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Effects of culvert modification on salmonid abundance and seasonal movements within a northern California coastal watershed
Fish passage improvement is used extensively throughout the Pacific Northwest to expand salmonid habitat to increase threatened salmon stocks. While culvert enhancement projects have been evaluated for physical responses, for example, the ability to withstand 100 year stream flows or support roads, few studies have documented the responses of salmonid populations. This study assessed the effects of two culvert replacements and one culvert enhancement on the abundance and movement of coho salmon (Oncorhynchus kisutch), steelhead (O. mykiss), and cutthroat trout (O. clarkii) within the Freshwater Creek watershed, a small, coastal watershed in northern California. All of the culverts in this study had been previously observed to be impediments to juvenile salmonid movement. Biological data was collected both before and after the culvert replacement on one stream, Graham Gulch. Only post-culvert enhancement data was collected for two other streams, McCready and Cloney Gulches. Biological data collection included out-migrant abundances of juvenile salmonids in the spring, summer juvenile abundances, and juvenile and adult movements. All responses were analyzed using an asymmetrical BACI (Before-After-Control Impact) design. Impact streams were three streams that were treated with culvert enhancement. Control streams were three streams that did not have any culverts present. Few significant differences were found between pre- and post-treatment dataset abundances in Graham Gulch. Spring out-migrant and fish movement data for the pre- and post-treatment dataset indicated that more juvenile trout used the impacted tributary for winter refugia after culvert replacement than before culvert replacement. The number of adult spawners using the tributaries post-culvert replacement did not appear to be increasing. Statistical and graphical analyses of the two-post culvert replacement datasets indicated that juvenile abundances did not differ between the treated tributaries and the control tributaries. Without a pre-culvert enhancement dataset it was difficult to assess the effects of culvert enhancement on salmonid populations.