Fish shrink during harsh winters

By Victoria Gill
Science and nature reporter, BBC News

In winter, young wild trout tend to shelter from energy demanding conditions

Fish can shrink during particularly harsh winters, according to researchers.

The scientists, based in Norway and Finland, discovered that juvenile brown trout reduced in length by as much as 1cm - a shrinkage of approximately 10%.

They say this could help the young fish to conserve energy when food is in short supply.

They describe, in the journal Functional Ecology, how the fishes' bodies "shortened".

This rare phenomenon has been seen before in some small mammals, including shrews, and in lizards.

We were faced with unexpected and previously undocumented observations... fish do shrink in harsh winter conditions

The most dramatic example is the marine iguana. This cold-blooded (or ectothermic) reptile lives in the Galapagos archipelago.

It has been found to shrink as much as 20% of its body length over the years of El Nino events when food availability dramatically decreases due to considerable temperature increase.

But this is the first study to show that fish can shrink.

The scientists described the condition that leads to this shrinkage as "over-winter anorexia" - whereby their appetite decreases dramatically over the autumn "transition period".

The team, led by Ari Huusko from the Finnish Game and Fisheries Research Institute in Paltamo used hatchery-raised salmonids - a group of ray-finned fish that includes salmon and trout.

They created experimental pools in which to study them.

To simulate the conditions of a cold winter, the water temperature and current were controlled and a covering was added to mimic ice coverage.

"We were faced with unexpected and previously undocumented observations... indicating that fish do shrink in harsh winter conditions," the researchers wrote.

"Young salmonids showed significant shrinking of individual body length, up to 10% of the body length, over the course of winter."

Shrinking mystery

The scientists do not yet know exactly what causes the shrinkage.

But they think that, like in shrews, it could be caused by a reduction in the volume of a jelly-like substance within vertebral discs of the creatures' spinal columns.

"This leads to flattening of these formations and thus into the shortening of the spinal column, and hence body length," the scientists explained in the Functional Ecology paper.

Dr Huusko told BBC News: "The apparent triggers for shrinkage were food and feeding stress in connection with environmental conditions generally poor for growth and survival.

"[This] raises several further questions about what kind of consequences shrinkage can have."