Some small Alaska fish have made a big change in response to a warming climate — they are breeding earlier and, in some cases, twice a year, according to new research.

Five decades of monitoring at Lake Aleknagik in the Bristol Bay region revealed that, as ice breakup dates arrived earlier and water temperatures increased, the threespine stickleback living there produced their offspring earlier in the summer.

The findings, detailed in a study published in the international journal Global Change Biology, come from long-term monitoring at the University of Washington's Alaska Salmon Program, which operates a field station at Lake Aleknagik, as well as stations at other Alaska sites.

Though ice-melt dates and temperatures vary greatly year to year, a long-term trend to earlier ice breakup and warmer waters is strong at Lake Aleknagik, said the study.

From 1963 to 2015, the mean date of spring ice breakup advanced by 11 days and mean June surface-water temperatures rose by 3.7 degrees Celsius (6.7 degrees Fahrenheit), according the study.

In cases of especially early ice melts, there were second waves of new stickleback born in late summers, making the Aleknagik stickleback the first documented case of any vertebrate species breeding multiple times a year in response to climate change, the study's lead author said.

That twice-a-summer breeding discovery was a big surprise, said lead author Rachel Hovel, a researcher at the University of Washington's School of Aquatic and Fishery Sciences.

"This is the first time that we've seen this strategy emerge in a population rather than already there," Hovel said. Though some insects breed multiple times a year, vertebrates do not, she said.

Whether the earlier and more frequent stickleback breeding is good or bad is not yet clear, she said.

For big fish like char that eat the small stickleback, it might be a good thing, increasing food supplies, she said.

But for salmon, there might be cause for concern, she said. Stickleback use the same territory and eat the same food that juvenile salmon do, she said. "There are potential competitive impacts of them breeding earlier and breeding more," she said.

The discovery of stickleback-breeding changes was, in a way, inadvertent. The University of Washington's Aleknagik field station, established in 1947, has focused on salmon using that area for spawning. But stickleback — big-eyed fish that grow to no more than about 4 inches in length — are about as plentiful as salmon in Lake Aleknagik, Hovel said. As a consequence, the surveys of fish netted at the lake over the past decades have monitored stickleback as well as salmon, she said.

The trend toward earlier stickleback breeding was noticed by the late Don Rogers, a longtime University of Washington salmon biologist and Bristol Bay expert, Hovel said. To confirm Rogers' anecdotal observation, Hovel and her colleagues were able to make use of long-term data collected in a consistent manner over five decades, she said. "The same net types have been used, the same sites have been used throughout," she said.

Stickleback are born in nests built of sand and plant matter and held together by a substance secreted from the adult males' kidneys, according to an Alaska Department of Fish and Game species profile. After females lay their eggs in the nests, the males fertilize and guard them until they hatch, according to the species profile.

Though they are small, stickleback are easy fish to study, Hovel said. Some live in saltwater environments, some in freshwater environments, and some move between the two, she said. They are known for their genetic complexity and rapid evolution in response to changing environmental conditions, she said. "They're really flexible," she said.

But Hovel worries that further study of climate change impacts to stickleback — and other species — may be at risk. Newly inaugurated President Donald Trump has dismissed climate change as a hoax. The National Science Foundation, itself funded by Congress, provided some of the money for the stickleback study.