U.S. Dept Commerce/NOAA/NMFS/NWFSC/PublicationsNOAA       Technical Memorandum NMFS-NWFSC-39       
Biological Surveys of the Trestle Bay       Enhancement Project 1994, 1996-97       

Susan HintonNorthwest Fisheries Science       CenterFish Ecology DivisionPoint Adams Biological Field       StationP.O. Box 155, Hammond, OR 97121 

Robert L. EmmettNorthwest Fisheries Science CenterFish Ecology       DivisionHatfield Marine Science Center2030 Marine Science Drive,       Newport, OR 97365 

Funded byU.S. Army Corps of EngineersPortland DistrictP.O.       Box 2946, Portland, OR 97208Contract E96960026January       2000U.S. DEPARTMENT OF       COMMERCEWilliam Daley, SecretaryNational Oceanic and Atmospheric       AdministrationD. James Baker,       AdministratorNational Marine       Fisheries ServicePenelope D. Dalton, Assistant       Administrator for Fisheries 
NOAA-NWFSC Tech Memo-39: Biological Surveys of the Trestle Bay Enhancement Project 1994, 1996-97 NOAA Technical Memorandum NMFS Series 
The Northwest Fisheries Science Center of the National Marine Fisheries Service, NOAA, uses the NOAA Technical Memorandum NMFS series to issue informal scientific and technical publications when complete formal review and editorial processing are not appropriate or feasible due to time constraints. Documents published in this series may be referenced in the scientific and technical literature.

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Reference throughout this document to trade names does not imply endorsement by the National Marine Fisheries Service, NOAA.

This document should be cited as follows:

Hinton, S. and R.L. Emmett. 2000. Biological surveysof the Trestle Bay enhancement project 1994, 1996-97.U.S. Dept. Commer., NOAA Tech. Memo. NMFS-NWFSC-39, 72 p.

Most NOAA Technical Memorandums NMFS-NWFSC areavailable on-line at the Northwest Fisheries Science Centerweb site (http://www.nwfsc.noaa.gov)

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National Technical Information ServiceU.S. Department of Commerce5285 Port Royal RoadSpringfield, VA 221611-800-553-8647orders@ntis.fedworld.gov

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EXECUTIVE SUMMARY
Trestle Bay is 11.3 RKm upstream from the mouth of the Columbia River in Oregon. The 244-ha bay consists of shallow subtidal and intertidal mudflats and intertidal marsh habitats. Trestle Bay formed as a result of the initial construction of the Columbia River south jetty which took place from 1885 to 1895. Sand accumulation around the jetty created the bay, which is surrounded by land on the west, south, and east sides and separated from the mainstem of the Columbia River by a 2,682-m rock jetty along the north side. Although the rock jetty is permeable to water, it has likely discouraged resident and migratory juvenile fishes (e.g., salmonids) from entering the bay. 

In August 1995, the jetty was modified by removing a 152-m section of rock, creating an unobstructed passageway between the bay and the lower Columbia River estuary. This allowed better flushing and made Trestle Bay available as foraging habitat for many marine and estuarine fishes and shellfish, and ultimately reestablished the bay as an integral part of the lower Columbia River estuarine ecosystem. In 1994, the U.S. Army Corps of Engineers contracted with the National Marine Fisheries Service to conduct pre- (1994) and post-breach (1996-97) surveys in and adjacent to Trestle Bay, to determine any physical or biological changes caused by the jetty breach.

Resource agencies were particularly interested in the use of the bay by juvenile salmonid species and changes in their benthic and epibenthic invertebrate prey after the breach. This is a concern because the Columbia River is critical habitat for several endangered salmonid species. Biological sampling was conducted to assess benthic and epibenthic invertebrate populations, fish species compositions, and sediment characteristics before and after the jetty was breached. Sampling was also conducted outside and adjacent to Trestle Bay to provide reference information. 

Estuaries and associated wetlands provide critical habitat for many marine and estuarine fish and shellfish species. The primary producers (algae and eelgrass) within these areas provide extensive cover and foraging habitat. Prior to the jetty breach, biological surveys within Trestle Bay found very high densities of benthic and epibenthic invertebrate species that are prey for many fishes that rear in the Columbia River estuary, yet few of those fishes were found inside the bay. In particular, no juvenile salmonids were captured in the bay during the pre-breach sampling period.

The effects of jetty modification were most apparent in the benthic invertebrate and fish populations in Trestle Bay, although the effects were difficult to discern because of confounding interactions. For benthic invertebrates, the decline in pre-breach to post-breach density inside the bay was more than twice that of the decline outside the bay, indicating a causal factor other than normal natural variation. Possibilities include either increased foraging activities by fishes after the jetty breach, or there was a physical disruption of the fauna related to a sustained freshwater flood that reduced salinities. For fishes, more species were common between the two sampling areas (in Trestle Bay and in the reference area) during post-breach surveys than during pre-breach surveys. This indicates that the jetty acted as a barrier, ultimately affecting the ability or willingness of some fish species to enter and utilize the Bay as foraging habitat. Juvenile salmonids, for example, were captured inside Trestle Bay only in surveys conducted after the jetty was breached. Fish densities were also more similar between the two sampling areas after the jetty was breached.

Although epibenthic invertebrate densities declined from 1994 to 1997, the breach did not appear to affect them significantly. Similarly, sediment characteristics remained essentially unchanged between the pre- and post-breach surveys.

The Trestle Bay project provided a rare opportunity to return ecologically functioning estuarine marshes, intertidal, and shallow subtidal habitats back to an estuary. The primary goal of restoring fish access to the bay, particularly for salmonids which could include federally listed endangered and threatened salmonid species, appears to have been met. Resource agencies should continue to look for similar project sites that could be modified at minimum cost, yet have long-term benefits for associated ecosystems.

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