What do fish and amphibians do in winter?

Overwintering in water seems obvious, yet most amphibians and reptiles (apart from the “armored frog”, that is the European pond turtle) spend this period on land. There are, however, exceptions, for example tadpoles that have not managed to metamorphose in time. Another is the common frog, which as an adult overwinters on the bottoms of streams and ditches, or the marsh frog, which does so in deep ponds and oxbow lakes. Both burrow into mud or the near-bottom layer of fallen leaves. Occasionally, the water frog and the moor frog also spend the freezing period in water.

To this day it is not known where agile frogs disappear in autumn. According to some researchers, their males overwinter in water and females on land. Others deny this. The matter has not reached a consensus among herpetologists. Sometimes we can observe awakened amphibians of the above-mentioned species moving along the bottom of a reservoir or watercourse. This happens quite often, as part of their search for the best microhabitat.

Hungry, cold and suffocating

In winter, amphibians are threatened by lack of oxygen just as much as by cold and hunger. That is why the vast majority of Polish species hibernate on land. This applies not only to arboreal tree frogs, heat-tolerant toads and poorly swimming fire salamanders, which drown during mountain floods, but even to species so strongly associated with water as newts, green frogs and fire-bellied toads. The latter never bask on the shore and leave their puddles only after they dry up. Yet even they begin in autumn to look for some shelter on land. Native amphibians, both tailless and tailed, spend the winter under tree roots, in piles of brushwood, heaps of stones, deep in turf or forest litter. Some of them are able to dig themselves quickly and deeply. The common spadefoot and the natterjack toad descend even to 1.5 m into the ground. The rest of our batrachofauna prefers ready-made shelters. In winter they crowd into abandoned burrows of rodents and sand martins, basements and dugouts, and increasingly also within culverts, bridges or tunnels. Only recently was it discovered that smooth newts can squeeze into the thin tunnels made by large earthworms.

Sleepers and activists among our fish

The fish of Poland are far more diverse than amphibians, including in how they spend the winter. Peaceful-feeding cyprinids such as roach and bream, as well as their herbivorous relatives introduced from East Asia such as grass carp and silver carp, gather in deeper parts of water bodies, where the temperature will always be +4°C. The biggest cold-sensitive members of this family, tench and crucian carp, burrow into the mud and fall into the deepest sleep. The ubiquitous predators of our rivers and lakes, namely pike, zander and perch, are instead very active, feeding on the sluggish representatives of other species. The increasingly rare burbot not only hunts but also reproduces in winter, as we have already mentioned in Wodne Sprawy.

In winter, native predators typical of fast-flowing rivers and mountain streams also remain very active. For salmon, sea trout and its resident cousin the brown trout, December primarily marks the beginning of spawning. At the turn of autumn and winter, further cousins of the true salmonids, the more plankton-feeding whitefish and vendace, also spawn. Among the remaining carnivores of our ichthyofauna there is no shortage of tireless migrants. In winter, the downstream migration of eel takes place, hence the closed season for this species from 1 December to 31 March. Standing apart from this group of winter activists is the European catfish. Although an even larger glutton than they are, it nevertheless spends the winter similarly to cyprinids, not feeding and mainly sleeping. This may be an ecological and physiological legacy of its tropical ancestors.

Bad either way

The diversity of life strategies among our fish makes their protection during works in river channels more difficult. Carrying out works that cause water turbidity in autumn and winter makes it possible to avoid conflicts with the spawning of most fish, especially cyprinids, perches, loaches and gobies, as well as grayling and huchen. Unfortunately, it potentially conflicts with spawning migrations and or the spawning itself, which are reflected in closed seasons, of several fish species valuable for the economy and nature, such as those mentioned above:

• sea trout (1 December to 28 or 29 February in the lower Vistula, 1 October to 31 December in the upper Vistula, its tributaries and most other rivers) and eel (1 December to 31 March);
• burbot (1 December to 28 or 29 February);
• European catfish (1 January to 31 May in most waters of Poland, except Lake Dąbie and internal marine waters);
• pike (1 January to 30 April).

Underwater journeys of Baltic fish

For marine fish, winter is not frightening. There is no shortage of depths with a constant temperature. It is also possible to swim away to warmer waters. This is how mackerel cope, for example, feeding intensively in the Polish Baltic after spawning, only to return to the Atlantic before winter. For arrivals from the Far North, trapped in the Baltic since the end of the Pleistocene, such as the eelpout and the fourhorn sculpin, winter is an excellent time for reproduction.