It has been speculated that conditions brought on by a warming climate may allow animals to breed more often in a single year. However, this has only been empirically shown in insects. The problem is that such predictions and vulnerability assessments require comprehensive and high-quality long-term datasets which are not widely available. But if such long term studies have been done they can deliver some exciting and perhaps concerning results.
A good example is a new study that for the first time documented multiple breeding cycles for fish in a single season due to climate change. Research conducted by the University of Washington showed that one of Alaska's most abundant freshwater fish species, the three-spined stickleback (Gasterosteus aculeatus) is altering its breeding patterns in response to earlier spring ice breakup and longer ice-free summers.
The data were collected from 1963 to 2015 in Alaska's Lake Aleknagik, home to one of the University of Washington's Alaska Salmon Program research stations. For 52 years, the abundance of juvenile sockeye salmon and other fish that live in the region's freshwater lakes was recorded by capturing fish along the lakeshore at 10 different sites every seven days between June and September. All fish were identified and measured.
While the program's monitoring was designed to track the commercially important sockeye salmon population, scientists also meticulously recorded every other fish present, including three-spine stickleback. Stickleback represent almost half of the fish found in Lake Aleknagik, with juvenile sockeye salmon nearly matching that percentage. Three-spine stickleback make up a large percentage of the fish communities in many northern lakes, so these findings could be relevant throughout the region.
Stickleback 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; males will build the nest and attract a female, who then lays the eggs. Males guard the nest until the fish hatch, usually after about two weeks. This behaviour made them great study objects for fish nerds like me. I remember keeping some of them in a tank at home fascinated by textbook knowledge becoming reality in front of my eyes.
By analyzing decades of data showing fish sizes throughout each summer, the colleagues could determine roughly when certain fish were born - a larger fish captured in August was indicative of an early season brood, while a smaller fish captured on the same day likely came from a brood that hatched later in the summer. Using these data and additional environmental data, they found that the fish 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.
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. 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.