as continuous. Environmental stress on the fish of the Great Lakes has 

 been of five general types: 



(1) Physical stress resulting from modification of the watershed 

 by deforestation, blockage of tributaries, and drainage of 

 marshes. This primarily affected anadromous species and be- 

 gan during the period of low human population density. 



(2) Biological stress caused by the introduction and colonization 

 of exotic species. Introduction of new species began before 

 1900 and continues today. 



(3) Chemical stress (first phase) in the form of oxygen-consuming 

 organic material dumped into tributaries and bays, and 

 increased plant nutrients in the inshore areas. 



(4) Chemical stress (second phase) caused by toxic chemicals such 

 as chlorinated hydrocarbons and heavy metals. 



(5) Thermal stress - more a future concern than a present concern. 



The direct effects of the environmental changes on the fish popula- 

 tions are seldom observed and perhaps rarely occur. Indirect effects of 

 these changes are often cited, but only occasionally quantified. In lakes 

 as large as the Great Lakes, cause and effect are separated in time and 

 distance to an extent that only after an event can the two be linked. This 

 is the situation with the continuous change in the abundance of Great Lakes 

 fish species. 



We all recognize the fact that the stresses, be they exploitation, des- 

 truction of spawning grounds, oxygen depletion, increased water temperature, 

 change in available food, or competition with introduced species, cannot 

 often be isolated and analyzed separately. This paper is a summary of the 

 changes that have occurred in the fish communities of the Laurentian Great 

 Lakes from the early 19th century to the present. The changing composition 

 of fish populations in the Great Lakes has been the subject in recent years 

 of many articles in scientific and popular publications. The most exhaus- 

 tive discussion of these changes occurred in a recent (1971) international 

 symposium on "Salmonid Communities in Oligotrophy Lakes" (SCOL). These 

 papers were published as a special issue of the Journal of Fisheries Re - 

 search Board of Canada in 1972. This publication contains seven papers ex- 

 clusively on the Great Lakes, including case histories of each of the five 

 Laurentian Great Lakes: Superior, Huron, Michigan, Erie, and Ontario 

 (Figure 1). Other comprehensive papers on changes in Great Lakes fish 

 species are by Smith (1964, 1958) and Christie (1974). 



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