The Lower Elwha Klallam Tribe, U.S. Geological Survey (USGS) and Washington Sea Grant are observing an increase of forage fish and Dungeness crab near the mouth of the Elwha River since the river’s two dams have been demolished.
Divers have noted continuous sand deposits in the Elwha nearshore, covering formerly cobble-dominated sub-tidal areas. This has resulted in the habitat shifting from a rocky bottom and kelp-dominated habitat to a soft-bottomed habitat suitable for clams, crabs, and other species.
“We first documented sand lance near the mouth of the river in 2012 after the dams started to come down in 2011,” said Matt Beirne, the tribe’s environmental coordinator and diver. Juvenile crab were also first seen in the new sand habitat just off the river mouth in 2013 during a dive survey.
Sand lance form schools in open water when feeding but also spend time buried in sand. Divers observed sand lance zipping in and out of the new sand near the river mouth, confirming the value of this habitat for sand lance.
“This is important because sand lance and other forage fish are such a critical part of the diet of adult salmon and other marine species,” Beirne said.
Researchers from National Oceanic and Atmospheric Administration have observed larger numbers of bull trout in the Elwha nearshore as well, which are possibly feeding on sand lance, smelt and other forage fish, Beirne added.
“We expected organisms to move in and take advantage of this new habitat, but it’s actually observing it happen and having the opportunity to investigate the details, such as tracking which critters move in and how quickly they show up, that makes it interesting,” said Ian Miller, coastal scientist with Washington Sea Grant.
In areas where the bottom is still rocky, divers noted a decline in kelp, possibly because the amount of light reaching the seafloor has been reduced by fine sediments that washed out of the river, said Steve Rubin, a USGS fishery biologist.
Even so, Rubin said, kelp have had some success reproducing as divers have noticed young kelp in late summer for the past two years and conditions may improve for kelp in the future as sediments finish clearing out from the former Aldwell and Mills reservoirs in the Elwha watershed.
“We expected that in the first few years following dam removal, the large influx of sediments would stress some organisms,” Rubin said. “But the hope is that the sediment will settle in the watershed in a way similar to pre-dam construction, and so will a diverse community of species inhabiting both sandy and rocky habitats.”