Beaver in Tidal Marshes: Dam Effects on Low-Tide Channel Pools and Fish Use of Estuarine Habitat

Abstract

Beaver (Castor spp.) are considered a riverine or lacustrine animal, but surveys of tidal channels in the Skagit Delta (Washington, USA) found beaver dams and lodges in the tidal shrub zone at densities equal or greater than in non-tidal rivers. Dams were typically flooded by a meter or more during high tide, but at low tide they impounded water, allowing beaver to swim freely while quadrupling pool habitat for fish compared to channels without dams. Seven fish species were caught in low-tide pools, including threatened juvenile Chinook salmon (Oncorhynchus tshawytscha), whose densities (by volume) averaged 3.2 times higher in low-tide pools than shallows. Accounting for the total contribution of pools and shallows to juvenile Chinook abundance, beaver pools tripled shrub zone channel capacity for juvenile Chinook salmon at low tide relative to herbaceous zone marsh without beaver pools. Current Chinook recovery efforts focus on restoring herbaceous zone tidal marsh for rearing juveniles, but this focus overlooks presently rare and poorly understood habitat, like tidal shrub marsh, that was historically common and likely important to beaver and small estuarine or anadromous fish.