Artificial reef under construction in Lake Michigan with Chicago in
background. (Photo courtesy of Illinois Department of Natural Resources.)
During November 1999, an artificial reef (approximately 244 m long x 6-9 m wide x 1-3 m high) was placed into Lake Michigan just south of Chicago to improve smallmouth bass fishing in the area. INHS researchers are currently investigating the effectiveness of this artificial reef for attracting or producing fish. The difference between attracting and producing fish is a critical one, both for managing the smallmouth bass fishery and for understanding the importance of artificial structures to regional population dynamics. For instance, if the artificial reef attracts but does not produce additional smallmouth bass, it may serve as a population sink if anglers harvest more smallmouth bass from the artificial reef than they would otherwise. Conversely, if the artificial reef produces additional smallmouth bass, that added production can offset angler harvest and may provide a more stable population of smallmouth bass than if the artificial reef merely attracted fish.
Artificial reef under construction in Lake Michigan with Chicago in
background.
(Photo courtesy of Illinois Department of Natural Resources.)
To address this question, Survey researchers are sampling both the artificial reef site and a nearby reference site to explore possible differences at these two sites before and after artificial reef construction. Researchers want to determine if the artificial reef (1) establishes a more productive food web than representative nonreef habitat, (2) increases spawning by fishes that are strongly associated with the reef, (3) provides habitat for adult sport fishes to use for feeding, resting, and spawning, (4) increases production of local sport fishes, and (5) produces a positive change in angler use and catch rates.
We studied the biotic and abiotic conditions of the artificial reef and reference sites during May-September 1999 to establish baseline data before reef construction. Both sites are located approximately 0.8 km offshore at a depth of 7.5 m and are separated from north to south by 1 km. We sampled several life stages of fish (newly hatched larvae, juveniles, and adults), zooplankton and aquatic invertebrates (both important food items for fish), and water temperatures at each site.
Bulldozing stones off barge to create artificial reef. (Photo courtesy
of
Illinois Department of Natural Resources.)
Preliminary data analysis from 1999 sampling indicates that the artificial reef and reference sites were comparable in abiotic and biotic characteristics before the reef was constructed. Water temperatures were similar throughout the spring and summer, with temperatures peaking around 25deg.C in late July. SCUBA surveys determined that the substrate at each site was a mix of sand, silt, and clay with outcroppings of hard substrate that support zebra mussels. Zooplankton density was similar between sites; zooplankton consisted primarily of calanoid and cyclopoid copepods, and rotifers, all important prey for fish. In addition, planktonic zebra mussel larvae, called veligers, were quite abundant in zooplankton samples from both areas.
Fish sampling at the artificial reef and reference sites identified similar larval and adult fish densities and composition before reef placement. Larval fish catches were characterized by a high percentage of alewife; yellow perch and Centrarchidae spp. appeared in lower percentages. Visual surveys of adult fish identified that the round goby, an exotic species, dominated the benthic fish assemblage at the artificial reef and reference sites, except for one date when 12 adult smallmouth bass were observed roving in the area. Generally, only adult round gobies were observed; however, in late August young-of-year round gobies were observed, indicating that round gobies are successfully spawning at both sites.
The 1999 observations suggest that zebra mussels and round gobies will be important components of the community associated with the artificial reef. Round gobies are abundant in the region and are currently limited to available structures (i.e., intermittent rocks and bedrock outcroppings); thus, the addition of a large amount of artificial structure will substantially increase suitable habitat, likely increasing carrying capacity for the round goby. Researchers also expect the zebra mussel to be an early colonizer because of the hard structure provided by the reef and the presence of veligers in the water column at the reef site.
Backhoe and bulldozer on barge at site of reef contruction in Lake
Michigan.
(Photo courtesy of Illinois Department of Natural Resources.)
Smallmouth bass will likely use the artificial reef, although the extent and intensity of that use is still unclear. Studies in Lake Erie have documented that smallmouth bass are attracted to similar artificial structures but no quantitative measure of their use of these structures has been completed. Therefore, the observation of smallmouth bass and round gobies indicates that the artificial reef may provide suitable habitat and prey for attracting smallmouth bass to the area. However, it will remain unclear if the reef attracts or produces smallmouth until additional comparisons are made after reef construction.
Data collection will continue at the artificial reef and reference sites during 2000 and 2001. In addition, anglers will be interviewed to determine if increased catch rates of smallmouth bass result from the addition of the reef. These data will provide an understanding of the role artificial reefs play in Lake Michigan and identify the effectiveness of this artificial reef for attracting or producing smallmouth bass.
Matthew J. Raffenberg and John M. Dettmers, Center for Aquatic Ecology
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