Photo Credit; Nathan Burnett
What do fish do in the winter? Part 2 Lakes-Canadians are pretty well adapted to winter. Come October, we’ve pulled out our toques and our coats, and placed shovels by every doorway in the house. Wildlife, too, starts to migrate to warmer climates, shed their summer fur for thick, winter coats, and hunker down to hibernate. But what about fish? Water freezes at 0 °C, but the fish continue to persist, frost after frost. So how do they do it?
In lakes, only the surface water freezes, leaving plenty of fluid water beneath it. That’s why people are able to go ice fishing during the winter. However, lake fish still face plenty of challenges in the winter, despite the fluid water. The biggest challenge is oxygen availability. In the summer, the constant movement of surface waters, along with abundant plant photosynthesis, creates plenty of fresh, dissolved oxygen for fish to breathe. However, in the winter, the surface is still, trapped by ice, and in the absence of light, plants consume oxygen instead of producing it. In small, shallow lakes, oxygen can decrease so much that fish begin to suffocate—a phenomenon known as hypoxia. When a fish suffocates and dies, it decomposes, which consumes more oxygen, and leads to more death. This can quickly snowball into the death of most or all lake organisms, also known as winterkill.
Lake fish have a variety of adaptations to hypoxic conditions. Some fish, like the Iowa Darter, will emigrate out of the lake into flowing waters before the ice forms. Others, like Brook Sticklebacks and Fathead Minnows, will stay within the lake, but move to the lake inlet, where freshwater from the streams flowing into the lake keep the dissolved oxygen content high. Many benthic fish, or fish that live at the bottom of a waterbody, move out towards the edges of the lake. Here, the shallow water is oxygenated down to the bottom, so they can rest on the lake bed and still be able to breathe. Mud minnows are unique; they can breathe gaseous air, and survive hypoxia by breathing the oxygen in air bubbles trapped beneath the ice.
Lakes lose oxygen from the bottom up, so for fish that remain within the heart of the lake, the logical solution is to move up, lingering near the surface ice, where oxygen in the ice slowly dissolves into the water. Pike take this to the next level, taking it upon themselves to create areas of high oxygen. They rest at a slight angle in the water, with their snouts almost touching the ice, and their tails pointed downwards. Then, they maintain slow, steady movements of their pectoral fins and operculum (the hard, protective covering for their gills), creating a weak current of warm water under the ice. This melts the ice, creating a dome of oxygen-rich water. While pike are normally a solitary fish, they can often be found resting in these domes in pairs or small groups and will create and utilize several of them throughout the winter.
However, moving up to the surface comes with another challenge. Since the ice is actively melting into the surface water, this is the coldest part of the lake, and the water closest to the ice can be less than 1 °C. This exposes fish to the risk of hypothermia. To combat this, fish tend to employ one of two strategies. One strategy is to conserve energy by entering a state of semi-dormancy. This is what Largemouth Bass do. They reduce their metabolism, activity, breathing rate, and primary body functions. However, in this state, they cannot process very much food, so their body whittles away at their fat reserves to stay alive. As a result, if the winter is too long or too harsh, fish can die of starvation. The second strategy is to increase their activity and breathing rates. This is what Yellow Perch do. However, this means that the fish must feed actively throughout the winter, which can be difficult when winter resources are already so scarce.
Winter is hard on all living organisms, but somehow, life finds a way. Come spring, the warming water nudges dormant fish back into action. The fish that migrated out of the lake are lured back into the rich, productive environment that it offers. There is an explosion of activity as fish hunt and spawn again, recuperating from their winter losses, and raising a new generation of hardy Canadians.
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