A warmer climate puts Lake Vättern’s char under pressure twice over

The large form of Arctic char was considered critically endangered when this report appeared in 2015, and its population in Lake Vättern had fallen sharply since the 1950s. Measures addressing overfishing and introduced competitors had brought some improvement, but a warmer climate posed another threat. Warm winters made food harder to find for newly hatched fry and larger fish alike, according to an SLU study published in Nature Communications.

Many plants and animals show direct effects of climate change. Predicting what warming means for a predator is more complicated, because the overall outcome depends on how other organisms in the food web respond. This study of Vättern’s char was one of relatively few to identify several interacting mechanisms affecting both fry and adults.

“As far as I know, this is the first study to address both direct and indirect climate effects on a freshwater top predator,” said study leader Tomas Jonsson of the Swedish University of Agricultural Sciences, SLU.

The researchers analyzed catch and climate records collected since the early 20th century alongside detailed knowledge of Vättern’s food web: which species eat which. Warm winters were followed by poorer char catches five to six years later. About four years after a winter with extensive ice cover, catches were unusually good. Catch records also showed a steep decline over the preceding 25 years in vendace, formerly the most important prey for adult char.

The study explained these trends through ecological mismatches affecting two age groups. In warm winters, char eggs develop faster and hatch too early. The fry exhaust their stored nutrients before zooplankton becomes plentiful in the lake. Older char are affected indirectly: vendace eggs also hatch prematurely, sharply reducing the availability of this important prey.

The apparent failure of char to adjust hatching time to the warmer climate of the preceding 25 years suggested that genetic variation in this trait was absent or insufficient. Because measures to limit climate change work slowly, helping the population grow could offer the best support for its adaptation. A growing population can develop greater genetic variation, improving its ability to respond to changing conditions.

“It may sound counterintuitive, but if we take the threat from warming seriously, management of Vättern’s char should focus on other factors that put the population under stress,” said Jonsson. “That includes limiting predation on eggs by introduced crayfish and competition from introduced salmon, as well as avoiding excessive catches by commercial and recreational fisheries.”

The research was a collaboration between SLU and the University of Skövde.

Vättern char. Photo: Jönköping County Administrative Board.