A longer growing season means a longer period of thermal stratification, which can be detrimental to coldwater species that rely on a limited layer of cool, oxygenated water near the thermocline. “When a lake remains stratified longer, oxygen depletion in the hypolimnion, the bottom layer below the thermocline, becomes more likely. As an anoxic hypolimnion expands upward, a warming epilimnion pushes downward, so the livable habitat zone for ciscoes shrinks and in the worst case disappears,” Jacobson explains. When thermal stress gets too high, die-offs occur.
How well do ciscoes adhere to their thermal niche? Andy Carlson, Minnesota DNR researcher, is using acoustic tags to track depth and temperature use of ciscoes in a thermally “marginal” Minnesota lake. In his study lake, Carlson says suitable conditions for ciscoes were restricted to a layer of water only about 2 to 3 feet thick, and that’s the depth range that the tagged ciscoes used. “Some individuals venture above and below that layer at times, but mostly they are sandwiched into that narrow zone,” Carlson says.
Further warming can squeeze ciscoes into an ever-shrinking zone. Extreme warming events several years in a row can have catastrophic consequences by eliminating a cisco population altogether.
Critical Habitats
“How declines in cisco abundance will affect trophy pike production is definitely a concern, especially on these marginal lakes.” Jacobson says. “What we consider the good pike lakes, those with deep, cold, clear water, tend to have good oxygen levels in the hypolimnion and should withstand the brunt of the expected levels of warming.
“In prime pike lakes we’re worried mostly about changes in water quality. Cultural eutrophication— human induced nutrient loading—can cause a hypolimnion to expand and become more anoxic, turning a good lake into a marginal one.
“Shoreline development is another concern. And beyond the shoreline, we should be protecting watersheds that drain into high-quality pike lakes. We may not be able to reverse climate trends and their direct impact on waters and fish, but through watershed protection we can exert some level of control,” Jacobson says.
Casselman explains that it’s not only the direct effects of warming that affect pike. “When water warms, evaporation also increases. We find that for every 1°C rise in water temperature there’s a 6 percent increase in evaporation. This affects runoff, wetland flooding, spawning habitat, and eventually pike recruitment. When recruitment declines, fewer pike are available to reach advanced age in the face of fishing mortality, which may mean fewer trophy pike.”
There are remarkable signs that pike may be adapting to changing conditions, Casselman says. “On the St. Lawrence River, for instance, pike are spawning deeper and earlier. Instead of spawning in 3 feet of water they are spawning in 6 feet, adapting to springs with weak runoff and little rain.”
That pike are adapting on these time scales is incredible, he says, but there likely isn’t going to be time for natural selection to keep up with the pace of the changing environment. “Pike aren’t going to fix things. We’re the ones that will have to adapt.”
