Dave it sounds like they are spawning there or in a nearby river. Those size fish are only a year or two old and generally dont migrate until 3 years old.
It takes 37 miles of river to be a viable spawning site with a current speed of 1 foot per second.
According to Bulak (1988), if a striped bass spawned at 70 degrees F in a low rainfall year, approximately 36 miles of flowing water is necessary for the eggs to hatch. But if a striper spawned at the same temperature during a high rainfall period, about 72 miles of flowing water from the spawning area is required. A flow velocity in the river of approximately one-foot per second is required to keep the eggs afloat.