Are Fish Populations in Hot Water Because of Climate Change?

Dr. Sapna Sharma, York University

Author: Lushani Nanayakkara

Why study climate change in Canadian lakes? Canada has about 3-7 million lakes which account for about 20% of the world’s freshwater. These lakes are a non-renewable resource that contains potable water as well as food sources. Additionally, they are economically, socially and culturally significant resources.

Hindcasting and extreme events

Ice dynamics

Lake Suwa in Japan has the longest ice record ever collected. Shinto priests have been collecting data about ice-on dates since 1442, resulting in 550 years of records. Frequency of lake extreme events (when lake does not freeze) has been on the rise from 1443 to 2005. In the last 250 years the lake did not freeze 3 times, over last 50 years it did not freeze 1 out of every 4 years, and over the last ten years it did not frozen 5 times.

Highlight 1: The Omiwatari or ice ridge formation is important in Shinto culture/religion because it represents the tail of the dragon the god uses to cross the lake to visit the goddess.

Long oscillatory cycles or large scale climate drivers have been changing since the industrial revolution. For example: there has been a loss of long-term cycles as the positive phase of the North Atlantic Oscillation (NAO) has stabilized with increasing CO2 concentrations. Duration of El Niño Southern Oscillation (ENSO) cycles has decreased from about 80-90 years during 900-1300 AD to about 2-8 years at present. Lakes have been melting earlier across the northern hemisphere over the past 100 years. Even more alarmingly, they are melting ~4x faster in the past 50 years relative to past 100 years.

So what is responsible for late ice-on and earlier ice-off? It may be the combined effect of the following factors: climate change (increases in air temperature coupled with increase in CO2), climate drivers (ENSO and NAO, and solar sunspots), and/or local weather (temperature, precipitation and snow cover).

Water temperature

Some clear trends have emerged from data for about 300 lakes world-wide. 90% of lakes are warming in the past 25 years, and ice-covered lakes are experiencing faster warming rates, including lakes in Ontario and Canada. Increasing water temperature can impact water quality, potentially leading to bottom-up effects of algae blooms and also reduced fish survival. There seem to be two main drivers of increased water temperature: climate change and solar brightening/dimming (northern hemisphere is becoming ‘brighter’ due to better control of pollutants, thus, reduction in cloud cover while the southern hemisphere is becoming ‘dimmer’ due to increase in pollutants and cloud cover).

Forecasting and uncertainty

Invasion of warm water fishes

Highlight 2: So how did smallmouth bass get to Ontario? Many years ago Ministry of Natural Resource officials moved bass via trains and planes and dumped them into lakes. Additionally anglers have been responsible, as well as natural migration through river networks.

The case of smallmouth bass: smallmouth bass are native to the eastern United States. Their growth rate is positively related to air temperature. Warmer summer peak temperatures favor increased smallmouth bass survival over-winter. Why are they a problem? Smallmouth bass invasions can lead to loss of biodiversity, as they are voracious predators and dramatically reduce the quantity of minnows in a lake. This can impact native cold-water fish in Canada like the lake trout, which is a top predator and a socio-economic commodity. Lake trout live in the deeper portions of the lake. Smallmouth bass and lake trout do not normally have overlapping habitat but do have food-web interactions. Lake trout generally depend on minnows for about 60% of their diet, and the rest is supplemented by zooplankton. But in presence of smallmouth bass they depend primarily on zooplankton (about 80%). This can result in lower growth, reproductive and survival rates. Smallmouth bass are currently found in southern parts of Canada, but under (unchanged) climate change scenarios bass may be found everywhere, threatening lake trout populations. Small lakes are even more at risk for vulnerable lake trout populations. Smallmouth bass invasions under unchecked climate change scenarios could result in reduced diversity in BC as well, including threats to salmon fisheries in addition to trout populations, with potentially widespread economic consequences.

Community dynamics

Species interactions and diversity

Under current greenhouse gas emission levels, walleye, a cool-water fish species, is predicted to shift its range northwards and be eliminated from the south. A cold-water species, cisco are also expected to become eradicated in south-central Ontario.

Highlight 3: smallmouth bass are predicted to invade most lakes in Ontario by 2070 under unchecked rates of climate change unless we control/stabilize greenhouse gas emissions.

What about interactions between walleye and smallmouth bass? Modeling under current climate change scenarios predict reduced co-occurrence in southern Ontario, increased smallmouth bass and reduced walleye occurrence in central Ontario, and novel interactions in northern Ontario. After an increase of about 4ºC both walleye and bass may co-occur in all habitable lakes. Data shows reduced walleye populations (up to 3 times less) where bass are already present, so walleye populations could become very vulnerable. Bass appear to be outcompeting native predators in addition to changing minnow population dynamics. Currently, Ontario lakes support high biodiversity including iconic, highly sought-after cool/cold-water predators with high socio-economic value. But projections under current climate scenarios predict loss of these iconic predators with concurrent impacts on sport and commercial fisheries because of changing ecological dynamics.

What can you do?

Please help prevent spread of all invasive species, not just fish species like Asian carp. We also need to mitigate greenhouse gas emissions at local, regional and global levels!

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