As a city grows, its impact on rivers doesn’t have to

Now, researchers have undertaken the first comprehensive study of how the infrastructure of U.S. cities alters rivers and their biodiversity. The effects are extensive, they reported yesterday in the Proceedings of the National Academy of Sciences, extending both upstream and downstream for tens or even thousands of kilometers like the branches of a nerve cell.

The researchers used data on river hydrology from the U.S. Geological Survey in a computer model to determine the effects of urban land transformation, electric power, and municipal water supplies on rivers nationwide. They also pulled information from a fine-grained database of species occurrence to assess the biodiversity of freshwater fish, mussels, and crayfish in urban-affected stream reaches.

The largest impacts are from urban land cover and electricity production, the researchers found. These aspects of infrastructure have altered at least 7% of the entire length of streams and rivers in the contiguous United States. “Although these estimates may not seem extensive, they result in very large biodiversity impacts,” the researchers write.

Urban infrastructure impacts 60% of all freshwater fish, mussel, and crayfish species in North America, over 1,200 species altogether. It has contributed to local extinctions of 260 species, sometimes in stream reaches distant from city boundaries themselves. It also influences 970 existing native species, 27% of which are threatened or endangered.

“Cities can impact far reaching areas due to the sheer intensity of resource demands,” the researchers write. For example, the energy and water infrastructure of Atlanta, Georgia stretches across four major river basins.

Moreover, the stresses from urban infrastructure are often compounded because multiple cities may be located on the course of the same river. Twenty-one cities lie along rivers downstream of Atlanta.

Surprisingly, however, the researchers found little connection between a city’s size and the severity of its effects on rivers. Atlanta’s infrastructure impacts 12,500 stream kilometers, and has contributed to 100 local extinctions of freshwater species. Las Vegas, Nevada has a similar population size, but impacts less than 1,000 stream kilometers and has contributed to only seven local extinctions.

Some of these differences have to do with differences in the layout of stream networks and the number of species found in the eastern compared to the western United States. But the findings also suggest that as a city grows, its area of hydrologic impact doesn’t necessarily have to expand along with it. For example, cities could check their impact on rivers by managing storm flows better, or choosing sources of electric power that minimize water use.

There are surely tradeoffs involved in such strategies (choices that take less of a toll on rivers may come with greater impacts on some other habitat). Still, the idea that city governments can not only cause ecological harm but also contribute to ecological integrity far beyond the city limits sounds like good news in an urbanizing world.

by  | Aug 22, 2017

Source: McManamay R. et al. “US cities can manage national hydrology and biodiversity using local infrastructure policy.” Proceedings of the National Academy of Sciences. 2017.


Recording fish song to make fisheries more sustainable

What do fish sound like?

When Gulf Corvina breed, their mating calls could be likened to an immense, underwater roar. Now, a group of researchers have found a way to use the deafening din to save these fish from exploitation. Using underwater microphones, they’ve developed a method for converting sound recordings of the fish’s calls into precise population estimates. Those could inform more accurate catch limits, they say, that would ultimately make corvina fisheries—and others—more sustainable.

Fish stocks worldwide are being depleted by overfishing, which often boils down to inaccurate population surveys that can lead to overly-liberal catch quotas. For the Gulf Corvina especially, overfishing over the last 20 years has taken its toll; the fish now has a vulnerable status.

Part of the problem for this species, the researchers explain in Scientific Reports, is that every year the entire population of two million corvina migrate to one spot in the Northern Gulf of California, Mexico, to spawn. There, males attract mates by producing their spectacular cacophony—so loud that fishers can easily locate them from the surface, and haul up more than a million over the course of three weeks. The researchers realized that if they could instead use the noise to monitor the population, there might be a solution for these fish.

Over the course of eight surveys in 2014, they used underwater hydrophones to record the corvinas’ roar at the Colorado River Delta where they spawn. The louder the din, the more fish there were assumed to be. But because of the way sound travels underwater it can be misleading, meaning the recordings alone couldn’t provide a dependable estimate. So the researchers paired them with sonar. This method pings sound waves into the water that bounce off objects and create a detailed picture of how many objects—i.e. fish—there are beneath the surface.

Sonar would be too costly to use for every population estimate. But in this case, the researchers only used it to sample the population size at different points in the survey, adding a layer of detail to the sound recording. If it worked, this would prove whether there was a link between more noise and more fish. And it did: “When all the fish are packed into the spawning grounds and males are chorusing during the peak spawning activity, we find a tight correlation between sound and abundance,” says co-author Brad Erisman from the University of Texas Marine Science Institute. “Now you can imagine a situation, if it’s predictable, [where] you can just have the underwater microphones out there,” he says. “Because you know this sort of sound intensity and this loudness corresponds to about this many fish. Then you have a very powerful monitoring capability.”

The researchers were thus able to determine that at peak spawning, there were between 1.53 and 1.55 million corvina in the delta. Compared to more traditional surveying tools, the advantage of this method, they showed, is its cost-effectiveness and efficiency. Hydrophones can easily and regularly be deployed to monitor the population, which could inform more accurate catch quotas and move the fishery towards sustainability.

As a tool, it also holds promise for other species. Commercial fish like pollock, cod, haddock, and grouper all produce calls during spawning. Now the researchers say they’re looking into how their method could be used to set sustainable catch limits for those species, too.

Source: Rowell TJ et al. “Estimating fish abundance at spawning aggregations from courtship sound levels.” Scientific Reports. 2017.