pest. Sea Grant's zebra mussel research has been conducted in six areas: (1) biology and life history, (2) effects on ecosystems, (3) socioeconomic analyses, (4) control and mitigation, (5) prevention of new introductions, and (6) reduction of spread. Limnologists at academic institutions have been involved in a wide range of research efforts within these categories. For example, limnologists have found Dreissenia polymorpha to be genetically highly diverse; this may allow it to spread and adapt to new environments. Laboratory-reared larvae have been observed to postpone settlement and attachment and to prolong their free-swimming period for an additional seven weeks beyond their normal time in this life form. These delays could have profound effects on the dispersal characteristics of zebra mussels. Although it is not possible to eliminate zebra mussels from water bodies where they have become established, limnologists are studying a variety of physical, chemical, and biological control measures. These include the use of ultraviolet radiation to kill larval forms of zebra mussels, application of high-voltage fields on water intake pipes, use of various chemicals to kill adults or prevent larval attachment, and isolation of bacteria that inhibit larval attachment or cause disease in zebra mussels.

Another invader of the Great Lakes from Europe via oceangoing vessels is Bythotrephes cederstroemi, a large crustacean zooplankton. This species is less conspicuous to the public eye than the zebra mussel but is nevertheless capable of causing significant economic damage. A voracious predator of other zooplankton, Bythotrephes was first observed in Lake Huron in December 1984, detected soon after that in Lake Erie, and present in all of the Great Lakes by 1988. It spread rapidly throughout Lake Michigan after its first appearance in the north end in 1986, and it has moved into lakes connected to the Georgian Bay of Lake Huron and into northern Minnesota, which is outside the drainage basin of the Great Lakes. Several groups of limnologists are investigating the role of Bythotrephes in the Great Lakes ecosystem (Lehman, 1991).

Changes in the zooplankton population of Lake Michigan immediately followed the increase of Bythotrephes. The abundance of Daphnia decreased sharply, with changes occurring in body size that would be expected from selective predation by Bythotrephes. Daphnia is an important food for several species of game fish, so the change may threaten the vitality of important fish populations. Bythotrephes is a poor food for small game fish because it has a stiff tail spine, a centimeter long in the largest individuals. However, it is eaten freely by the alewife (Alosa pseudoherengus), an undesirable exotic fish species in the Great Lakes. Since the production of a full-grown Bythotrephes involves the consumption of many Daphnia, the net effect on the production of game fish could be negative. Apart from its indirect effect on fisheries, Bythotrephes is a direct nuisance to net fisherman. Its long tail spines stick to nylon nets, and animals accumulate on nets in slimy masses, making the nets difficult and unpleasant to



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