Which of the following is a common aquatic pest in Washington State?

Eurasian watermilfoil is recognized as a common aquatic pest in Washington State due to its aggressive growth and ability to quickly dominate water bodies. This invasive species can disrupt local ecosystems by outcompeting native aquatic plants, altering habitat for fish and other wildlife, and impeding recreational activities such as boating and fishing. The growth patterns of Eurasian watermilfoil can lead to decreased water quality and can create dense mats on the water surface, which hinder light penetration and affect oxygen levels.

In contrast, the other options represent native or non-invasive species that do not have the same level of impact on aquatic ecosystems in Washington State. Common cattail, while prevalent in wetlands, is typically not considered a pest, as it can play a role in stabilizing shorelines and providing habitat. Bluegill sunfish are a native fish species that contributes to the biodiversity of aquatic systems, rather than being classified as a pest. Horseweed, primarily found in terrestrial areas, is not an aquatic plant and does not have the same implications for water bodies as Eurasian watermilfoil. Therefore, identifying Eurasian watermilfoil as a common aquatic pest is crucial for effective aquatic pest management strategies in Washington State.