What Are Aquatic Nuisance Species and Why Are They a Threat?

Aquatic Nuisance Species (ANS) are organisms that cause harmful impacts on aquatic ecosystems or on activities that depend on those ecosystems, such as agriculture, aquaculture, and recreation. According to the U.S. Fish & Wildlife Service (fws.gov), ANS — also called Aquatic Invasive Species — can occur in freshwater, estuarine, and marine waters, and are considered one of the most significant threats to the waters of the United States.

What Counts as an Aquatic Nuisance Species

The term covers any organism — plant, animal, or pathogen — that is introduced or spreads beyond its natural range and produces harm in a water environment. Two features define the category:

• Harmful impact: The organism damages aquatic ecosystems, or the human activities that rely on them.
• Aquatic setting: It lives or spreads in freshwater, estuarine, or marine waters.

Because the definition is based on impact rather than species type, ANS include a wide range of organisms rather than a single group.

Why ANS Are Considered a Threat

The Service frames ANS as one of the most significant threats to U.S. waters. The harm works through two connected channels:

1. Impacts on aquatic ecosystems

When an invasive organism establishes itself, it can disrupt the natural balance of a waterbody — competing with or displacing native species and altering habitat and food webs.

2. Impacts on dependent activities

The Service specifically names activities that depend on aquatic ecosystems:

This dual framing matters: ANS are not only an ecological concern but also an economic and practical one for people who use and depend on U.S. waters.

Where ANS Occur

ANS are not limited to one type of water. The Service notes they may occur in:

• Freshwater — lakes, rivers, and streams
• Estuarine — coastal waters where rivers meet the sea
• Marine — ocean environments

That breadth is part of why they are treated as a national-scale threat rather than a local problem.

Key Takeaways

• ANS = Aquatic Nuisance Species, also known as Aquatic Invasive Species.
• They are defined by the harm they cause to aquatic ecosystems or to activities dependent on them, including agriculture, aquaculture, and recreation.
• They occur in freshwater, estuarine, and marine waters.
• The U.S. Fish & Wildlife Service identifies them as one of the most significant threats to the waters of the United States.

For the Service's own framing and related programs, see the Aquatic Nuisance Species Task Force pages on fws.gov.