Vertebrate response to a tidal marsh restoration in Humboldt Bay, California

A 5.5-ha abandoned log pond (originally salt marsh) at the end of Park Street, Eureka, California was chosen as the off-site mitigation area for the destruction of 6.8 ha of wildlife habitat during the construction of the Woodley Island marina. Passive saltmarsh restoration was attempted by breaching a dike separating the log pond from Freshwater Slough (an estuary of Humboldt Bay) in December 1980, thus allowing the periodic tidal intrusion of salt water into 3.8 ha of the area. This study measured the response of amphibians, reptiles, birds, and mammals to the change in vegetation and other habitat conditions brought about by the reestablishment of tidal action. Previously, in August 1979 an interior dike had been constructed within a portion of the mitigation area to enhance an existing 0.7-ha freshwater marsh, and the effect of this change on the vertebrate life also was appraised. Frogs disappeared and snakes declined in numbers within the tidal portion of the mitigation area after the dike was breached. Savannah Sparrows (Passerculus sandwichensis) were the most common birds in the grassland of the dried log pond before breaching, and declined after breaching. Several species of shorebirds and the Snowy Egret (Egretta thula) increased their use of the tidal portion of the mitigation area after breaching. Small mammals such as California Vole (Microtus californicus), Vagrant Shrew (Sorex vagrans), and Western Harvest Mouse (Reithrodontomys megalotis) were common before breaching, but declined in numbers in the first month after breaching. Use of the area by the most common large mammal, the Domestic Dog (Canis familiaris), increased after breaching because people used the flooded marsh to train retrievers. Within the freshwater marsh five species of birds showed significant increases in numbers during subsequent corresponding seasons, whereas one species showed a significant decrease in numbers. The success of the restoration after breaching was not fully known at the end of 1.5 years of study, but the trend was towards a renewed, vigorous salt marsh. In addition, the interior dike construction enhanced the freshwater marsh. In the interim the tidal and freshwater marshes provided valuable foraging areas for many species of wetland birds, and the freshwater marsh also served as a nesting area for certain bird species.