Lake Superior is the largest of the Great Lakes of North America, and among freshwater lakes, it is the world's largest by surface area and the third-largest by volume. It holds 10% of the world's surface fresh water. It is bordered by Ontario, Canada, to the north, Minnesota to the west, and Wisconsin and the Upper Peninsula of Michigan to the south. Lake Superior is the most northerly and most westerly of the Great Lakes chain, and the highest in elevation. It drains into Lake Huron via St. Marys River.
More than 80 species of fish have been found in Lake Superior. Species native to the lake include banded killifish, bloater, brook trout, burbot, cisco, lake sturgeon, lake trout, lake whitefish, longnose sucker, muskellunge, northern pike, pumpkinseed, rock bass, round whitefish, smallmouth bass, walleye, white sucker and yellow perch. In addition, many fish species have been either intentionally or accidentally introduced to Lake Superior: Atlantic salmon, brown trout, carp, chinook salmon, coho salmon, freshwater drum, pink salmon, rainbow smelt, rainbow trout, round goby, ruffe, sea lamprey and white perch.
Lake Superior has fewer dissolved nutrients relative to its water volume than the other Great Lakes and so is less productive in terms of fish populations and is an oligotrophic lake. This is a result of the underdeveloped soils found in its relatively small watershed. It is also a reflection of relatively small human population and small amount of agriculture in its watershed. However, nitrate concentrations in the lake have been continuously rising for more than a century. They are still much lower than levels considered dangerous to human health; but this steady, long-term rise is an unusual record of environmental nitrogen buildup. It may relate to anthropogenic alternations to the regional nitrogen cycle, but researchers are still unsure of the causes of this change to the lake's ecology.
As for other Great Lakes fish, populations have also been affected by the accidental or intentional introduction of foreign species such as the sea lamprey and Eurasian ruffe. Accidental introductions have occurred in part by the removal of natural barriers to navigation between the Great Lakes. Overfishing has also been a factor in the decline of fish populations.