Juvenile Salmonid Assemblages at the Mirror Lake Complex in the Lower Columbia River before and after a Culvert Modification

Abstract

This study examined salmonid assemblages upstream of a culvert connecting the Mirror Lake Complex with the lower Columbia River before and after the culvert was modified to improve habitat connectivity and fish passage. Initially the culvert limited water flow between the Columbia River and the Mirror Lake Complex. The outlet and interior of the culvert were reconfigured to create a more “natural” and suitable passageway for salmonids through the removal of riprap and the strategic placement of boulders, cobbles, gravels, baffles, and weirs. Prior to the culvert modification, three sites were sampled monthly between April and August of 2008, 5.0 and 0.5 km upstream of the culvert and immediately downstream of the culvert. After the culvert modification, the same sites were sampled from 2009 to 2012, with two additional sites added in 2010. Sites near the culvert supported Chinook Salmon Oncorhynchus tshawytscha, Coho Salmon O. kisutch, and Chum Salmon O. keta, while sites further from the culvert supported unmarked Coho Salmon and Rainbow Trout O. mykiss, steelhead (anadromous Rainbow Trout), and Cutthroat Trout O. clarkii. Clear trends in salmonid occurrence were not observed, although densities of Chinook Salmon tended to be higher in years postmodification than before modification. Culvert modifications should focus on alleviating site‐specific fish passage conditions to result in substantial changes to habitat connectivity.

Changes in Fish Assemblages at the Mirror Lake Complex in the Lower Columbia River Before and After a Culvert Modification

“…In addition, in contrast to the upper sites, summer water temperatures were above those suitable for salmonids as well as some native fish species (Marine and Cech 1998;McCullough 1999). At the lower MLC sites, water temperatures during the summer months were consistently above 20°C, whereas the water temperature at the upper sites rarely exceeded 20°C (Sol et al 2019). While our premodification survey established that there were signs of disturbance at the lower sites of the MLC, we saw few major changes in fish assemblages at the MLC that could be linked to the modification of the culvert.…”

“…Study sites.-Study sites are described in detail in Sol et al (2019). Briefly, prior to the culvert modification in the late summer of 2008, three sites were monitored at the MLC for fish assemblage and abundance: lagoon, Mirror Lake, and Young Creek (Figure 1).…”

“…Fish collection methods.-Fish sampling was generally initiated in April and continued monthly through August. Water level at the MLC can vary throughout the year (Sol et al 2019). Due to the variability in water level at the sites, several types of gear were used.…”

“…Dam construction has changed the timing and magnitude of river flow, affecting water depth in the Columbia River (Kukulka and Jay 2003;Sanderson et al 2009). Accordingly, water levels in the MLC changed drastically across seasons, coinciding with the Bonneville Dam discharge fluctuations (Parametrix 2006;Sol et al 2019). Modifications made at the FISH ASSEMBLAGE BEFORE AND AFTER CULVERT MODIFICATION culvert may allow more water through the culvert to inundate upper sites, but unusually high Columbia River flows in 2011 and 2012 may have been equally or even more important.…”

“…It was hypothesized that culvert modifications would facilitate fish passage between the Columbia River and the MLC, so juvenile salmonids could use the areas for feeding and rearing, and would also facilitate migration of juvenile Coho Salmon O. kisutch from the upper sites. Sol et al (2019) showed that the restoration of the culvert at the MLC was successful to some extent. The lower sites (i.e., the sites closest to the Columbia River) supported primarily Chinook Salmon O. tshawytscha and a variety of native and nonnative fish species, with summer water temperatures above those favorable for salmonids.…”