Located in North America, Lakes Superior, Michigan, Huron, Erie and Ontario are geologically young, glacial lakes holding nearly 20% of the world’s supply of freshwater. Whereas the lakes share a common glacial origin, their morphology, climate, and surrounding land use differ substantially. The Laurentian Great Lakes watershed covers more than 777,000 km2. The basin lies in the temperate zone and undergoes large seasonal temperature changes. Air temperatures in winter average -2. 5 oC to -22.5 oC (Fuller et al., 1995). Average summer surface water temperatures range from 10.0 oC in the northern lakes to 25.0 oC in southern lakes (Fuller et al., 1995).
Phosphorus levels across the lakes have declined but some localized areas still experience high nutrient loadings due to watershed inputs from tributaries, particularly in areas with extensive agricultural land use. Lakes Superior, Michigan, and Huron are oligotrophic; Lake Ontario combines mesotrophic nearshore areas and oligotrophic offshore areas; and Lake Erie consists of three distinct basins: the deep, oligotrophic/mesotrophic eastern basin, the mesotrophic central basin, and the shallow eutrophic western basin. Dissolved oxygen is not a problem in these lakes except in Lake Erie where the central basin hypolimnion (Edwards et al., 2005) and sometimes (ephemerally) the western basin (Bridgeman and Schloesser, 2003) experience periods of hypoxia or anoxia during the summer months.
Fish and Fisheries
The Laurentian Great Lakes currently support 179 fish species from 29 families (Coon, 1999). The historic commercial fishery consisted of native fish species included lake sturgeon, lake herring, lake whitefish (Coregonus clupeaformis) and lake trout (Salvelinus namaycush). Technological advances in fishing gear, predation by non-indigenous sea lamprey (Petromyzon marinus) and degraded waterways led to a decline in the fisheries throughout the Great Lakes by the early 1900s. Reduction in sea lamprey abundance, brought about by a lamprey control program, paved the way for the success of massive fish stocking programs beginning in the 1960s. During the 1970s and 1980s, improvements in water quality and coordinated inter-jurisdictional fisheries management led to dramatic reductions in stresses associated with habitat, invasive species (primarily sea lamprey), and exploitation. While some species were extirpated or remain at low levels, many exploited species have responded favorably to these management actions. Lake whitefish remain the most harvested single species throughout the lakes whereas percid species continue to be the most economically valuable commercial fishery and contribute substantially to the recreational fishery (Bronte et al., 2003; Dobiesz et al., 2005; Madenjian et al., 2002; Mills et al., 2003; Ryan et al., 2003). Only Lake Superior maintains a naturally reproducing lake trout population in sufficient numbers to allow commercial fishing of this native species.
Current commercial fish harvests average 24.9 million kg/yr. In 2000, the landed value was estimated at over $17 million US but processing raises this value by 1.5 to 2.5 times (Seagrant, 2009). Lake whitefish was the most harvested species and accounts for slightly less than half of the total commercial catch. While several species are fished commercially, yellow perch had the highest value per pound in 2000 of over $2 (Kinnunen, 2003). In Canadian waters of Lake Erie, the commercial fishery remains strong and represents about two-thirds of the Great Lakes harvest. The fishery supported 750 fishermen who harvested approximately 2,300 tonnes of fish in 1991, valued at $59 million US (Kinnunen, 2003).
Interest in the recreational fisheries resurged in the 1970s as non-native Pacific salmon were introduced to control nuisance prey fish species, primarily the alewife (Alosa pseudoharengus) (McCrimmon Jr., 2002). This fishery created or aided related services (e.g., bait shops, charter boats, marinas, etc.) and improved recreational industries (e.g., boat building, tackle manufacture) (Regier et al., 1999; US Army Corps of Engineers, 2008). Recreational species include salmonids, walleye, and yellow perch as well as many nearshore species (McCrimmon Jr., 2002).
The value of the recreational fisheries far exceeds that of the commercial fisheries. US Great Lakes anglers spent $1.3 billion on fishing trips and equipment in 2001 (Anon, 2001). If secondary impacts such as lodging, restaurants, and marinas are added, the total economic impact exceeds $19 billion in sales and $6.4 billion in income for the nearly 250,000 jobs supported by the recreational fishing industry (US Army Corps of Engineers, 2008).
Population Growth and Economy
The human population in the Great Lakes basin rose rapidly beginning in the 1800s (Beeton, 1969) but slowed between 1970 and 1990 to less than 1% for the combined US and Canadian population; by late 1999, more than 33 million people resided in the basin. The most populated areas occur along the shorelines in large metropolitan areas of Chicago, Illinois, Detroit, Michigan, and Toronto, Ontario. Among US coastal regions, the Great Lakes region contains 18% of the total coastal population, making it the third most populated coastal area (Crossett et al., 2009).
The Laurentian Great Lakes basin has been shaped by two centuries of human activity. Settlement and population growth during the early 19th century brought deforestation associated with the timber industry and agricultural land clearing (Beeton, 1969). Anthropogenic activities still impact the lakes’ ecosystem, but intensive management of water quality and fisheries has had positive effects (Ebener, 1997; Knight, 1997; Neilson et al., 2003). Concentrations of toxic chemicals have declined in the waters but fish consumption advisories for legacy contaminants, especially Hg, are still issued for many sport-caught species (U.S. EPA Office of Water, 2004). Phosphorus abatement programs have reduced phosphorus loadings throughout the Great Lakes, although some localized areas experience seasonal eutrophication (Neilson et al., 2003). The lakes have been impacted by at least 180 non-indigenous species introductions since the opening of the St. Lawrence Seaway in 1959 that opened the lakes to oceangoing ships (Grigorovich et al., 2003; Ricciardi, 2001).