On a global scale, aquatic systems are warming. Average temperatures in the world’s oceans have been steadily increasing over the past 50 years. Given that the observed warming trends are predicted to continue, our job, as fisheries scientists, is to understand how fish are presently being affected by this warming and predict how fishes may respond to continued warming in the future. However, some of our predictions are not turning out as we expected.
Most cool-water fish species that live in temperate climates (the area on Earth between the tropics and poles, which includes much of North America, Europe, and Asia) grow faster in the summer because of the warmer water temperatures (see ‘why are fish cold-blooded’). That is why summer is often referred to as the ‘growing season’ by fisheries biologists. Also, larger fish generally make more and better eggs than smaller fish. For these reasons, most climate change research on temperate, cool-water fishes has focused on understanding how longer warmer summers would affect fish growth, with the expectation that fish would grow more and produce more offspring. However, when summers are longer that means winters are shorter and there seems to be some unexpected consequences from these shortened winters.
Recently, my colleagues (Drs. Stuart Ludsin, Elizabeth Marschall, and Konrad Dabrowski at The Ohio State University) and I have been working with yellow perch (Perca flavescens), a temperate, cool-water fish, to understand how short winters might affect reproduction. Due to their sweet, mild flavor, yellow perch are highly sought after by recreational anglers and commercial fishers. Yellow perch are widely distributed across North America. Also, a closely related species, the Eurasian perch (Perca fluviatilis), occurs across much of Europe and northern Asia. Like many temperate fishes, yellow perch develop eggs during winter and spawn during the spring.
In Lake Erie, the southernmost of the Laurentian Great Lakes, yellow perch populations have generally declined over the past several decades. During this time the frequency of short, warm winters with little ice cover on Lake Erie has also increased. Historical Lake Erie data over the past 4 decades shows many more juvenile yellow perch following long, cold winters than short-warm ones. As female yellow perch develop eggs during winter, we were curious to know if these short, warm winters resulted in fewer juvenile yellow perch the following year.
As it turns out, yellow perch don’t fare well when winter is warm. Laboratory experiments found that female yellow perch exposed to a short winter produce smaller eggs than females exposed to a long winter. Furthermore, these small eggs hatched at lower rates and produced smaller larvae than large eggs, produced after a long, cold winter. Previous research has shown that smaller larvae are more likely to starve or be eaten by a predator than larger larvae.Additionally, yellow perch females in both Lake Erie and the laboratory did not fully adjust their spawning time following a short, warm winter. Instead, female yellow perch spawned at warmer water temperatures when spring arrived exceptionally early. If spawning does not occur at the appropriate temperatures, yellow perch larvae may be hatched into an environment with low levels of zooplankton (their main food source), leading to starvation.
While this result was unexpected for yellow perch, what about other fish species? It turns out that fishes from the tropics to the arctic also show reduced reproduction following exposure to warm temperatures. Anemonefish from the Great Barrier Reef in Australia produce smaller eggs, smaller larvae, and fewer batches of eggs when consistently exposed to warmer conditions. At the other extreme of the spectrum, arctic char in Sweden experience reduced egg viability in response to warmer temperatures. Others that show this same negative response to warm temperatures include Atlantic halibut, Atlantic salmon, and rainbow trout.
Given that warming trends in Lake Erie and other aquatic systems are expected to continue, scientists around the globe are currently working to understand the implications of a warmer climate on fish growth, survival and reproduction, keeping in mind that results might not turn out as they had expected.
By: Troy Farmer, a postdoctoral researcher at Auburn University
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