Summertime’s come and gone, and winter has arrived in the Northern Hemisphere. Inland waters across North America are cooling down, and many will soon be covered with ice.

Winter can be hard on fishes, being poikilotherms that can’t regulate their own body temperature (with some exceptions). So just how do they make it through this challenging season?

Firstly, freshwater fishes are rarely threatened by freezing to death because of one of water’s most unique properties. Like most substances, water’s density increases as temperatures drop. However, it’s most dense just above its freezing point; density increases between 4° and 0°, causing ice to float. Were it not for this property, lakes and rivers would freeze from the bottom-up and fish at more northern latitudes would be seasonally extirpated.

But not all fishes are not equally thermally adapted, and many cannot tolerate very cold or very warm temperatures. Thermal tolerance is an important controller of fish distributions, but within their range, fishes have adapted several ways to deal with cold temperatures:

Movement. When temperatures drop, many fish make seasonal movements to thermal refugia. Thermal refugia can be particularly important in northern climes, where warmer groundwater helps keep aquatic habitats ice-free.

Alternative enzymes. Enzymes are proteins that catalyze metabolic reactions, but their efficiency is temperature-dependent; enzymes perform best at an optimal temperature. To maximize their efficiency across a range of temperatures, some fishes have evolved alternative enzyme systems called isozymes to perform the same function at different optimal temperatures. Other fishes utilize allozymes—different forms of the same enzyme that are controlled by different alleles that allow reactions to function at different temperatures.

Antifreeze. In polar marine systems, high salt concentrations reduce water’s freezing point to around -2°–colder than the freezing point of most species’ bodily fluids. In these habitats, several species (most noteably the Notothenoidei but also cods, sea ravens, and some herrings) have antifreeze proteins that prevent blood from crystalizing. This allows them to survive in the harsh, ice-covered polar regions.

Salt Concentration. Other species beat the super-cold salty water at its own game. Marine fishes must constantly export ions to maintain osmotic balance. However, retaining more ions can help to reduce a fish’s freezing point and survive in below-zero temperatures.

But in the end, some fish simply can’t handle the cold. Threadfin Shad, in particular, are subject to large winter die-offs, which can be an important source of winter forage for many predators. Large-scale winter fish die-offs often occur in instances of extreme temperature fluctuation. For example, ~100,000 drum died in the Arkansas River in 2011 after a particularly chilly cold front front blew through following several warm weeks. Lastly, many ponds and small lakes are experience winterkill–massive fish die-offs that occur not because of cold, but because ice limits oxygen exchange. The fish in the photo above probably suffocated and floated to the surface where they became lodged in ice.

Stay warm, folks.

—Brandon Peoples