A freshwater fish in Alaska has been discovered to have altered its breeding habits in response to climate change, a behavioral trend that may impact the ecology of northern lakes.
A University of Washington study arrived at this finding after analysing the reproductive patterns of the three-spine stickleback for more than a century in Alaska's Bristol Bay region. It showed the fish breeding earlier and more often in response to earlier spring ice breakup and longer ice-free summers.
This is the first study to document multiple breeding cycles for fish in a single season due to climate change, said lead author Rachel Hovel, a postdoctoral researcher in the university’s School of Aquatic and Fishery Sciences.
“Climate change literature features many predictions and vulnerability assessments, but we don’t have many opportunities to actually observe species’ responses over time, as this is very data-intensive. Our ability to detect multiple breeding in fish is attributed to our comprehensive and high-quality long-term dataset,” said Hovel.
As part of the university's Alaska Salmon Program, the data in the study recorded the abundance of juvenile sockeye salmon and other fish in the freshwater lakes in the Lake Aleknagik region from 1963 to 2015. During this period, fish were captured along the lakeshore at 10 different sites every seven days between June and September. All fish were identified and measured.
Stickleback represent almost half of the fish found in Lake Aleknagik, with juvenile sockeye salmon nearly matching that percentage. They are born near the shore, then move to the middle of the lake to feed on zooplankton. Adults return to the shore in the summer to spawn.
By analyzing decades of data showing fish sizes, Hovel and her team determined roughly when certain fish were born; a larger fish captured in August indicated an early season brood, while a smaller fish captured on the same day was likely came from a brood that hatched later in the summer.
The researchers discovered that three-spine stickleback spawned earlier in years when ice breakup occurred earlier, and in some years, the fish produced more than one brood. Given the short summers in Alaska, most stickleback have time and stamina for only one brood, but increasingly they are rearing two broods a summer as climate change ushers in earlier springs.
Researchers don’t yet know if breeding more often and earlier is beneficial for three-spine stickleback, but it appears that over the long term, the fish is likely to increase in abundance. This is likely to impact the juvenile sockeye salmon, its primary competitor in many Alaskan coastal lakes. The two species share the same habitats in lakes and generally consume the same things. Hovel explained that “If stickleback are increasing in abundance because of their modified reproduction strategy, this can have ecosystem implications for the productivity of species we commercially care about, like sockeye salmon."
“We don’t know exactly what this means for demographics of this species,” Hovel added, referring to the three-spine stickleback. “It could also mean that fish are living shorter lives because there’s a higher physiological cost to breeding more than once. In the lower-latitude extent of their range, fish mature earlier and die earlier.”