Abstract

Under the 2004 Biological Opinion for operation of the Federal Columbia River Power System released by the National Marine Fisheries Service, the U.S. Army Corps of Engineers (USACE), the Bonneville Power Administration (BPA), and the Bureau of Reclamation (Reclamation) were directed to restore more than 4,047 hectares (10,000 acres) of tidal marsh in the Columbia River estuary by 2010. Restoration of Crims Island near Longview, Washington, restored 38.1 hectares of marsh and swamp in the tidal freshwater portion of the lower Columbia River. The goal of the restoration was to improve habitat for juveniles of Endangered Species Act (ESA)-listed salmon stocks and ESA-listed Columbian white-tailed deer. The U.S. Geological Survey (USGS) monitored and evaluated the fisheries and aquatic resources at Crims Island in 2004 prior to restoration (pre-restoration), which began in August 2004, and then post-restoration from 2006 to 2009. This report summarizes pre- and post-restoration monitoring data used by the USGS to evaluate project success. We evaluated project success by examining the interaction between juvenile salmon and a suite of broader ecological measures including sediments, plants, and invertebrates and their response to large-scale habitat alteration.

The restoration action at Crims Island from August 2004 to September 2005 was to excavate a 0.6-meter layer of soil and dig channels in the interior of the island to remove reed canary grass and increase habitat area and tidal exchange. The excavation created 34.4 hectares of tidal emergent marsh where none previously existed and 3.7 hectares of intertidal and subtidal channels. Cattle that had grazed the island for more than 50 years were relocated. Soil excavated from the site was deposited in upland areas next to the tidal marsh to establish an upland forest. Excavation deepened and widened an existing T-shaped channel to increase tidal flow to the interior of the island. The western arm of the existing ‘T-channel’ was extended westward and connected to Bradbury Slough to create a second outlet to the main river. New intertidal channels were constructed from the existing ‘T-channel’ and tidal mudflats became inundated at high tide to increase rearing habitat for juvenile salmonids. The restoration action resulted in a 95-percent increase in available juvenile salmon rearing habitat.

We collected juvenile salmon and other fishes at Crims Island and a nearby reference site using beach seines and fyke nets annually from March through August during all years. Benthic invertebrates were collected with sediment corers and drift invertebrates were collected with neuston nets. Juvenile salmon stomach contents were sampled using lavage. Vegetation and sediments characteristics were surveyed and we conducted a topographic/bathymetric survey using a RTK (real time kinematic) GPS (global positioning system).

The fish assemblage at Crims Island, composed primarily of threespine stickleback (Gasterosteus aculeatus), non-native banded killifish (Fundulus diaphanus), peamouth chub (Mylocheilus caurinus), subyearling Chinook salmon (Oncorhynchus tshawytscha) (hereinafter referred to as subyearlings), and small numbers of juvenile chum salmon (Oncorhynchus keta), did not differ appreciably pre- and post-restoration. Subyearlings were the primary salmonid collected and were seasonally abundant from April through May during all years. The abundance of juvenile salmon declined seasonally as water temperature exceeded 20 °C in the Reference site by mid-June; however, subyearlings persisted at the Mainstem site and in subtidal channels of the Restoration site through the summer in water temperatures exceeding 22 °C. Residence times of subyearlings in Crims Island backwaters generally were short consisting of one or two tidal cycles. Median residence time was longer in the Restoration site than in the Reference site pre- and post-restoration.

Small (mean = 55.7 millimeters) subyearlings primarily consumed dipteran adults and larvae in backwater habitats, while large (mean = 60.0 millimeters) subyearlings consumed Daphnia and Corophium in nearshore mainstem habitats. At all sites, chironomid larvae, Corophium, and oligochaetes were dominant in the benthic invertebrate community, whereas chironomid adults and aphids were dominant in the drift invertebrate community. Based on feeding indexes, subyearlings fed more intensively in the Reference site than in the Restoration and Mainstem sites prior to restoration. However, post-restoration, subyearlings fed more intensively in the Restoration site than in the Reference site and their diet was more similar to that of fish found in the Reference site. Although invertebrate density at the Restoration site did not change significantly post-restoration, invertebrate diversity initially decreased before returning to pre-restoration levels. However, the overall abundance of chironomids actually increased in the Restoration site post-restoration. Although juvenile salmon can feed preferentially, our results indicated that subyearlings fed most intensively on chironomids in backwater habitats and insect diversity, although important to overall ecosystem function, did not appear to limit their abundance.

Although we were unable to estimate salmon abundance pre-restoration, a 95-percent increase in available habitat coupled with the large numbers of subyearlings with high condition factors collected post-restoration indicate that the project was largely a success in creating suitable rearing habitat for subyearlings. Catch data indicated that more subyearlings per hour were accessing restored habitat compared to unrestored habitat. We estimated total subyearling numbers ranging from 11,000 to 13,000 in the Restoration site post-restoration. Our before-after-control-impact paired series (BACIPS) study design permitted data analysis using t-tests to evaluate response to restoration and indicated that restored habitat contained larger subyearlings than unrestored habitat. Future monitoring efforts could benefit from sampling additional reference sites and evaluating higher-order metrics, such as survival or growth, to gauge restoration success.

Further creation of shallow water rearing habitat will likely benefit subyearlings compared to other juvenile salmon life histories because subyearlings tend to migrate seaward using productive shallow backwaters instead of mainstem habitats more so than yearling Chinook salmon and large subyearlings. Much research in the lower Columbia River is directed toward the survival of large subyearlings using mainstem habitats. Estimating survival and growth of small subyearlings using restored habitats can help in planning and implementing off-channel restoration projects in the future.

First posted February 23, 2007