Texas Parks and Wildlife Department officials say the fish -- identified as shad -- are dying due to a lack of oxygen levels in the water.
This type of fish is particularly sensitive to varying oxygen levels. As the temperature rises, oxygen levels drop.
More fish are expected to wash ashore, though it's unknown how many.
For residents and visitors to the island, the smell was the worst part.
"It smells really fishy out here today," said Amanda Guirguis, who was vacationing from Dallas. "It grossed me out. I didn't want to be in the water."
According to officials, there is no danger to the public. The water was tested and it is safe to get in, they said.
Cleanup was expected to begin around 3am Monday along the Galveston seawall.
Stay with Eyewitness News and abc13.com for updates on their progress.