Both site-and landscape-scale processes play important roles in the biological communities of rivers. Understanding the influences of these processes on fish abundance can help direct management and research efforts toward appropriate habitat variables and scales. We used multiple linear regression analysis of a regional fish and habitat database to determine the feasibility of using geographical information systems (GIS)-derived landscape-scale habitat variables to explain the spatial variation in the density of five sport fish species (Chinook salmon Oncorhynchus tshawytscha, steelhead O. mykiss, brown trout Salmo trutta, brook trout Salvelinus fontinalis, and white sucker Catostomus commersonii) in the rivers of Michigan's Lower Peninsula. We compared these models with those developed using site-scale variables traditionally measured in the field. Landscape-scale riverine habitat variables obtained through GIS analysis and modeling of catchment characteristics accounted for 18-69% of the variation in fish density. Landscape estimates of mean July water temperature were negatively correlated with the density of brook trout, brown trout, and Chinook salmon. Drainage area was negatively correlated with the density of steelhead and white suckers, and 90% exceedence flow yield (a measure of flow stability) was positively correlated with the density of Chinook salmon and steelhead. Site-scale habitat variables explained less (12-57%) of the variation in fish density than landscape-scale variables. In the site-scale models, depth was negatively related to all species' densities, and the percentage of soft substrates was positively correlated only with white suckers. Although there was still much unexplained variation in density, our models provide insight into key habitat variables that influence fish density patterns on a large scale.
Lake Michigan has experienced many ecological changes as a result of introductions of non-native species. Arguably the most significant was that of alewife (Alosa pseudoharengus), yet studies on diet overlap with native species are lacking. We analysed diet trends of alewife, spottail shiner (Notropis hudsonius) and yellow perch (Perca flavescens) collected in summer and fall 2000-2007 near Waukegan, IL, in 3-10 m depths. Mean percentage composition by dry weight for 23 prey taxa was used in multivariate analysis to test whether diet differed across species and size classes. We also tested whether zooplankton and benthic invertebrate community composition changed over time. Fish diets were similar over all years but differed seasonally. In summer, diets of large alewife were similar to both small alewife and small yellow perch, with Bosminidae, chironomid larvae and copepods as primary common prey. During fall, alewife and yellow perch size classes exhibited strong intraspecific diet overlap, while there was low diet overlap between species. Primary distinctions between species' diets in fall were higher consumption of amphipods by yellow perch and dreissenids by spottail shiners compared to alewife, which consumed higher proportions of zooplankton. Overall, high yellow perch diet overlap with alewife during summer and with their larger conspecifics during fall could lead to negative implications for yellow perch growth before the critical overwintering period. Detailed insights into diet overlap and prey availability are critical first steps in understanding competitive interactions between native and non-native fish that dominate the nearshore community in southwestern Lake Michigan.
In November 1999, an artificial reef composed of granite rubble was built in southwestern Lake Michigan to attract smallmouth bass Micropterus dolomieu. Adult fish communities were sampled at the artificial reef site and a nearby reference site before (1999) and after reef construction (2000)(2001)(2002)(2003) to determine whether the artificial reef attracted sport fishes. The total number of fish observed along scuba transects was higher at the artificial reef than at the reference site during 2000-2003. Smallmouth bass, rock bass Ambloplites rupestris, round goby Neogobius melanostomus, and yellow perch Perca flavescens were most commonly observed by divers at the artificial reef site, whereas the round goby was the most prevalent species observed at the reference site. Mean annual total gill-net catch per unit effort (CPUE) did not differ at the two sites after reef construction. Freshwater drum Aplodinotus grunniens, gizzard shad Dorosoma cepedianum, yellow perch, and salmonines were commonly caught at both locations. The presence of several of these taxa was related to water temperature but not location. Smallmouth bass presence was related to location; CPUE was greater at the artificial reef than at the reference site during 2000-2002. Rock bass CPUE also was greater at the artificial reef than at the reference site during 2002. Smallmouth bass association with the reef was seasonal and correlated with temperature. Although anglers were aware of the artificial reef, fishing effort and success were low, in part because few anglers targeted black bass Micropterus spp. Because water temperature strongly influences the use of structure by centrarchids in deep, cold lakes like Lake Michigan, care must be taken to site artificial reefs in zones of the most suitable water temperature for these species.
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