Temperature changes can have complex effects on fish communities
Climate change and global warming are not universally accepted. I get that. My intent is neither to support nor refute climate change, but rather share how small changes in temperature — one of many climate variables predicted to change — can affect sportfish. While it seems intuitive that a warming climate would result in a longer growing season and faster growth for warmwater sportfish like largemouth bass, the results indicate this may not be the case.
The study system was West Long Lake, a 62-acre lake in north-central Nebraska with a simple fish community composed primarily of bluegill, yellow perch, largemouth bass and northern pike. Northern pike, like largemouth bass, are piscivorous, and both species feed on bluegill and yellow perch.
A multi-agency team of fisheries scientists used three regional climate-change models — models that predicted changes at the county level rather than generalized global or continental models that are not expected to be locally accurate — to predict annual temperature regimes for West Long Lake in 2040 and 2060. The models predicted different annual temperature regimes, but all forecast mean monthly temperature changes compared to present averages were minus 0.4 °F to plus 4.2 °F from April through October 2040 and plus 2.0 °F to 6.3 °F during April through October 2060.
Largemouth bass and northern pike growth and food consumption were estimated from bioenergetic models, which use well-established physiological parameters to estimate energy used for metabolism, activity and waste excretion to predict species- and size-specific food consumption and growth of predator fish. Water temperature and energy content of forage are input data. The models consider temperatures at which feeding begins, optimal temperature and temperatures above which feeding slows and stops.
Because the bioenergetic models used temperature inputs predicted by three climate-change models, ranges of growth and food consumption were predicted for both 2040 and 2060. All changes were compared to model outputs for temperatures measured in 2001.
Average daily food consumption for largemouth bass increased from less than 1% to 67% among months. The greatest increase in food consumption occurred in spring and fall. Total April-to-October food consumption increased 5% to 15% for 2040 predicted temperatures and 20% to 30% for 2060 predicted temperatures.
The change in average daily food consumption for northern pike ranged from a decrease of 18% to an increase of 76% among months. Increased food consumption occurred in spring and fall, but food consumption decreased in the summer. Total April-to-October food consumption increased 5% to 15% for 2040 predicted temperatures and 20% to 30% for 2060 predicted temperatures.
Growth of largemouth bass from April to October in 2040 was 1.3% less to 4.3% more than 2001 growth rate. By 2060, growth would decline 0.1% to 4.1% compared to 2001. Northern pike growth from April to October would decline 3.3% to 27.5% by 2040 and 17.7% to 45.3% by 2060.
Understand temperature effects
Both metabolism and food consumption rise with warmer temperatures. Positive growth occurs when energy consumed exceeds energy burned for metabolism; fish lose weight (negative growth) when energy burned to support metabolism exceeds energy consumed. At temperatures above optimum, metabolism continues to increase, but food consumption declines, and fish can stop growing and lose weight.
Optimum temperature for northern pike, a coolwater fish at the southern edge of its range in West Long Lake, is 68 to 75°F. Optimum temperature for largemouth bass is 82 to 84°F. Northern pike would be expected to have a longer period of negative growth than largemouth bass in a warming environment, exactly as the results predicted.
A shortcoming of the research is failure to include growth before April and after October. Under historical temperature conditions, water temperature is cool and growth is slow before April and after October in this temperate-zone lake. But in a warming environment, growth of both species would resume earlier in the spring and continue later in the fall than during the 2001 temperature scenario. Thus, predicted declines in growth during an entire year would be lower for both species. Conversely, increases in food consumption during an entire year would be greater.
While the lack of increase in growth despite greatly increased food consumption is important, the change in food consumption is potentially more consequential. The greatly increased food consumption from April to October may cause an imbalance between predators and their food supply unless the increases in temperature resulted in substantial increases in production of bluegills and yellow perch, the primary forage.
The total amount of forage consumed is important, but equally or more important is when the food consumption increases. While the total increase in consumption for the April-to-October growing season was always less than 30%, average daily consumption rates were up to 60% greater during May and October for both largemouth bass and northern pike. These spikes in consumption are likely to influence the population dynamics of the prey species. These declines in prey abundance resulting from greater bass and pike consumption will further reduce the growth of the bass and pike and could lead to smaller average sizes.
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