Three of the five dolphins returned to the Shrewsbury River after they were spotted Saturday in Sandy Hook Bay.
Bob Schoelkopf, co-director of the Marine Mammal Stranding Center in Brigantine, says it appears they were frightened by noise caused by construction work being done on the bridge.
Four dolphins then tried to cross yesterday but turned back, although it's not clear why.
Three dolphins had been confirmed dead out of the original group of 16 that spent the summer and fall in the Shrewsbury and Navesink rivers. Authorities don't know what happened to the others.