New research from Nord University highlights what study authors describe as an argument that “fish are not just passive victims of environmental change but active participants in a feedback loop that can reshape entire river systems.”
We already know that dams, hydropower generations, fish ladders, and other forms of diversions are causing rivers to change. Water temperatures get warmer more quickly, and staple insects like mayflies and stoneflies are in decline. What hasn’t been closely monitored, before this study, was how fish are reacting to all those changes on an evolutionary level.
Xiatong Cai, the lead author of the study which was published in Water Resources Research, told Phys.org that, “Fish don’t just live in rivers—they change them, and they are changing in response to us. If we ignore evolution, we risk designing river systems that work against both nature and our own long-term goals.”
Cai’s research showed that some fish have already evolved in the face of hydropower demand. They specifically looked at Atlantic salmon in Norway. In rivers with lower-than-normal flows due to hydropower demand, larger salmon likely have a harder time of accessing historical spawning ground. Smaller fish, however, can more easily access those areas.
And since body size is a trait that’s inherited from parents, those salmon are evolving to be smaller and smaller as the years pass, thanks to what has essentially become selective breeding for smaller fish due to hydropower demands.
Cai also pointed out that the riverbed itself is likely going to change due to smaller fish spawning. Larger fish move more silt when creating redds; smaller fish don’t. That distribution of silt throughout the river is key in transferring nutrients throughout the system. With smaller fish spawning, we may see less and less of that moving foward.
The rest of Cai’s research is worth diving into, because it asks us to think not only how can we restore habitat and fish, but how are we going to respond to decades of these fish evolving to live in nonnatural environments? Are we losing genetics in fish that may not be recoverable by the time a fishery can be fully restored to its natural state?