Using Lake Trout as a Surrogate of Ecosystem Health for Oligotrophic Waters of the Great Lakes

Abstract

The Great Lakes Water Quality Agreement (1978) between Canada and the United States provided for the accommodation of ecosystem objectives for the Great Lakes basin ecosystem. Accordingly, the two parties to the agreement, under the aegis of the International Joint Commission, acted in concert to define ecosystem objectives for the Great Lakes, and to provide a vehicle for practical application of these objectives. For the oligotrophic portions of the Great Lakes, the lake trout was selected as an exemplary organism for the detection of well-being or “health” of the system. The suitability of the lake trout derived from the fact that it occupied a sensitive, integrative node at the top trophic level of the system. There, in its role as a terminal predator, the lake trout acted as a major controlling factor over the remainder of the cold-water community. The lake trout, as the organism of choice, best satisfied a comprehensive set of qualifying criteria for indicator organisms. A secondary organism, Pontoporeia hoyi, was selected as complementary to the lake trout as it was indicative of oligotrophic system quality at another sensitive, integrative node, namely the mud-water interface of the demersal zone. To provide a practical means for management application of the lake trout as an oligotrophic ecosystem surrogate, a Dichotomous Key was devised and subsequently upgraded to a menu-driven, computer-based expert system. This system was designed explicitly for ecosystem managers to rapidly detect the relative state of health of an oligotrophic ecosystem. It focused specifically on questions dealing with the niche characteristics and habitat requirements of the lake trout. An optional printed output not only described the relative state of the system, but in addition, underlined the areas of greatest degradation and pointed out the current information needs. Preliminary testing of the Dichotomous Key on each of the Great Lakes has shown its value as a first logical and necessary step in the process of system rehabilitation.