I think that one of the reasons that people are fascinated with fish is that they are hard to observe. Fish live in a world that is difficult to visit and largely mysterious to us air breathers, even though that world might be only a few feet underneath your boat or dock. While the underwater world might seem like an alternate dimension, the under ice world is even more distant from people’s understanding. Anyone that has stood on a frozen lake has wondered what is going on underneath them, but under ice behavior is notoriously hard to study because it’s cold and dark down there.
A cool new study by researchers from Sweden and Norway has shed some light on what trout are doing under the ice. They built stream channels in the lab that had a window on one side (to observe the fish) and added ice cover to the top of the channels to simulate a frozen river environment. In each channel they added 4 brown trout and observed their feeding and swimming behaviour.
The researchers found that adding ice cover (while keeping the water temperature constant) influenced a variety of trout behaviors. Trout living under ice cover had decreased stress levels and spent more time actively swimming as opposed to holding on the stream bottom. The researchers suggested that the presence of ice could cause fish to become more active because ice decreases the risk of predation (at least from birds).
Additionally, trout living under ice had more aggressive interactions with one another than those in open streams. This effect could be due to increased activity in general from fish under ice. It is interesting that fish still had lower stress levels under ice despite the increase in aggressive interactions, which suggests that for these fish the decreased predation risk was more important than increased social aggression in determining stress levels.
Since global warming is reducing the duration that rivers are ice covered, studies like this help us understand how the changing climate will influence fish behaviour. Previous studies have already shown that salmonid production is higher in ice-covered rivers than in rivers that are ice-free, and now we know that fish are more stressed when their icey ceilings melt away.
Citation: J Watz et al. 2015. Ice cover alters the behavior and stress level of brown trout Salmo trutta. Behavioral Ecology 26: 820-827
Find the paper here