FOR IMMEDIATE RELEASE – Madison, Wis.
When it comes to water temperatures, fish can be a lot like Goldilocks. Some water bodies are too warm, others too cold, and a handful are “just right” – providing the perfect thermal habitat for a species to flourish.
As lakes across the Midwestern United States continue to warm due to climate change, it is often assumed that there will be a sort of balance reached, with warm-water species expanding their range into the places where cold and cool-water species struggle to survive.
But, a study published November 27th in the journal Nature Communications, says such assumptions may be overly optimistic.
When it comes to climate change and preferred temperatures for fish, we often talk about “winners and losers,” says Olaf Jensen, a professor at the University of Wisconsin-Madison’s Center for Limnology and co-author of the study. But impacts aren’t felt evenly. In fact, “the losers have lost a lot and the winners have had only small gains,” Jensen says.
This imbalance in the fortunes of different species means that fisheries managers will need to be nimble in how they respond to shifting fishery dynamics in a warming world, says Luoliang Xu, a postdoctoral researcher at the Center for Limnology and lead author of the paper.
To get to the bottom of this thermal habitat imbalance, Xu and his team analyzed a database of modeled temperature profiles for more than 12,000 lakes across the Midwest from Minnesota to Arkansas and North Dakota to Ohio. They then combined those water temperatures with data on the preferred thermal habitats for 60 different freshwater fish species and looked at how many days of the year each lake had at least some optimal water temperatures for each species.
Using powerful computers to model and sift through this immense amount of data, a trend emerged showing that the number of days cold-water species had to enjoy their optimal conditions were fading much faster than optimal warm-water days were growing.
The team identified two hypotheses that likely explained much of this dynamic.
The first is that there is a seasonal timing of when habitat is available. In general, warm water species of fish only have a short summer window when lake temperatures finally reach their preferred conditions, whereas cool and, especially, cold water species are closer to their temperature thresholds for much of the year and any spring or fall warming can push those temperatures beyond their optimal ranges.
Secondly, warming temperatures have made the entire water column of a lake more similar from a temperature standpoint. In the warmer months of the year, lakes stratify into a warm, upper layer of water and a cold, bottom one. But slight increases in wind and evaporation have led to warm water mixing further into the lake and shrinking the amount of cooler water available for species that prefer it.
The paper’s authors are quick to point out that what they’ve studied is only optimal temperature conditions and not actual population dynamics of fish. Variables like the age of a fish or different species’ ability to withstand less-than-ideal conditions will also play a part in how warming impacts populations.
But, Xu says, the take-home message still holds – cold water species are seeing their optimal conditions decrease much faster than warm water species are seeing theirs increase. And that, he says, has a lot of implications for fisheries management.
“We have some management strategies being widely discussed that when warming happens in a lake, we can shift our management focus from species that don’t do well in warming waters to species that benefit from the warming,” Xu says.
While such a policy isn’t entirely unviable, he cautions, fisheries managers will need to rethink how to direct management efforts and identify which species to focus on to ensure that their work best protects populations while maintaining fishing opportunities.
That is a much more complicated narrative than “we may not have as many walleye but we can still fish for bass” says Jensen. When it comes to climate change, the success of the winners is unlikely to make up for the other things we lose.
MEDIA CONTACTS:
Luoliang Xu – lxu287@wisc.edu
Olaf Jensen – ojensen@wisc.edu, 608-262-3014
PDF of article available here.