Asian clam Mass Die-off

On July 24, 2025, the City of Lake Stevens observed a significant die-off of invasive Asian clams (Corbicula fluminea) along the lake’s shoreline, prompting an immediate investigation. City staff collected observational reports, reviewed water quality data, and examined whether an herbicide treatment conducted on July 22 could have contributed to the incident. The analysis revealed that the die-off was most likely triggered by elevated water temperatures, as no abnormalities in herbicide use or application methods were found. Scientific studies support that Asian clams experience thermal stress at temperatures above 77°F (25°C) and may suffer mortality above 86°F (30°C), making them vulnerable during periods of sustained heat (McMahon, 2002; Cherry et al., 2005).

Asian clams are an invasive species known for rapid reproduction and altering nutrient cycles in freshwater ecosystems. When large populations die off, they release organic matter that can contribute to nutrient spikes, algal blooms, and water quality issues. The lake had reached nearly 80°F on July 17—one week prior to the die-off—coinciding with a known thermal threshold for clam stress. While aquatic herbicides such as Diquat, Aquathol, ProcellaCOR, and Galleon were applied in compliance with EPA guidelines, studies confirm they do not cause acute toxicity to mollusks like Corbicula fluminea at approved concentrations (Madsen et al., 2004; Sprecher & Getsinger, 2000).

Events like this have been reported across North America and Europe in similar warm, low-oxygen conditions, including the Flint and Choctawhatchee Rivers, Lake George, and the Ebro River Basin. These die-offs are becoming more common as climate change increases air and water temperatures. The City continues monthly monitoring of lake nutrients and algae and encourages the public to report any unusual algae blooms or odors. Residents can help protect the lake by reducing fertilizer use, planting native vegetation, and supporting local climate resilience efforts.