Because of this, boaters are being encouraged not to take their boats from infested waters to the St. Croix because of the endangered populations of native mussels inhabiting the river. Despite this effort, approximately 40 juveniles were found last month on a plate sampler by the National Park Service above the site of the endangered mussel bed. In the Illinois River, the number of adults was low, but veliger numbers were up relative to last year. However, zebra mussels can still be found in the lower Mississippi River and also in the Atchafalaya River basin. The annual cycle of warm summer temperatures in the river appears to be too high for successful reproduction and long-term survival.
Recruitment probably comes from reproduction upriver or reintroduction by barges.
A record spawning event took place in Lake Dardanelle on the Arkansas River this Spring according to one biologist at Arkansas Tech University. Water clarity has increased giving way to the growth of rooted submergent macrophytes not normally present. The U.S. Fish and Wildlife Service found a 10-30 fold increase late in 1996 over the previous year in the densities of zebra mussels in the Ohio River. Although they have not begun this Summer's monitoring, preliminary observations have not revealed any recruitment so far for 1997. In the upper Ohio Valley of Pennsylvania, the Corps of Engineers reported new locations for zebra mussels.