One of the Great Lakes' most prized fish is increasingly under stress from climate change, and its decline is at risk of getting worse.
Winters in the region are becoming a lot more unpredictable, and a new study foreshadows what the unpredictability may mean for walleye, which are important commercially, recreationally and ecologically. As apex predators at the top of the food chain, walleye keep the food web in check. They are also a part of the cultural identity of Ojibwe tribes.
The research team looked at population data from 194 lakes across Wisconsin, Minnesota and Michigan, and found that the freshwater fish is becoming out of sync with lake ice-off, the usual cue that it's time to spawn.
Walleye populations have already been declining for decades, said Zach Feiner, author on the study and scientist for the Wisconsin Department of Natural Resources as well as the University of Wisconsin-Madison.
Climate change adds another layer of problems, Feiner said.
So why do walleye spawn after ice-off? And how are changes in ice-off dates affecting walleye? Here are some takeaways from the study.
More:This will almost certainly be the warmest winter ever in Wisconsin's recorded history
Walleye spawning is synced with the rest of the food web
Walleye spawn every year in the spring shortly after the ice thaws. Warmer water helps phytoplankton, microscopic plant-like organisms that form the base of the food web, bloom. Zooplankton, which eat the phytoplankton, emerge just after the phytoplankton bloom.
Zooplankton are the main food source for walleye fry after they hatch. Spawning is timed so the fish will hatch when zooplankton are most abundant.
That is what happens in a “normal” year. But in an abnormal year, where ice-off comes earlier or later, there can be a “mismatch” in timing where walleye hatch, but there are not enough zooplankton around and the walleye do not have enough food.
Walleye experience year-to-year 'whiplash'
The researchers looked at walleye survey data from state natural resource departments and the Great Lakes Indian Fish and Wildlife Commission along with spring harvest counts from several Ojibwe tribes. The commission also participated in the study.
The researchers found that while walleye are spawning slightly earlier in the spring, ice-off dates are shifting three times faster. This resulted in a mismatch in timing in almost every single lake.
Feiner said that walleye populations are also experiencing “whiplash” where the ice thaws extremely early one year followed by a year where ice-off is a lot later.
The freshwater fish “isn’t keeping up with the pace of climate change,” he said.
Ojibwe tribes have already observed timing changes
Much of the upper Great Lakes region is on track to have the warmest winter on record, and it’s the second year in a row where the lakes have experienced historic low ice cover.
The region has experienced the strongest winter warming trends in the country since the 1970s, bringing increased year-to-year variability as well as an overall decline in ice cover.
For Ojibwe tribes, warming winters, dwindling ice thickness and year-to-year whiplash is putting important treaty resources at risk. For instance, the Ojibwe have a treaty-protected right to spearfish spawning walleye in the ceded territories.
Bazile Minogiizhigaabo Panek, a member of the Red Cliff Band of Lake Superior Chippewa, previously described this timing mismatch to the Journal Sentinel. He explained that when he was a child his grandfather taught him that walleye spawn when spring peepers make their first calls. But now, because of warmer weather and water temperatures, those events don’t line up like they used to.
Making lakes more resilient is key
The timing mismatch is not always a problem year-to-year, Feiner said, because adult walleye can always spawn again the next year when the conditions may be better and the fry may have a better chance of surviving.
But walleye populations have been declining for decades, he said, and populations are likely to experience a lot more “bust” years.
And although the team didn’t look at other fish for this study, Feiner believes other fish that spawn in the spring, like perch, muskie, pike and bass, are likely experiencing the same timing troubles.
Other mainstay fish in the Great Lakes fishing industry may also be at risk when there is a lack of ice. For instance, some fish, like lake whitefish, spawn in the fall in nearshore areas of the lakes where the eggs incubate over the winter. When ice is not there to protect the eggs, strong winds and waves stir up the sediment and reduce the number of fish that hatch in the spring.
Feiner said the best way to maintain walleye populations is by finding the lakes that are doing well and protecting them from land use, overharvesting and invasive species. This will help make them more resilient to the added stressors of climate change, he said.
More:Climate change is making conditions harder for Wisconsin trout. But there is hope.
More:Tribal members in Wisconsin spear fish through ice not just to feed, but to teach community
Caitlin Looby is a Report for America corps member who writes about the environment and the Great Lakes. Reach her at clooby@gannett.com or follow her on X@caitlooby.
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