Freshwater Fish

A little rise in water temperature here, a small drop in water level there, a few more days of drought . . . does it really add up to all that much?

For trout, walleye, bass, pike and other freshwater fishes, it adds up to a lot. In fact, put it all together and you have a crisis that could take the magic out of those graphite wands.

While predictions of global warming are serious for all freshwater fish, they are dire for trout. “It doesn’t take a big jump in the temperature of a stream to wipe out a population of brookies,” says Steve Moyer, Trout Unlimited’s vice president for governmental affairs. “And it happens so fast — seems like they disappear while you’re switching your fly from a nymph to a dry.”

As snowpack decreases in volume and melts earlier in the season and as temperatures shoot up in midsummer, stream flows will plummet. “Trout in streams and rivers at upper elevations will be under a great deal of stress,” says Dr. Jack Williams, Trout Unlimited’s senior scientist. “At lower elevations they’re just not going to survive without a lot of help.”

While more able than trout to adapt to increased water temperatures, bass, bluegill and other warm-water species will face other threats from global warming. For example, changes in precipitation patterns — heavy, floodproducing rains interspersed with extended droughts — will cause major fluctuations in water levels. In lakes, rivers and reservoirs, these fluctuations could dramatically reduce the survival rate of eggs, larvae and fry. “Occasional extreme events have always been part of the weather cycle, but with global warming these exceptions could become the rule,” says John Lott, fisheries chief at the South Dakota Freshwater Fish Department of Game, Fish and Parks. “If so, recruitment into adult fish among all freshwater species would be seriously jeopardized.”

Current projections of the effect of global warming on freshwater fish include the following:

• Nationally, up to 42 percent of current trout and salmon habitat will be lost before the end of the century, with the South, Southwest and Northeast experiencing especially severe reductions.

• In regions most affected by global warming, trout and salmon populations will be slashed by 50 percent or more. Many trout species already listed as threatened or endangered will become increasingly vulnerable to extinction.

• In the Pacific Northwest, up to 40 percent of the salmon population will disappear.

• In localized, high-mountain areas of the West, bull trout will suffer reductions of up to 90 percent. In the lower elevations of the Appalachian Mountains, as much as 97 percent of the wild trout population will die.

• Sea-level rise will push salt water into rivers, over lowlying land and into freshwater lakes. This salinity will contract suitable habitat for warm-water fishes that require water with low or no salinity.

• Across the nation, increased water temperatures in rivers and lakes will promote the continued expansion of noxious, exotic plant species such as giant salvinia, hydrilla and Eurasian watermilfoil.

• Because fish recruitment in rivers and lakes is frequently tied to water levels, extreme droughts and floods will cause large fluctuations in the quality of recreational fisheries.