On fish in rivers during winter: the challenges posed by ice and a warming climate

This report is meant to raise general awareness of what is faced by fish in rivers during the winter. As the water temperature drops in the fall, rivers begin to freeze. This occurs at the surface, initially along the shorelines, then away from them. Where the water currents are low enough, a uniform ice cover develops. Higher currents, especially in areas where the water is not deep, will lead to a broken ice cover, which can accumulate in places and form an ice jam. There can also be open water all winter long. Ice can form below the water surface as well, either within the water column or along the riverbed. River ice causes an increase in water levels upstream because of the friction that exists between the flowing water and the bottom surface of the ice cover. A rough bottom surface and thick ice cover can cause significant flooding upstream. The break-up of ice jams may induce a sudden increase in water levels downstream. There are approximately 180 fish species inhabiting Canadian rivers. Some of the most common ones are part of the salmon and trout Family, the minnow and carp Family, and the perch Family. Winter is a difficult time of the year for fish. Depending on geographical location, cold water conditions may last four to eight months. The challenges begin as the water temperature goes down to the freezing point. Fish are cold-blooded animals, and their body temperature is near that of the surrounding water. This causes a reduction in their metabolism. They are then less able to move, and seeking low-energy environments becomes a priority. Ice action is detrimental to fish in a number of ways. It can obstruct the channel and prevent them from seeking a suitable environment, or it can occupy those environments. Underwater ice poses several challenges for fish. For example, if it forms over egg nests, it can inhibit overwinter development of eggs. Surface ice can damage these and other critical habitats if it reaches the river bed or via gouging by drifting pieces. The changes in water levels and water currents during winter are also a liability for fish. Drastic events, such as an ice jam break-up, can have dire consequences for fish. Winter conditions, however, can also favor fish. An ice cover is a solid surface that does not exist in the summer and promotes calm environments when fish need it. It also constitutes a refuge against land-based predators because it is both a visual and a physical barrier. Human activities can directly impact fish ecology – this is briefly reviewed also. The salient concern is the presence of structures built across river channels, such as hydroelectric dams. These are a significant barrier to fish mobility; they are also responsible for modifying the waterway, with a loss in the complexity and natural variability it had before dam construction. The cyclic nature of water releases from hydroelectric dams, which is meant to meet energy demands, is utterly unnatural from an ecological perspective. Human activities can also indirectly impact fish ecology via climate change, primarily through an increase in air temperature, which has an important influence on the formation of ice and river dynamics during winter. In general, a reduction in the length of the ice season is expected, with later freeze-ups and earlier break-ups. Fish ecology will be affected by two concurrent sets of circumstances: water temperatures and ice action. Water temperatures are generally expected to rise, and fish adapted to cool or cold water are vulnerable to climate conditions that lead to warmer waters. Overall, an increase in water temperature will impact the entire food web which the fish depend on. Foreseeing the climate impact on fish in rivers is the subject of ongoing investigations by scientists from several fields of expertise.

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