PART VIII — AQUATIC ECOSYSTEMS 



failed. Even though a salmon fishery 

 no longer existed and the problems of 

 mill dams and pollution had been 

 eliminated in most streams, the lower 

 flows and warmer waters continued, 

 indicating that removal of timber 

 and agricultural development within 

 the drainage had created conditions 

 that made the region unsuitable for 

 survival of the Atlantic salmon. 



Effects of Marine Invaders — Elim- 

 ination of the Atlantic salmon, which 

 was the major fish predator of Lake 

 Ontario, created conditions favorable 

 for the entrance of the alewife, 

 which was the first and most de- 

 structive marine invader. As the sal- 

 mon were declining, the alewife was 

 entering the Lake Ontario drainage 

 via the Hudson River and the Erie 

 Canal. By 1868, alewives had be- 

 come abundant in the Canal and in 

 the Finger Lakes, which drain into 

 Lake Ontario. Large schools of small 

 alewives were reported in Lake On- 

 tario in 1873 — by which time the 

 Atlantic salmon had been reduced 

 greatly. The lake trout was another 

 major fish predator in Lake Ontario, 

 but it, too, was declining during this 

 period, possibly due to heavy ex- 

 ploitation. 



More recent experience in the other 

 Great Lakes has shown that the small, 

 landlocked alewives are unable to 

 thrive when any of the Great Lakes 

 is densely populated by larger fish 

 predators. Without predators, how- 

 ever, the alewife in Lake Ontario 

 was able to increase rapidly; it had 

 become the most abundant fish by 

 1880. Furthermore, by 1900, the ale- 

 wife had greatly reduced or virtually 

 eliminated all of the previously abun- 

 dant small species of Lake Ontario 

 that depended on plankton during 

 at least part of their lives; as past 

 studies have shown, the alewife has 

 a strong competitive advantage over 

 native freshwater fish that also feed 

 on zooplankton. 



The alewife used the lake much 

 less efficiently than native species, 

 causing a reduction in the total 



amount of fish in the lake. The 

 previously abundant native species 

 had occupied all zones of the lake 

 during the entire year. In contrast, 

 the alewife ranged throughout the 

 lake in dense schools but occupied 

 different portions of the lake in vari- 

 ous seasons; under its dominance, 

 the vast deep-water region repre- 

 senting 70 to 80 percent of the area 

 of the lake was unoccupied by other 

 species during most of the year. 



The parasitic sea lamprey was the 

 second marine invader of the Great 

 Lakes. It had free access to Lake 

 Ontario via the St. Lawrence River 

 but it did not become established in 

 Lake Ontario until the 1880's. Con- 

 ditions that made the lake unfavor- 

 able for the Atlantic salmon appar- 

 ently made it suitable for the sea 

 lamprey. The inland ranges of the 

 two species do not overlap — the 

 Atlantic salmon favors drainages that 

 have durable, cool streams suitable 

 for its fall spawning, while the 

 spring-spawning sea lamprey favors 

 streams that become warm following 

 the spring runoff. 



Once the Atlantic salmon, which 

 fed on small fish, was eliminated, 

 the sea lamprey, which feeds on 

 large fish, became the dominant pred- 

 ator; once established in Lake On- 

 tario, it prevented any sustained re- 

 surgence of lake trout, whitefish, or 

 larger species of deep-water ciscoes 

 (commonly called chubs by fisher- 

 men of the Great Lakes). Thus, the 

 combined effect of the invasions of 

 the alewife and the sea lamprey was 

 to reduce drastically the fishery pro- 

 ductivity of Lake Ontario. 



If it were not for the Welland 

 Canal, which provides a waterway 

 that bypasses Niagara Falls and al- 

 lows access to the upper lakes, the 

 destructiveness of these marine in- 

 vaders would have been limited to 

 Lake Ontario. Both the sea lamprey 

 and the alewife were able to nego- 

 tiate the Welland Canal, however. 

 The sea lamprey reached Lake Erie 

 by 1921, was established in Lake 



Huron in 1932, Lake Michigan in 

 1936, and Lake Superior in 1946. 

 The alewife first appeared in Lake 

 Erie in 1931, Lake Huron in 1933, 

 Lake Michigan in 1949, and Lake 

 Superior in 1954. 



Neither the sea lamprey nor the 

 alewife became a severe problem in 

 Lake Erie, which had few suitable 

 spawning streams for the lamprey 

 and had substantial populations of 

 predators to keep alewife abundance 

 low. 



Conditions in Lakes Huron, Mich- 

 igan, and Superior favored the lam- 

 prey and alewives, however, and 

 these lakes were to suffer fates sim- 

 ilar to that of Lake Ontario. The 

 influences of the lamprey and alewife 

 occurred in the reverse order. As the 

 lamprey became established in each 

 of the upper lakes, it destroyed the 

 lake trout that was the major fish 

 predator of the upper Great Lakes. 

 Loss of the lake trout was followed 

 by establishment and rapid increase 

 of the alewife population. In Lakes 

 Michigan and Huron, the destruc- 

 tion of large fish by the sea lamprey 

 and small fish by the alewife became 

 as severe as in Lake Ontario. 



Development of a chemical method 

 of sea-lamprey control was first ap- 

 plied in Lake Superior and prevented 

 the complete collapse of the lake- 

 trout population that had occurred 

 in Lakes Michigan and Huron. Al- 

 though present control methods have 

 not been sufficient to permit restora- 

 tion of significant spawning stocks 

 of lake trout in Lake Superior, sus- 

 tained introductions of hatchery- 

 reared trout have held the alewife 

 in check. 



The status of Lake Superior re- 

 mains uncertain, however, as sea lam- 

 preys are thriving; the sparser popu- 

 lations resulting from the control 

 measures have enabled the lamprey 

 to reproduce more prolifically than 

 ever. The remnant lamprey pop- 

 ulations pose a serious threat to 

 rehabilitation of a reproducing popu- 



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