Human responses to climate change could further exacerbate the negative impact on aquatic ecological systems. For example, human responses to a warmer climate and the variety of precipitation scenarios, as well as human population increases, are likely to place greater demands on freshwaters to meet water needs for drinking, industry, and irrigation, along with an increase in water management projects (see Chapter 4). Potential results would include fewer free-flowing streams and greater fluctuations in water level. These changes would cause a loss of ecosystem services and products from "natural/ unmodified" lakes and streams. Conflicts between developers and those wishing to reduce development pressure on lakes and streams probably would intensify as freshwater becomes either more scarce or more abundant.

One adaptation to climate change would entail poleward transportation of fish, mollusks, and other less vagil organisms across watershed boundaries to cooler waters (Magnuson et al., 1997). Historical introductions and range expansions of species have resulted in extinction or extirpation of preexisting fauna owing to ecological interactions, especially predation by the invader. Well-known examples include the sea lamprey, alewife, smelt, and zebra mussel to the Laurentian Great Lakes in North America (Mills et al., 1994); the peacock bass in Panama (Zaret and Paine, 1973); and the Nile perch into Lake Victoria (Kaufman, 1992). Other examples being documented include the decline or extirpation of cisco and yellow perch in small Wisconsin lakes in the United States with the arrival of rainbow smelt (Hrabik et al., 1998) and the loss of cyprinids (minnows) and the decline of lake trout with the introduction of black bass and rockbass into Ontario lakes in Canada (Casselman, 2000; Jackson, 2000; Vander Zanden, 2000). The history of invasions exemplifies the homogenization of continental faunas (Rahel, 2000), losses in native organisms, and large changes in ecosystem processes and structure—all with economic consequences.

Fisheries (capture, aquaculture, and recreational) and associated introductions would exacerbate biodiversity and exotic problems in a warming climate. A substantial number of inland fisheries are associated with introduced species and stock enhancement. Tilapia, carp, grass carp, and rainbow trout are commonly introduced or stocked. An estimated 38% of the recreational catch is of non-native fishes. The practice of introducing species apparently has resulted in 1,354 introductions of 237 species into 140 countries from pre-1900 to the mid-1980s (FAO, 1988). Despite recognition of the importance of wild native species and increasing appreciation of the importance of biodiversity, impacts of climate change are likely to be managed by introducing better adapted species, if past management practices prevail. In the United States, an average of 38% of the recreational fishery is for non-native species, 75% of which now reproduce naturally (Horak, 1995). As climate change impacts affect the abundance of native (and non-native) species, it is probable that the past precedent of introducing non-native species would be applied to maintaining catches and recreational fishing stability.

Moving species poleward to adapt to changing climate zones for lakes and streams is fraught with scientific uncertainties and human conflict. The exotic can become a superabundant pest species, with negative effects on native organisms—including extirpation and extinction. The invasion ecology of organisms is not a predictive science; many surprises would be expected. The case has been made that managing with exotics increases the instability of the fish community and fish management problems and includes many unexpected consequences (Magnuson, 1976). Introduction of warmer water fauna on top of regional fauna that are having increasing problems from warming climates is likely to be a controversial adaptation.

Climate changes may provide development opportunities for aquaculture if traditional wild fisheries are less stable and markets favor the stability of the aquaculture product. Aquaculture represents a mobile technology that potentially can move with changing climate to the best conditions for culture (see Figure 5-7).

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