Climate Change in Context: Bass Mass Causes Lake Ache

(Above: Juvenile smallmouth bass rely on fat reserves built during the summers to survive on throughout the winter and early-spring. Photo Credit: August Rode)

This is the third article in a series about climate change impacts on various ecosystems. To read the other articles in this series, click here.

Imagine you’re vacationing on a paradise island. In my mind, this island has an endless supply of pastries, coffee, and perfectly ripe fruit. Your island probably has a similar bounty. A seemingly endless supply of my favourite food is what would make me feel like royalty.

In our plentiful small lakes, bass form the royals.

Both largemouth and smallmouth bass are a popular sport fish, attracting a large fishing industry across Ontario. Right now, many Ontarians are getting out into nature to pull bass from the water as the season just opened at the end of June. In the HSA, bass are present in our lakes. Many of us have caught a smallmouth bass or know someone very passionate about bass fishing.

Large-sized fish need a large-size meal plan to support themselves. The preferred diet of smallmouth bass usually has crayfish and minnows on the menu. This diet plan is so well established that my Royal Ontario Museum Freshwater Fishes of Ontario field guide features a smallmouth bass alongside a crayfish on the cover.

July is the favourite time for bass fishing. Not only are the days long and evenings warm, but the fish are biting, too! The youngest bass are now preparing for the winter by building up fat stores. During winter, bass enter a hibernation-like state, relying on built energy reserves to pull them through until summer — much like we did during this past March, April, and May. Preferred fatty prey are most abundant in July when waters are warmest. As air and water temperatures increase with climate change, there is an increase in bass winter survival, causing more and more bass roaming the lakes the following summer.

Minnows are the prime roast in the bass world, full of rich proteins and fats. A study out of Queen’s University analyzed the long-term fish census between 1969-1979 and in 2014 and found that across all studies in lakes in eastern Ontario bass species were increasing and minnow species were decreasing, both at shocking rates (Finigan et al., 2018). The same results are shown in a study from University of Guelph that found that game species, particularly bass species, are dominating lakes across Ontario (Cazelles et al., 2019). Increasing temperatures were deemed the most controlling factor, though some habitat changes and stocking activities are involved as well (Finigan et al., 2018; Cazelles et al., 2019).

Warmer temperatures are helping to inflate bass numbers and are drastically deflating minnow numbers, causing ecological impacts throughout the lake. There are several different ecological cascades that could occur over the short- and long-term.

These changes in fish communities may seem small and inconsequential — fish are fish, right? Surprisingly, the behaviour and diets of these two different fish families have local and global impacts.

The simplest and most likely change to lake ecosystems is the complete fish die-off. When a top predator increases in abundance, there is suddenly pressure on the rest of the supporting food web. More fish are demanding food that the system cannot sustainably produce. Stressing the system as such means something will break. The hungry bass will likely clear out the minnow and crayfish in the lake, which will damage the rest of the ecosystem until there is no longer any structure left to hold the bass up.

Another possible outcome, in addition to the fish collapse, is unrestricted algae growth. Minnows eat algae and small crustaceans that also eat algae. If the smallmouth bass were to reduce the minnow numbers sufficiently, algae will slowly grow more abundant. Warmer temperatures will also expediate this process. In the long-term, this would result in a completely sterilized lake because the decaying algae attracts large amounts of microbes that use up all the oxygen in the water, leaving none behind for any other organism.

Globally, this is important because this switches how the lake operates regarding climate change. Bass domination may very well be a cause of climate change (Eby et al., 2006). When lakes are abundant with algae-eating fish, the aquatic plants grow better and use up all the carbon dioxide in the water to create oxygen, causing the lake to pull more carbon dioxide from the overlying atmosphere. When fish-eating fish dominate the lake, the algae-eating fish are less abundant or absent, allowing for less controlled, or completely uncontrolled, algae growth, reducing the plant growth and thereby producing more carbon dioxide within the lake, releasing it to the atmosphere. When bass dominate a lake so much that the minnow species disappear, the lake causes climate change and perpetuates the bass-domination process.

Not only are bass expected to become detrimentally successful in the lakes they currently occupy, they are also expected to move northwards into new lakes. Bass have been found to have expanded their range into northern Ontario over the last few years. In Wisconsin, bass have pushed north, displacing other top fish predators like walleye (Hansen et al., 2017). Colonizing and dominating new lakes will only perpetuate their ecosystem impacts at quicker rates every year.

Climate change in causing bass to become kings and queens of their lake, and of all lakes. Much to their demise. The bass are their own worst enemy, it seems.

So, all in all, pick up the fishing rod and get on the lake! The fish are biting, and the environment is counting on it. It is time to dethrone the ruler for a more stable kingdom.

Further reading

Cazelles, K., Bartley, T., Guzzo, M.M., Brice, MH., MacDougall, A.S., Bennett, J.R., McCann, K.S. (2019). Homogenization of freshwater lakes: Recent compositional shifts in fish communities are explained by gamefish movement and not climate change. Global Change Biology, 25, 4222-4233. doi 10.1111/gcb.14829

Eby, L.A., Roach, W.J., Crowder, L.B., Stanford, J.A. (2006). Effects of stocking-up freshwater food webs. TRENDS in Ecology and Evolution, 21(10), 576-584. doi 10.1016/j.tree.2006.06.016

Finigan, P.A., Mandrak, N.E., Tufts, B.L. (2018). Large-scale changes in the littoral fish communities of lakes in southeastern Ontario, Canada. Canadian Journal of Zoology, 96, 753-759. doi 10.1139/cjz-2017-0080

Hansen, G.J.A., Read. J.S., Hansen, J.F., Winslow, L.A. (2017). Project shifts in fish species dominance in Wisconsin lakes under climate change. Global Change Biology, 23, 1463-1476. doi 10.1111/gcb.13462

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