extent of summer rearing for juvenile salmon and steelhead. The composition of fish species observed in our surveys was closely associated with stream temperatures, and river segments with cooler temperatures were associated with juvenile salmon and steelhead rearing. Interestingly, our results support the concept that local adaptation or acclimation to the thermal environments may occur in different sub basins as mean daily August temperatures that supported high proportions of juvenile salmon and steelhead varied by up to 3.9° among survey areas.

Increasing stream temperatures from climate change are likely to further limit suitable summer rearing locations for juvenile salmon and steelhead and may facilitate upstream expansion of competing and or predatory native and non-native fish species. Over four years, we have accumulated a comprehensive spatial data set that combines information on fish, habitat, and temperature in the Chehalis River basin. Future work will further examine the relationships of stream temperature and landscape variables with fish occupancy and density, and explore variables associated with the lower spatial extent of juvenile salmon and steelhead summer rearing habitat.