Federal scientists have placed underwater microphones in the rivers near the Jersey Shore to record sounds that might be scaring the dolphins from heading back out to sea.
The first round of data they're analyzing was removed from the devices last week.
The dolphins passed underneath the Route 36 bridge between Sea Bright and Highlands on their way into the river earlier this summer.
They must go back out the same way in order to reach Sandy Hook Bay and the ocean.
Scientists are looking at whether noise from construction on the bridge might be making the dolphins hesitant to pass by it.