Man's activities have caused great changes in Lake Michigan in the past 120 years. Although changes in water chemistry and lower biota have been generally modest (except locally), those in native fish stocks have been vast. Exploitation, exotic fish species, and eutrophication and other forms of pollution all have played a role in bringing about the changes (mostly declines in abundance) in fish populations. Exploitation resulted in a noticeable reduction in abundance of certain native species (especially whitefish) soon after the establishment of the commercial fishery in the 1840's. By the 1930's the sturgeon and the two largest deepwater ciscoes (Coregonus nigripinnis and C. johannae) became severely depleted. Other species- whitefish (Coregonus clupeaformis), lake trout (Salvelinus namaycush), and lake herring (C. artedii)- remained important commercially, but at a lower level of production than originally; greatly increased fishing effort and efficiency were required to maintain even these decreased catches. The catch of intermediate-size ciscoes held relatively stable, but again only through sharply increased fishing effort and efficiency. The earliest serious effects of exotic fish species on native fish stocks may have been during the 1930's, when smelt (Osmerus mordax), first became abundant. Powerful influences by exotics were not obvious, however, until the 1940's, when the sea lamprey's (Petromyzon marinus) predation on several species, particularly the lake trout, became critical. In the 1950's the sea lamprey was joined by the alewife (Alosa pseudoharengus), another exotic strongly deleterious to several native fish. The alewife apparently inhibited reproduction of deepwater ciscoes, yellow perch (Perca flavescens), deepwater sculpins (Myoxocephalus quadricornis), emerald shiners (Notropis atherinoides), and perhaps others (through competing with young, or feeding on them). At the same time, however, the alewife as a prolific forage fish has made possible the highly successful introduction of several species of salmonines. The effects of accelerated eutrophication and other pollution, although not always as easy to identify as the influences of other factors, were nevertheless clearly important as early as the mid-1800's. The first conspicuous contamination of Lake Michigan was by sawmill wastes, which covered spawning grounds in streams and around stream mouths. This type of pollution was particularly destructive to whitefish. Other forms of stream degradation (e.g., dams, deforestation of watersheds) although not strictly "pollution," must also have been detrimental to stream spawners. Heavy pollution in southern Green Bay (a large area of the bottom of which is now covered with anoxic gray sludge) probably has resulted in reduction in abundance of several species, e.g., lake herring and walleye (Stizostedion v. vitreum).
Exploitation was largely responsible for the changes in Lake Michigan fish stocks before the invasion of the smelt, and probably before the invasion of the sea lamprey. The lamprey and alewife, however, have exerted a greater impact than the fishery on native fish populations in recent decades. Accelerated eutrophication and other pollution, although important, have not equalled the other factors in causing changes in native fish populations.