November’s Lake of the Month, Sparkling Lake, is a 157-acre lake with clear waters and natural shorelines in the middle of the Northern Highland State Forest in Vilas County. Sparkling Lake is part of a large cluster of lakes in Vilas, Iron, Oneida, Forest and Price counties called the Northern Highland Lake District. This area has one of the densest concentrations of lakes in the world and it’s a hotspot for limnology research. Sparkling is one of 11 Wisconsin lakes that scientists have been monitoring since 1981 as part of the North Temperate Lakes Long Term Ecological Research Program, funded by the National Science Foundation and administered by the University of Wisconsin’s Center for Limnology.
This month we highlight Sparkling Lake for its innovative research. A partnership between the Center for Limnology and DNR allowed for a long-term aquatic invasive species control effort in Sparkling Lake. Because Sparkling is a Long-Term Ecological Research Lake, there are more than 30 years of water quality data for the lake as well as records on water levels, aquatic plants and fish and crayfish populations. Sparkling was invaded by rusty crayfish (Orconectes rusticus) in the 1970s and long-term monitoring records indicated they reached relatively high densities by the late 1980s.
Once introduced into lakes, rusty crayfish often mow down aquatic plants that served as fish habitat, feast on bottom-dwelling invertebrates and fish eggs, and out-compete native crayfish, which can significantly change a lake. Certain fish species, such as pumpkinseed (Lepomis gibbosus) and bluegill (Lepomis macrochirus), may see population declines.
In 2001, a study by the Center for Limnology began to evaluate the effectiveness of intensive rusty crayfish trapping and changes in fishing regulations as a means of crayfish population control. The study was designed to assess whether invasive species’ impacts could be reversed or at least managed on a whole-lake scale. This study included the extensive harvesting of rusty crayfish from 2001-2008 by the Center for Limnology researchers. At the same time, DNR changed the total daily bag limit and length of smallmouth bass (Micropterus dolomieu) and walleye (Sander vitreus), known predators of juvenile rusty crayfish, to limit angler harvesting of these predators. In addition, DNR stocked high numbers of large juvenile walleye each year in hopes of increasing predation on rusty crayfish and rainbow smelt, another troublesome invasive.
The lake transformed quite quickly. According to DNR Fisheries Biologist, Steve Gilbert, “I wouldn’t expect a lake to respond that fast to [restoration efforts], but Sparkling Lake has.” Researchers used long-term monitoring records to compare plant, fish and invertebrate populations from years when rusty crayfish were abundant to years after the population crashed due to harvesting. Scientists found that as the number of rusty crayfish decreased, aquatic plants began to re-establish quickly. The number of bluegill, pumpkinseeds and native crayfish are on the rise in Sparkling Lake. Four years after the harvesting of rusty crayfish ended, their populations have remained low. Researchers believe that allowing bluegill and pumpkinseed populations to rebound has helped keep the rusty crayfish in check; bluegill and pumpkinseed have been shown to prey on juvenile crayfish. Read the entire paper to learn more and be sure to check out amazing photos on the Center for Limnology’s blog.
This research highlights how management efforts can alleviate the harmful effects of invasives, even if complete eradication is not possible. Is the intensive harvesting of invasive species coupled with an increase in predators a viable solution to manage invasives in small lake ecosystems? Can the lessons learned in this study be transferred to other invasive species scenarios? These are all questions to contemplate. A big thanks to the Center for Limnology and DNR fisheries biologists for making us think.