Graduate Thesis Or Dissertation
 

Restoring the Columbia River Estuary : Chinook Salmon Recovery and Invasive Species Management

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  • As highlighted in this study, shallow-water habitats and the prey they provide are important for juvenile salmon in the Columbia River estuary, but the spread of invasive species and large-scale changes to the estuary influence how these habitats are utilized. Reed canarygrass (Phalaris arundinacea) (“PHAR” hereafter) is a highly invasive aquatic plant species that affects a number of emergent wetland habitats in the upper Columbia River estuary, and is of concern to those trying to restore important shallow-water habitats for juvenile salmon. The presence of PHAR undoubtedly reduces plant diversity, but this study seeked to elucidate how PHAR affects juvenile Chinook Salmon rearing success relative to natural emergent vegetation. The study further evaluated the importance of wetland-deriver prey to juveniles found in shallow-water habitats along a habitat gradient (i.e., back-water channel, confluence, and main stem). We conducted research at a floodplain wetland restoration site in the tidal-fluvial portion of the estuary, where a water control structure is used to manage water levels and the spread of PHAR. The goal of the research was to compare two microhabitats within the floodplain: areas dominated by PHAR and areas dominated by natural emergent vegetation. Using hatchery-raised juvenile Chinook salmon, we designed a series of feeding experiments in artificial enclosures to determine whether invasive PHAR and natural emergent vegetation provide similar foraging and growth opportunities for juvenile salmon. We further identified differences in physical and biological parameters of each habitat (e.g., temperature, dissolved oxygen, and invertebrate community) and examined feeding characteristics (e.g., diet composition and modeled growth rates) for juveniles through a stomach content analysis. Additionally, we conducted beach seining and diet analysis for fish collected along a habitat gradient and compared the diet composition and modeled growth rates between the sample areas. In the floodplain wetland, invertebrate prey compositions from the 2015-2016 (March- June) fallout and emergence traps were similar, but prey abundances and diversity varied seasonally. Across both years, for emergence traps, the average total density (per m²) of invertebrates in the natural vegetation and in PHAR was 154±18 m² and 225±34 m², respectively, and for the fallout traps was 664±95 m² and 662±64 m², respectively. Salmon diets were mostly similar, and a large proportion of diets in both habitats were made up of Copepoda/Cladocera (60-98% composition by biomass). Growth during the net pen experiment differed significantly between the two vegetation types (Kruskal-Wallis: p<0.001), with fish growing more in the natural emergent vegetation. In 2015, juveniles grew an average 6.4 mm FL in the natural emergent vegetation, compared to 4.7 mm FL in PHAR, and consumed fewer Copepoda/Cladocera and more Diptera over the 10 days. In 2016, juveniles grew an average 9.2 mm FL in the natural emergent vegetation compared to 7.6 mm FL in PHAR. In 2016, fish from both habitats consumed a higher abundance of Copepoda/Cladocera and grew more, despite poor water quality conditions at the site. Of the subyearlings (n=170) and yearlings (n=14) collected in habitats adjacent to the floodplain wetland, 48% were of known hatchery origin. Modeled potential growth rates for subyearlings were similar on average between February and May in the back channel, confluence, and main stem, ranging from 0.067–0.07 g/g/d, but the rates varied seasonally. Growth potential was higher in the back channel habitat earlier in the sampling season (February-March), as the modeled daily growth rates were 0.08 g/g/d compared to 0.06 g/g/d in the main stem. All fish collected prior to May were relatively small and unclipped, highlighting the seasonal benefits of shallow-water habitats to support a variety of life-history traits. When comparing modeled growth rates of the juveniles reared at the floodplain wetland compared to the in adjacent habitats, estimates from the floodplain wetland were higher across all months. Restoration of shallow-water habitats is important for the overall health of the estuary, but the effects of large-scale ecosystem changes (e.g., flow regulation and spread invasive species) on salmon recovery and estuary-management decisions are uncertain. For example, water control structures to limit the spread of PHAR also reduce fish access to floodplain habitats, and fewer high-flow events further limit fish access and may facilitate the spread of invasive species. PHAR has been shown to reduce overall plant diversity and, as observed in this study, may provide habitat that is less suitable to juvenile Chinook Salmon. The high density of important prey for salmon found at the site, regardless of the presence of PHAR, and high potential growth measured at the site, emphasize the importance of restoring shallow-water habitats that are accessible to juvenile Chinook Salmon in the upper estuary.
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