If the invasive Asian carp that are knocking Great Lakes’ door ever make it into the world’s largest freshwater ecosystem, a new study suggests the destructive fishes will find much more habitat suitable for spawning than previously thought.
The bighead and silver carp that are threatening to invade the Great Lakes from the Mississippi River Basin spawn in flowing water and lay eggs that develop while tumbling downstream.
Based on observations of the streams where the carp have successfully spawned in their native range, scientists have generally thought that a 100-kilometer stretch of free-flowing water was needed to keep the eggs suspended long enough to hatch.
Among the first efforts to predict where the carp might find suitable spawning grounds in the Great Lakes Basin was a 2007 GIS assessment that identified 22 tributaries with at least 100 kilometers between their mouth and first barrier to fish passage.
But a new study from the U.S. Geological Survey based on intensive in-stream data collection and new knowledge of Asian carp egg development indicates that uninterrupted stream segments much shorter than that could do the job.
“Based on our data, as little as 25 kilometers can be suitable for these eggs to be able to drift all the way to the point of hatching,” said Ryan Jackson, a hydrologist with the USGS Illinois Water Science Center and coauthor of the report. “It’s been a little bit eye-opening. And it definitely opens up the possibility of many more tributaries that may be suitable for spawning in the Great Lakes that we never anticipated based on the 100 kilometer criteria.”
The problem with the 100-kilometer rule is that it was based solely on observations of what seemed to work for the fish in other streams, not a scientific understanding of the hydraulics and water quality of the streams and the biology of the eggs.
The new USGS study, on the other hand, is based on an intensive data collection effort that gave researchers a detailed account of stream velocity and water quality along long stretches of four Great Lakes tributaries. The study also benefits from recent research on Asian carp egg buoyancy and development time.
“You can combine the two, and then you can start to paint a picture what minimum conditions are required to keep this egg in suspension and how long it’s going to take to travel downstream,” Jackson said. “And in that time that it travels downstream, how fast it’s developing and how long it’s going to take to hatch.”
USGS crews surveyed the Milwaukee River in Wisconsin, St. Joseph River in Michigan, and Maumee and Sandusky rivers in Ohio. The Milwaukee and St. Joseph are among the first tributaries that the carp might encounter if they invade the lakes through the Chicago Area Waterway System. The Maumee River’s headwaters can occasionally flood and connect with the Wabash River, which holds Asian carp. The bay at the mouth of the Sandusky River has produced positive tests for Asian carp eDNA.
Crews on each river surveyed the streams with acoustic Doppler current profilers. They measured transects of the channel roughly every mile between the first barrier to fish passage and the mouth. The ADCPs also took readings while traveling downstream to the next transect site. They also collected continuous water quality data, including temperature, which has the most influence on egg development time.
The high-resolution current data allowed researchers to hunt for settling zones, or areas where the water slows down enough for eggs to fatally fall out of suspension. Although all four tributaries had areas that might trap eggs, they weren’t prevalent enough to make a successful trip downstream impossible.
“While those settling zones may trap some eggs, if the fish chose to spawn in certain locations within those rivers under the conditions we measured, there is certainly enough distance and enough turbulence to keep those eggs in suspension until they hatch,” Jackson said.
Though the study only assesses four tributaries, the authors also report a protocol for other researchers to conduct similar surveys on other tributaries. The data have also contributed to the development of an Asian carp egg transport model, called FluEgg, that can help managers determine whether their tributaries could support carp spawning.
The hope, of course is that Asian carp never make it as far as these Great Lakes rivers. Meanwhile, it’s best to be proactive and put as many tools and as much data in front of scientists and planners as possible, Jackson said.
“It would be really nice if they didn’t get through,” he said. “But the fact that they’re knocking on the door is evidence to me that there is a high potential that they could get through, and I think we need to be ready. Having tools like this in place and studies like this done I think really helps decision makers prepare in the long run.”
Top image: Silver Carp (Credit: D. O’Keefe, Michigan Sea Grant)