Fish can suddenly die in Hilton Head ponds and lagoons. Here are the likely reasons why

Have you seen or smelt dead fish on Hilton Head Island?

You may have witnessed a fish kill, the dying of a pond or lagoon’s fish. And it’s not the work of a maniacal fisherman. They occur naturally when conditions become harmful for fish species.

Here’s what you need to know about fish kills and why they occur:

What are the causes?

Fish kills happen when a pond or lagoon’s environment changes due to a variety of factors. Some fish species can survive these conditions while others can’t. Here are some common causes:

Chemicals

• Chemicals enter waters through rainfall.
• Application of chemicals near or in a pond or lagoon.

Low dissolved oxygen

• Fish suffocate in water with low dissolved oxygen levels.
• Rapid changes in temperature can cause turnover. This is when a pond or lagoon’s water layers mix. This decreases the dissolved oxygen in the pond. Turnovers are most common in fall and spring.
• Heavy rainfall can also decrease dissolved oxygen levels.

Algal blooms

• Algal blooms steal oxygen from fish. They can also clog their gills.

Salt Water

• Abnormal high tides can leak saltwater into freshwater ponds, killing freshwater-only fish species.

Hilton Head’s ponds and lagoons

The majority of ponds and lagoons on Hilton Head Island are man-made. These systems, such as the one at Palmetto Dunes, were created to prevent erosion, and to collect stormwater.

Freshwater stormwater ponds are the most susceptible to fish kills. They collect both water runoff and the contaminants that mix in. They are also prone to stagnation and collect run-off fertilizer, creating a perfect environment for algal blooms that harm fish.

Hilton Head’s lagoons contain brackish water, a mix of salt and fresh water. Lagoons connect to the ocean, allowing fish to enter and leave. Ponds are often isolated, preventing fish from seeking survivable water.

Most species living in brackish water can survive in both fresh and saltwater. Pond fish only survive in freshwater. Fish kills still occur in brackish lagoons.

What you can do

If you see a large fish kill, report it to South Carolina’s Department of Natural Resources by calling 1-800-922-5431. And check here for details you can provide DNR to assist them.

Fish can be saved. During a fish kill at Shelter Cove Community Park in May 2025, Town of Hilton Head staff relocated fish to safer ponds. Pond aerators were also used to boost dissolved oxygen levels.