Wetlands Ecol Manage (2012) 20:137–150 DOI 10.1007/s11273-012-9250-x ORIGINAL PAPER Chemical and physical factors associated with yellow perch abundance in Great Lakes coastal wetlands: patterns within and among wetland types Aaron D. Parker • Matthew J. Cooper Carl R. Ruetz III • David P. Coulter • Donald G. Uzarski • Received: 3 March 2011 / Accepted: 23 January 2012 / Published online: 19 February 2012 Ó Springer Science+Business Media B.V. 2012 Abstract Great Lakes coastal wetlands provide important spawning and nursery habitat as well as abundant food resources for yellow perch (Perca flavescens). We examined multiple years of fyke-net data from wetlands along Lakes Huron and Michigan to describe yellow perch distribution in drowned river mouth (DRM) and coastal fringing systems. Principal components analysis and multi-response permutation A. D. Parker (&) M. J. Cooper C. R. Ruetz III Annis Water Resources Institute, Grand Valley State University, 740 West Shoreline Drive, Muskegon, MI 49441, USA e-mail: [email protected] Present Address: A. D. Parker Carterville Fishery Resources Office, U.S. Fish and Wildlife Service, 9053 Route 148, Marion, IL 62959, USA M. J. Cooper Department of Biological Sciences, University of Notre Dame, 107 Galvin Life Sciences, Notre Dame, IN 46556, USA procedures indicated that DRM wetlands (yellow perch CPUE = 0.2) were eutrophic systems that often exhibit high temperatures and periods of hypoxia, whereas coastal fringing wetlands (yellow perch CPUE = 32.1) were less productive. Among the coastal fringing systems, Saginaw Bay (Lake Huron), displayed characteristics of being more productive and had more yellow perch. Most yellow perch captured in Saginaw Bay were age-0, suggesting that it was an important nursery habitat. Among DRM ecosystems, we found that the downstream lake macrohabitats contained more yellow perch than upstream wetlands; however, there was no significant difference in abiotic characteristics to explain the higher catches in lakes. We hypothesize that yellow perch were more prevalent in wetlands with intermediate productivity during summer because these systems provide abundant food resources without the harsh conditions associated with highly eutrophic wetlands. Keywords Coastal fringing Drowned river mouth Great Lakes Perca flavescens Wetlands Yellow perch D. P. Coulter D. G. Uzarski Department of Biology, Institute for Great Lakes Research, CMU Biological Station, Central Michigan University, 156 Brooks Hall, Mount Pleasant, MI 48859, USA Introduction D. P. Coulter Department of Forestry and Natural Resources, Purdue University, 195 Marsteller Rd, West Lafayette, IN 47907, USA Coastal wetlands occupy a relatively small portion of the Laurentian Great Lakes shoreline, yet many different fish species use these systems as spawning, 123 138 feeding, and resting areas (Jude and Pappas 1992; Albert 2003; Uzarski et al. 2005). Over the last 100 years, about 50% of the historic Great Lakes coastal wetlands have been lost to agricultural conversion or drainage (Krieger et al. 1992). The remaining wetlands face anthropogenic disturbances mainly in the form of nutrient pollution (Uzarski et al. 2005; Cooper et al. 2006) and fragmentation (Gyekis 2006; Uzarski et al. 2009). Coastal wetlands typically form along the shorelines of the Great Lakes where some protection from wind and wave activity is present. Areas of the Great Lakes shoreline that are exposed to the full force of wind, waves, heavy ice formation, and the associated erosive forces do not develop wetlands. Because much of the Great Lakes shoreline is continuously exposed to one or a combination of those erosive forces, coastal wetlands only make up a small percentage of the overall shoreline and have a clumped distribution on the landscape. Nearshore barriers (such as shoals or sand spits) and/or gently sloping, shallow bathymetry that attenuate wave energy typically provide enough protection to facilitate wetland formation (Albert 2005). Numerous hydrogeomorphic wetland types exist along the Great Lakes shoreline. Three broad coastal wetland classes are lacustrine, riverine, and barrier-enclosed (Albert et al. 2005). Within these groups, finer classifications also were devised yielding a total of 17 unique hydrogeomorphic wetland types (Albert et al. 2005). Yellow perch (Perca flavescens) have been documented in many wetland types throughout the Great Lakes, including open and protected embayments (lacustrine; Brazner 1997; Uzarski et al. 2005), barrier-beach lagoons (barrier-enclosed; Brazner et al. 1998, 2001; Tanner et al. 2004), and both open and barred drowned river mouth (DRM) wetlands (riverine; Chubb and Liston 1986; Stephenson 1990). Open embayment wetlands maintain direct surfacewater connection to nearshore waters and are strongly influenced by both short- and long-term fluctuations in lake-water level (Keough et al. 1999). Open embayments exposed to high wave energy generally contain little detrital material, whereas protected embayments contain more organic matter (Burton et al. 2004; Albert et al. 2005; Cooper et al. 2009). Barrier beach lagoons form behind sand barriers which may or may not obstruct pelagic surface water connections to the wetlands (Keough et al. 1999; Albert et al. 2005). Most 123 Wetlands Ecol Manage (2012) 20:137–150 rivers discharging into eastern Lake Michigan have flooded mouths, forming coastal lakes with wetlands in their upper reaches, which are described as barred DRM systems (Wilcox et al. 2002; Albert et al. 2005). These are the only wetlands that form along the eastern side of Lake Michigan. Fish community structure in relation to environmental variables has been assessed across Great Lakes coastal wetlands (Brazner and Beals 1997; Uzarski et al. 2005). However, much remains to be learned about the distributional patterns for yellow perch in wetlands related to chemical/physical variables. Single-species assessments, especially of game fish, have the potential to complement community studies and aid in conservation by highlighting the importance of wetlands to individual fish species (Casselman and Lewis 1996). Yellow perch are a native fish to the Great Lakes region where they are both ecologically and economically important. Yellow perch occupy a wide variety of habitats where they play an important role in food webs (Becker 1983) and energy cycling (e.g. Brazner et al. 2001). Yellow perch can be one of the most prevalent fishes inhabiting Great Lakes coastal wetlands (Stephenson 1990; Jude and Pappas 1992; Brazner et al. 2001). The majority of yellow perch found in these systems are larvae (Chubb and Liston 1986; Höök et al. 2001; Tanner et al. 2004; Gyekis 2006), age-0 (Brazner 1997; Brazner et al. 1998; 2001), or spawning adults (Stephenson 1990). The prevalence of these particular life stages in coastal wetlands suggests that these habitats are important for yellow perch reproduction and recruitment. Furthermore, some yellow perch that spawn in Great Lakes coastal wetlands have been shown to eventually emigrate to adjacent nearshore habitats where they are commercially and recreationally exploited (Brazner et al. 2001; 2004; Parker et al. 2009a). Parker et al. (2009b) found that the wetlands of Saginaw Bay, Lake Huron provided abundant food resources and that yellow perch in those areas made the ontogenetic niche shift to piscivory by age 1, which is earlier than reported by others (e.g. Keast 1985). Great Lakes coastal wetlands are typically warmer than the deeper, adjacent waterbodies, which leads to earlier hatch dates and thus, longer growing seasons in these systems for yellow perch (Jude and Pappas 1992). Longer growing seasons can lead to larger sizes entering winter and are important for preventing Wetlands Ecol Manage (2012) 20:137–150 overwinter mortality from starvation, intolerance to environmental extremes, and predation (Sogard 1997). Previous studies have indicated the importance of multiple types of coastal wetlands for yellow perch throughout the Great Lakes by documenting their abundance, however, none explored the distribution of the species across wetland types or regions along with abiotic variables. Therefore, given that (1) yellow perch is an ecologically and economically important species in the Great Lakes, (2) coastal wetlands appear to be a critical habitat for yellow perch in many areas throughout the Great Lakes, and (3) few studies have explained yellow perch distributions across wetland 139 types or within coastal wetlands across regions, we describe the distribution of yellow perch and associated abiotic variables using a multi-year sampling record of wetland fish that included 62 unique sites along Lake Michigan and Lake Huron (Fig. 1). This makes up the majority of the wetlands along the Michigan shoreline of these Lakes. Yellow perch have been documented in Muskegon Lake, a DRM lake, during all life stages from larval (personal observation) to adult (Ruetz et al. 2007; Bhagat and Ruetz 2011). Chubb and Liston (1986) documented larval yellow perch in a DRM wetland; however, yellow perch from age-0 to the adult stage Fig. 1 Locations of fish sampling sites in drowned river mouth (triangles) and coastal fringing (circles) wetlands along Lakes Michigan and Huron. Sixtytwo unique sites were sampled in total however, some individual points on the map represent several different sites located in close proximity to each other 123 140 have not been documented in high numbers in these wetlands. Because of this apparent disparity between DRM wetlands and lakes, we compared yellow perch catches between the two habitat types for a subset of sites that were sampled simultaneously. Our objectives were to describe yellow perch abundances throughout wetlands along Lakes Michigan and Huron related to chemical/physical properties. Specifically, our goals were to: (1) describe yellow perch abundances in DRM and fringing Great Lakes coastal wetlands, (2) describe yellow perch abundances in fringing wetlands across four regions, and (3) directly compare yellow perch abundances in DRM coupled lake and wetland systems of eastern Lake Michigan. We also describe chemical and physical variables collected at the same time that fish were sampled. The purpose of describing abiotic variables along with yellow perch abundances throughout the Great Lakes, particularly the coastal fringing systems, is to generate hypotheses. Although our spatial distribution of wetland sites is clustered by the nature of wetland formation, we sampled a large array of wetlands that effectively covered a large gradient of anthropogenic disturbance. Because of the wide array of wetlands that we sampled, we feel that reasonable conclusions about yellow perch abundances in wetland types and regions can be made. Methods Study sites We described yellow perch abundances in Lake Michigan barred DRM wetlands as well as open and protected embayment wetlands of northern Lakes Michigan and Huron, and Saginaw Bay, Lake Huron (hereafter combined and referred to as ‘coastal fringing wetlands’; Fig. 1). Yellow perch abundances are described amongst coastal fringing wetlands but not directly compared because of population genetic differences that exist throughout the Great Lakes that may confound direct comparisons over large spatial scales (Miller 2003; Parker et al. 2009a; SepulvedaVillet et al. 2009). We did, however, directly compare yellow perch abundances between neighboring DRM lake and wetland habitats for 3 years when both habitats were sampled simultaneously. Parker et al. (2009a) found that yellow perch populations appear to 123 Wetlands Ecol Manage (2012) 20:137–150 be homogenous within DRM systems, therefore, there should be no concern about genetic differences confounding adjacent lake-wetland comparisons. Because Great Lakes coastal wetland formation is limited to areas where adequate protection from wind, wave, and ice erosion is provided, our sites were unevenly distributed throughout our sampling region. The margins of Saginaw Bay and portions of Grand Traverse Bay are gently sloping and contain numerous lacustrine, open-embayment wetlands (Albert 2005; Albert et al. 2005). The substrate of Saginaw Bay and Grand Traverse Bay wetlands generally consists of a combination of sand, silt, clay, and some gravel, with relatively little organic sediment compared to other types of Great Lakes coastal wetlands (Albert 2005; Albert et al. 2005; Nelson et al. 2009). Emergent vegetation at our study areas in Saginaw Bay and Grand Traverse Bay was dominated by bulrushes (Schoenoplectus spp.), which formed nearly monodominant stands at most sites. Coastal fringing wetlands along the northern shore of Lakes Michigan and Huron and along the islands of the Beaver Archipelago consist of both open and protected embayments, many of which are distributed among rock till/island complexes (Burton et al. 2004; Uzarski et al. 2004; Albert et al. 2005). Like Saginaw Bay and Grand Traverse Bay, emergent vegetation at our study sites in northern Lakes Michigan and Huron and the Beaver Archipelago was dominated by bulrushes. The areas that we sampled contain the majority of the fringing wetlands on the Michigan shoreline of Lakes Huron and Michigan. The DRM systems along the eastern shore of Lake Michigan have direct surface water connections to Lake Michigan via channels, are hydrologically influenced by Lake Michigan, and receive riverine inputs (Keough et al. 1999; Wilcox et al. 2002; Jude et al. 2005). Some DRM systems, such as Muskegon Lake, receive large riverine inflows based on watershed area. These systems accumulate thick organic sediments, often [2 m deep (Albert 2003; Albert et al. 2005). Emergent vegetation within the Lake Michigan DRM wetlands that we sampled was heterogeneous and contained intermixed stands of yellow pond lily (Nuphar advena), arrow arum (Peltandra virginica), arrowhead (Sagittaria spp.), bur reed (Sparganium spp.), water lily (Nymphaea odorata), and cattail (Typha spp.). Wetlands Ecol Manage (2012) 20:137–150 141 For the direct comparisons of yellow perch between adjacent DRM lake and wetland habitats, the two systems were delineated by identifying the confluence of the main tributary river with the lake. Upstream areas from the confluence were considered ‘‘wetland’’ and downstream areas considered ‘‘lake’’ macrohabitat (Cooper et al. 2007a, 2009). Within the lake and wetland macrohabitats, we sampled three microhabitats: monodominant stands of emergent lily vegetation (‘‘lily,’’ either N. advena or Nympheae odorata), submerged aquatic vegetation (‘‘SAV’’, usually dense beds of Myriophyllum spicatum, which also contained intermixed Ceratophyllum demersum, Potomogeton crispus, and other Potomogeton spp.), and bare sediment (‘‘bare’’). Microhabitats were randomly chosen within each macrohabitat based on availability. Three replicate nets were fished in each microhabitat (18 nets total per system). Fish sampling Fish were sampled for multiple years in DRM and coastal fringing wetlands (Table 1). The sampling protocol used was the same among all sites and years and took place from June to September (Uzarski et al. 2005; Cooper et al. 2007a, 2009). At each site, a minimum of three fyke nets were set with the net lead bisecting a stand of emergent vegetation. Small fyke nets (mouth opening: 0.5-m 9 1.0-m) were fished in water depths of 0.2–0.5 m and large fyke nets (mouth opening: 1.0-m 9 1.0-m) were fished in water depths of 0.5–1.0 m. Both sizes of fyke nets had 7.3-m leads, 1.8-m wings (set at a 45° angle from the lead), and 4.8-mm mesh. Since net height was the only difference between the two net sizes, data from both net sizes were pooled (Uzarski et al. 2005). All nets were soaked for one net-night (usually 24 h, see Uzarski et al. 2005 for complete description of sampling methods). All fish captured were identified, enumerated, and released. Size measurements were made on a subset of fish collected because only fish abundances were a question of concern for the initial studies from which these data originated. Therefore, all size classes of yellow perch captured, ranging from young-of-theyear (YOY) to adult were pooled. No larval fish were targeted or collected. From 2004 to 2006, DRM lakes and their associated wetlands were sampled simultaneously (Cooper et al. 2007a, 2009), allowing us to test whether yellow perch were more abundant in lakes or adjacent wetland habitats. Four DRM complexes were sampled in 2004: Pentwater, White, Muskegon, and Kalamazoo. In 2005 and 2006, two additional systems were sampled: Lincoln and Pigeon. To control for habitat within the lake and wetland systems, we sampled the same microhabitats (lily, SAV, and bare) within each lake and wetland macrohabitat to avoid confounding (Cooper et al. 2007a, 2009). Chemical/physical measurements Twelve chemical/physical variables were measured at each site. Sulfate, chloride, ammonium, nitrate, soluble reactive phosphorus, and alkalinity were measured from samples collected with 1-L acid-washed polyethylene bottles following standard methods (American Public Health Association 1998). Dissolved oxygen, percent dissolved oxygen saturation, pH, temperature, specific conductance, and turbidity were measured in situ using a HydroLab DataSonde 4a Table 1 Yellow perch total catch and catch per unit effort (CPUE) in coastal fringing and drowned river mouth wetlands Years sampled Yellow perch catch Nets fished CPUE Coastal fringing 2001–04, 06, 08, 09 18,801 585 32.1 Drowned river mouth 2000–06 81 494 0.2 Beaver Archipeligo 2008–09 0 42 Grand Traverse Bay 2004 2 18 0.1 Northern Lakes Huron and Michigan 2001–04, 06, 08, 09 2,188 320 6.8 Saginaw Bay 2002–04, 06–09 16,611 205 81.0 All wetlands Coastal fringing by regions 0 Yellow perch total CPUE in coastal fringing wetlands is further divided into descriptions by regions 123 142 (Hach Corporation, Loveland, CO, USA). All chemical/physical measurements were taken during daylight hours at mid-depth from a boat before fish sampling began. Statistical analyses Separate principal components analyses (PCAs) were used to explore variation in the 12 chemical/physical variables between DRM and coastal fringing wetlands, among coastal fringing systems based on region, and between DRM lake and wetland habitats. The DRM lake chemical/physical data were excluded when comparing wetland types. Chemical/physical variables were averaged over all years that a site was sampled. To interpret PCAs, we labeled component scores in the PCA bi-plots by wetland type (coastal fringing (n = 47), DRM (n = 15)), and by region in coastal fringing wetlands (Beaver Archipelago (n = 5), Grand Traverse Bay (n = 3), northern Lake Michigan-Huron (n = 18), and Saginaw Bay (n = 21)), or microhabitat type in DRM systems (lake lily, SAV, bare, wetland lily, SAV, bare). Principal components were calculated using a correlation matrix, which gives equal weighting to all variables and was preferable over a covariance matrix in our study because the units of measurement differed among the variables used (Noy-Meir et al. 1975; McGarigal et al. 2000). To generate hypotheses as to whether yellow perch abundances were related to abiotic conditions, linear and quadratic regression models were run on transformed [loge(n ? 1)] yellow perch CPUE versus PC (principal component)-1 scores for the coastal fringing wetlands. We tested for statistical differences between coastal fringing and DRM wetlands with a multi-response permutation procedure (MRPP; Mielke 1984; Zimmerman et al. 1985). Euclidean distance measures and a natural weighting (n/sum[n]), recommended by Mielke (1984), were used in the MRPP, and we defined significance as a = 0.05. Since six comparisons were made within the coastal fringing systems based on region (Beaver Archipelago, Grand Traverse Bay, northern Lake Huron-Michigan, and Saginaw Bay), we Bonferroni-corrected for multiple comparisons, in order to guard against a Type I error, and significance was defined as a = 0.008. Chemical/ physical variables were also compared using MRPP between DRM (n = 15) and coastal fringing wetlands 123 Wetlands Ecol Manage (2012) 20:137–150 (n = 37) on a subset of data for years (2001–04, 2006) when both system types were sampled. Among coastal fringing wetlands, we also compared chemical/physical variables between northern Lake Michigan-Huron (n = 20) and Saginaw Bay (n = 17) and Saginaw Bay (n = 8) and Beaver Archipelago (n = 5) on a subset of data for years (northern Lake Michigan-Huron and Saginaw Bay: 2002–04, 2008; Saginaw Bay and Beaver Archipelago: 2008–09) when both regions were sampled. These comparisons were made to guard against confounding temporal and spatial variation in the previous comparisons because not all wetland types or regions were sampled in all years (Table 1). To compare yellow perch catch per unit effort (CPUE; defined as catch per net night) in DRM lakes and wetlands from 2004 to 2006, we used a three-way split-plot analysis of variance (ANOVA) to test whether catches differed between macrohabitats (lake and wetland), microhabitats (lily, SAV, and bare), and their interaction (Montgomery 1991). System served as the blocking variable, macrohabitat was the whole plot, and microhabitat was the subplot (Cooper et al. 2007a). We used the system–macrohabitat interaction as the whole plot error term and the system–macrohabitat–microhabitat interaction as the subplot error term. Yellow perch CPUE was averaged over the years sampled for the analysis: 3 years for Pentwater, White, Muskegon, and Kalamazoo and 2 years for Lincoln and Pigeon. Yellow perch CPUE was transformed [loge(n ? 1)] prior to analysis to homogenize variance based on residual plots. When significant interactions were found, Tukey’s HSD tests were performed post hoc to identify differences. Results We captured a total of 81 yellow perch in 7 years of sampling (0.2 fish net-night-1) in DRM wetlands, whereas we captured 18,801 yellow perch in 7 years of sampling (32.1 fish net-night-1) in coastal fringing wetlands. Among the coastal fringing wetlands, most yellow perch were captured in Saginaw Bay (Table 1). Coastal fringing wetlands were sampled in 2008 and 2009, whereas DRM wetlands were not. In 2008 and 2009, 12,438 yellow perch were captured (129.6 fish net-night-1) in coastal fringing wetlands. If the 2008 and 2009 sampling years are excluded, yellow perch catch in coastal fringing wetlands was still Wetlands Ecol Manage (2012) 20:137–150 143 6 A A 8 - Coastal fringing - DRM 6 % D.O. D.O. pH 4 SO4 TEMP CL COND 2 NO3 TURB ALK NH4 0 -2 PO4 -4 -4 between DRM and coastal fringing wetlands when a subset of the entire analysis was done that only compared years in which both wetland types were sampled (t = -5.035, p = 0.003). The PCA bi-plot representing the 47 coastal fringing sites showed a clear distinction between northern Lakes Michigan and Huron sites (including Grand Traverse Bay and the Beaver Archipelago) and Saginaw Bay sites in PC-1, which explained 27% of the variability in the correlation matrix (Fig. 2b). The MRPP confirmed our visual interpretation of differences in chemical and physical conditions between regions (t = -11.439, p \ 0.001). Subsequent MRPP pairwise comparisons revealed significant differences (after Bonferroni-correction for multiple comparisons) between Saginaw Bay and northern Lakes Huron and Michigan, Beaver Archipelago and Saginaw Bay, and Beaver Archipelago and Grand Traverse Bay (Table 2). Similarly, when comparisons were made on a subset of data from regions that were sampled at the same time, we found that chemical/ physical conditions in northern Lakes Michigan and -2 0 2 4 6 8 Principal Component 2 19% Principal Component 2 19% considerably higher than DRM wetlands with 6,363 (13.01 fish net-night-1) caught in coastal fringing wetlands compared to 81 (0.2 fish net-night-1) caught in DRM wetlands at the same time. The PCA bi-plot representing all 62 sites showed a clear distinction between DRM and coastal fringing wetlands in PC-1, which explained 29% of the variability in the chemical/physical correlation matrix (Fig. 2a). Principal component-1 was best explained as a gradient of anthropogenic disturbance (i.e. increasing chloride, conductivity, and turbidity along PC-1) and heterotrophic productivity/organic sediment accumulation (i.e. decreasing dissolved oxygen and pH along PC-1). DRM wetlands received higher PC-1 scores and were associated with higher chloride, conductivity, alkalinity, and turbidity, while coastal fringing wetlands received lower PC-1 scores and were associated with higher dissolved oxygen and pH. The MRPP revealed a significant difference in conditions between DRM and coastal fringing wetlands confirming our interpretation of the PCA bi-plot (t = -3.809, p = 0.010). We also found the same general pattern B B 4 2 SO4 Principal Component 2 (19%) COND CL TEMP PO4 NO3 NH4 0 ALK TURB -2 -4 -4 -2 0 2 4 6 Principal Component 1 27% - Lake bare - Lake lily - Lake SAV - Wetland bare - Wetland lily - Wetland SAV %D.O. CC - Saginaw Bay - N Lakes Huron-Michigan - Grand Traverse Bay - Beaver Archipelago pH Principal Component 1 29% 4 D.O. % D.O. D.O. pH 2 SRP TEMP NH 4 0 CL SO 4 -2 SPC NO 3 ALKTURB -4 -4 -2 0 2 4 Principal Component 1 (38%) Fig. 2 Principal components analyses of 12 chemical/physical variables for a all wetlands b coastal fringing wetlands, and c drowned river mouth lake and wetlands. Arrows eigenvectors multiplied by 10 to scale to the bi-plot area (TEMP temperature, DO dissolved oxygen, %DO percent dissolved oxygen saturation, COND specific conductance, pH, TURB, turbidity, ALK alkalinity, NH4 ammonium, NO3 nitrate, SRP soluble reactive phosphate, SO4 sulfate, CL chloride) 123 144 Wetlands Ecol Manage (2012) 20:137–150 Table 2 MRPP test statistics (t) and p values for pair-wise comparisons of chemical/physical variables between different coastal fringing wetland regions Beaver Archipelago Grand Traverse Bay Northern Lakes Huron and Michigan -4.10, 0.004 -2.91, 0.018 Grand Traverse Bay Saginaw Bay -8.99, <0.001 -2.64, 0.025 -1.13, 0.125 Northern Lakes Huron and Michigan -16.41, <0.001 p values in bold indicate significance at a = 0.008 Huron differed from Saginaw Bay (t = -9.828, p \ 0.001) and Beaver Archipelago differed from Saginaw Bay (t = -2.646, p = 0.02). Similar to the PCA of all 62 sites, PC-1 of the coastal fringing analysis was best explained as a gradient of anthropogenic disturbance (i.e. increasing chloride, conductivity, and turbidity along PC-1) and heterotrophic productivity/organic sediment accumulation (i.e. decreasing dissolved oxygen and pH along PC-1). Saginaw Bay wetlands received higher PC-1 scores and were associated with higher chloride, conductivity, and turbidity, while northern Lakes Michigan and Huron wetlands received lower PC-1 scores and were associated with higher pH and dissolved oxygen. Both the linear (r2 = 0.18) and quadratic regression models (r2 = 0.26) of yellow perch CPUE versus PC-1 scores for coastal fringing wetlands were significant (p \ 0.001; Fig. 3), although the quadratic regression model provided a better fit. DRM lakes versus wetlands Yellow perch were captured in all DRM lake and wetland microhabitats over the 3 years they were simultaneously sampled (Fig. 4). Overall, DRM lake macrohabitats tended to have higher yellow perch CPUE than their associated wetlands (Table 3). Within the lake macrohabitats, yellow perch were most abundant in the SAV microhabitats. Only two yellow perch were caught in the four systems sampled in 2004. In 2005 yellow perch were collected in all microhabitats except wetland bare substrate and wetland SAV and in 2006, yellow perch were collected in all microhabitats. The interaction between microhabitat and macrohabitat was marginally significant; macrohabitat explained most variability in yellow perch catch (Table 3). Yellow perch CPUE at lake-SAV sites was significantly greater than at all other microhabitats (Table 4). 6 40 Yellow perch catch Yellow Perch CPUE (ln + 1 transformed) 50 5 4 3 30 20 2 10 1 0 BARE LILY Lake 0 -4 -2 0 2 4 BARE LILY SAV Wetland 6 Principal Component 1 Scores Fig. 3 Regression of coastal fringing wetland yellow perch catch and principal component 1 scores fitted with linear and quadratic regression lines 123 SAV Fig. 4 Yellow perch catch (±1 standard error) showing differences between macrohabitats (lake, wetland) and among microhabitats (bare bare sediment, lily lily habitat, SAV submerged aquatic vegetation habitat) in Lake Michigan drowned river mouth systems Wetlands Ecol Manage (2012) 20:137–150 145 Table 3 Split-plot analysis of variance results for the effect of macrohabitat (lake and wetland), microhabitat (lily, bare, and SAV), and the interaction of macrohabitat and microhabitat on Source of variation average yellow perch catch in drowned river mouth systems from 2004 to 2006 df MS F p 10.30 0.024 System 5 0.71 Macrohabitat 1 2.17 Whole plot errora 5 0.21 Microhabitat 2 0.61 1.67 0.213 Microhabitat 9 Macrohabitat 2 1.11 3.07 0.069 20 0.36 Subplot error b System was used as the blocking variable a Whole plot error term was the system 9 macrohabitat effect b Subplot error term was the system 9 macrohabitat 9 microhabitat effect p values in bold indicate significance at a = 0.05 Table 4 Tukey’s HSD p values for pair-wise comparisons of average yellow perch catch per net-night per microhabitat in drowned river mouth systems from 2004 to 2006 Lake bare Lake lily Lake lily Lake SAV Wetland bare Wetland lily Wetland SAV 0.410 0.047 0.357 0.441 0.230 0.008 0.920 0.956 0.696 0.006 0.009 0.003 0.877 0.771 Lake SAV Wetland bare Wetland lily 0.656 p values in bold indicate significance at a = 0.05 When comparing the chemical/physical properties of DRM wetlands and lakes using PCA, conditions at some lake sites appeared more similar to wetlands and received PC-1 and PC-2 scores that were within the range of wetland sites (Fig. 2c). Accordingly, the MRPP comparing lake and wetland chemical/physical characteristics was not significant (t = -1.3, p = 0.10). Discussion All wetlands We found that yellow perch were much more abundant in coastal fringing wetlands than DRM wetlands. When 2008 and 2009 catches in coastal fringing wetlands were removed so that timing was consistent between both wetland types, there was a large reduction in the total number of yellow perch caught. However, the CPUE for yellow perch in coastal fringing wetlands was still 65 times higher than in DRM wetlands when keeping sampling years consistent. Although we did not measure all of the yellow perch that we sampled, the vast majority that we caught in the coastal fringing wetlands were less than 6 cm standard length (SL), which would be age-0 (e.g. Fitzgerald et al. 2001). Thus, given our observations on the size of yellow perch captured, and large difference in yellow perch CPUE in DRM versus coastal fringing wetlands, we strongly suspect that even if we had measured a different metric such as yellow perch biomass, we would still find the same general distribution pattern throughout Great Lakes coastal wetlands. For instance, Parker et al. (2009b) captured 1,092 yellow perch throughout the Saginaw Bay wetlands in July–August, 2004 and retained all fish greater than 6 cm SL for diet analysis. Only 43 (3.9%) of the 1,092 yellow perch captured were greater than 6 cm SL, and age analysis revealed that 42 were age-1 and one was age-2 (Parker et al. 2009b). Because of the dominance of age-0 fish sampled, we propose that coastal fringing wetlands serve as important nursery habitat for young yellow perch. 123 146 However, the role of DRM wetlands as nursery habitats is less clear because so few yellow perch were captured in those habitats. We propose that the difference in yellow perch CPUE between DRM and coastal fringing wetlands is likely a real phenomenon resulting from different habitat characteristics rather than differences in fyke net capture efficiencies between the coastal fringing and DRM wetlands. Fyke nets have been used successfully to capture fish in a variety of littoral habitats (Wilcox et al. 2002; Uzarski et al. 2005; Ruetz et al. 2007). Moreover, the fyke nets were soaked in the same depths at both habitats (0.5–1 m), which adequately sampled all wetland areas characterized by emergent vegetation. Most adjacent deep-water areas ([1 m) were devoid of emergent vegetation and were not sampled because we were interested in sampling fish that were in the wetland habitats. Yellow perch in DRM systems most likely inhabit deeper areas ([1 m) adjacent to the wetlands. Overall, DRM wetlands were more eutrophic than coastal fringing wetlands and had lower dissolved oxygen concentrations and pH. Our PCA results were similar to Uzarski et al. (2005). Uzarski et al. (2005), found three groupings using consistent sites and parameters: sites with low PC-1 and PC-2 scores tend to be the least disturbed, and those with high PC-1 scores and either high or low PC-2 scores, tend to be more disturbed areas. When DRM and coastal fringing wetland comparisons were made to only include years when both types were sampled, DRM wetlands were still more productive/eutrophic than coastal fringing wetlands. DRM wetlands have a tendency to be eutrophic because of sediment and detritus from the rivers that is deposited in these delta-like systems (Albert 2003; Albert et al. 2005; Jude et al. 2005). DRM wetlands also are not subject to waves or pelagic mixing from Lake Michigan, thus minimizing the flushing of organic material from the wetland (Jude et al. 2005; Nelson et al. 2009). Interestingly, and consistent with Cooper et al. (2007b), we found that DRM wetlands with the highest conductivity, turbidity, and chloride did not have the highest dissolved nutrient concentrations, which contribute to eutrophication. This may be due to different sources of solutes (e.g. urban versus agricultural land uses) or because we measured only dissolved forms of nutrients. In the most eutrophic and impacted systems, a large proportion of nutrients were likely sequestered in biomass 123 Wetlands Ecol Manage (2012) 20:137–150 during the summer when we sampled (Uzarski et al. 2004, 2005). Eutrophication of DRM wetlands and the periods of low dissolved oxygen concentrations that are associated with it, especially at night, may repel yellow perch, which others have found as well (Coble 1982; Suthers and Gee 1986). In contrast, Cooper (2009) found that night-time dissolved oxygen measurements in Saginaw Bay and northern Lakes Huron and Michigan coastal fringing wetlands were rarely hypoxic (\30% saturation), which is preferred by yellow perch (Coble 1982; Suthers and Gee 1986) and noon temperatures never exceeded 29°C, which are lethal to yellow perch (Hokanson 1977). Coastal fringing wetlands We found that among coastal fringing wetlands, yellow perch were more prevalent in Saginaw Bay. Saginaw Bay had more eutrophic/productive characteristics (higher conductivity, chloride, turbidity) and warmer temperatures than wetlands in northern Lakes Michigan and Huron and Beaver Archipelago. Furthermore, Saginaw Bay was more eutrophic/productive than northern Lakes Huron and Michigan and Beaver Archipelago when both systems were sampled in the same year. Although we did not directly measure ecosystem productivity, others have established that Saginaw Bay is a very productive ecosystem (e.g. Sprules and Munawar 1986). Saginaw Bay had lower dissolved oxygen concentrations than the other coastal fringing wetlands, which is characteristic of heterotrophically-productive systems (Wetzel and Likens 2000). Brazner (1997) found high numbers of yellow perch in the coastal fringing wetlands of Green Bay, which is a shallow, productive embayment (albeit mostly at the southern end of the bay) of Lake Michigan, located at similar latitude to Saginaw Bay. According to Eshenroder (1977), despite Saginaw Bay’s eutrophic status, frequent wind-mixing of the water column maintains adequate oxygen concentrations for fish. Yellow perch in higher latitudes generally have shorter growing seasons than those in the lower latitudes, which is primarily due to differences in water temperature (Power and Van Den Heuvel 1999). Warmer temperatures lead to early hatch dates for yellow perch before the initial spring zooplankton bloom (Fitzgerald et al. 2001), which are important prey during larval (Bremigan et al. 2003) and juvenile Wetlands Ecol Manage (2012) 20:137–150 stages (Parker et al. 2009b). Zooplankton were not regularly sampled at the same time as yellow perch; however, Gyekis (2006) collected zooplankton from coastal fringing wetlands in Grand Traverse Bay, Saginaw Bay, and northern Lakes Huron and Michigan during the summer of 2004. Similar amounts of zooplankton biomass were sampled in northern Lakes Huron and Michigan and Saginaw Bay, but Grand Traverse Bay had very low biomass (Gyekis 2006). Gyekis (2006) did not sample zooplankton until July 2004, so while the amounts of zooplankton in Saginaw Bay and northern Lakes Huron and Michigan were similar by July, early peak zooplankton levels may be different across systems. Yellow perch in Saginaw Bay wetlands consumed zooplankton until they were about 3.5 cm SL, fed mainly on macroinvertebrates at 5–6 cm SL, and became primarily piscivorous by age 1 (Parker et al. 2009b), which is earlier than typically reported (e.g. Keast 1985), suggesting that these wetlands may provide abundant invertebrate and small fish prey. Thus, hatching during the spring zooplankton bloom in regions with a relatively long-growing season, such as Saginaw Bay, could benefit yellow perch because attaining a large size is important for preventing overwinter mortality from starvation, intolerance to environmental extremes, and predation (Sogard 1997). However, Fitzgerald et al. (2004) proposed that overwinter mortality was not a significant source of yellow perch loss in southeastern Lake Michigan. We hypothesize that water temperature and productivity are important factors that explain the high abundance of juvenile yellow perch in Saginaw Bay wetlands. We found that yellow perch CPUE peaked in the moderately-productive coastal fringing wetlands along a disturbance gradient and decreased in the more-impacted systems. A quadratic relationship along a disturbance gradient seems most appropriate, rather than yellow perch abundance continuing to increase as disturbance increases. This is also evidenced by the lack of yellow perch in the more disturbed/eutrophic DRM wetlands. Increased nutrient concentrations result in increased algal and zooplankton biomass (e.g. Vanni 1987). However, excess eutrophication has been shown to be detrimental to yellow perch (Schaeffer et al. 2000; Tyson and Knight 2001). We propose that moderately-productive coastal fringing wetlands are most strongly preferred by yellow perch. 147 DRM systems Catch per unit effort was variable from 2004 to 2006 when directly comparing yellow perch CPUE in connected DRM lakes and wetlands. Despite the variability among years, yellow perch CPUE tended to be lower in the wetland macrohabitat as well as lakelily and lake-bare microhabitats relative to the lakeSAV microhabitat in DRM systems. In 2005, the Muskegon and White lake-SAV microhabitats had high yellow perch CPUEs (81 and 53.67 fish netnight-1, respectively), followed by Pigeon (8 fish netnight-1), and then Pentwater (0.33 fish net-night-1), Lincoln (0 fish net-night-1), and Kalamazoo (0 fish net-night-1). While yellow perch were completely absent from some lakes, there also was not just one high CPUE at one lake that singly increased the average CPUE. This coupled with the fact that we sampled the same microhabitats in both the wetland and lake macrohabitats, so as not to confound comparisons, leads us to conclude that yellow perch are more abundant in DRM lakes than wetlands. Our conclusion that yellow perch are more abundant in DRM lake macrohabitats is further supported by observations in the littoral zone of Muskegon Lake (a DRM lake) where high numbers of age-0 and adult yellow perch have been captured using boat electrofishing and fyke netting (Ruetz et al. 2007; Bhagat and Ruetz 2011). By comparing microhabitats within both lake and wetland macrohabitats, we were able to provide further insight into yellow perch habitat use within DRM systems. We propose that yellow perch prefer lake-SAV because lake-lily sites most likely become hypoxic at night due to organic matter accumulation while the SAV sites offer more protection from predation than the bare sites (e.g. Rozas and Odum 1988). Chubb and Liston (1986) found most larval yellow perch in wetlands associated with the Pentwater DRM system during April and May, suggesting that reproduction occurs in DRM wetlands. Moreover, larval yellow perch were absent from the bayou portions of this wetland complex by July, but they were present in the main channels, which were cooler and more oxygenated (Chubb and Liston 1986). We observed that conditions did not become hypoxic (lowest dissolved oxygen concentration over a 24-hour period was 5.91 mg L-1 (51.8% saturation) at 9.0°C) in a DRM wetland associated with the Muskegon River during April, 2005 (DGU ‘‘unpublished data’’). 123 148 Furthermore, yellow perch egg skeins tend to become entangled on the upper portions of macrophytes, logs, and other substrates when spawning (Scott and Crossman 1973), which keeps the eggs of percids suspended above the hypoxic sediment (Regier et al. 1969). There is also evidence that Perca species may intentionally wrap their egg skeins around plant stems (Treasurer 1981; Patrick Hudson, USGS ‘‘personal communication’’). Most of our sampling in DRM wetlands occurred in July and yielded few yellow perch. We hypothesize that soon after yellow perch larvae hatch in DRM wetlands they disperse downstream to the lake ecosystems. No differences were found between DRM lake and wetland macrohabitats when comparing chemical/ physical variables, despite an apparent dissolved oxygen gradient (Snodgrass et al. 1996) along PC-2. This is probably because some of the lily habitats within the lakes shared the same characteristics as wetlands. Nelson et al. (2009) did not find significant differences between DRM lakes and wetlands when comparing dissolved oxygen, total dissolved solids, turbidity, pH, and chlorophyll a (all measured during daytime). However, Nelson et al. (2009) did find that DRM wetlands had greater organic sediment depth, less water movement, and more hypoxic conditions (measured at nighttime) than adjacent lake habitats. Organic sediment depth and water movement in wetland habitats, which we did not measure, most likely affect yellow perch distribution among certain habitats (discussed below) and may be the principal reasons why we found low numbers of yellow perch in DRM wetlands. Low to hypoxic nighttime dissolved oxygen concentrations have been observed in DRM wetlands (Chubb and Liston 1986; Nelson et al. 2009). Additionally, we observed dissolved oxygen concentrations fluctuate, in a 24-hour period, from 1.1 mg L-1 (13.1% saturation) at 14.3°C in the early morning to 9.5 mg L-1 (121.6% saturation) at 27.0°C in the afternoon in the DRM wetland associated with the Muskegon River during August 2005 (DGU ‘‘unpublished data’’). The optimal temperature for adult yellow perch is 24.7°C with lethal temperatures ranging from 29 to 34°C (Hokanson 1977). Coble (1982) found more yellow perch in areas of a large river where dissolved oxygen was C5 mg L-1, and Suthers and Gee (1986) found that juveniles completely avoided sections of a prairie marsh that had 123 Wetlands Ecol Manage (2012) 20:137–150 mean dissolved oxygen concentrations B1.5 mg L-1. During summer, hypoxia most likely repels yellow perch from DRM wetlands at night, and they may avoid those habitats during the day because of high temperatures. We hypothesize that nighttime hypoxia, caused by a combination of low water movement and deep organic sediment, and high daily water temperatures limit the use of some DRM-wetland habitats by juvenile yellow perch during summer. Conclusions We observed that Great Lakes yellow perch are substantially more abundant in coastal fringing wetlands than DRM wetlands. Among the coastal fringing wetlands that we sampled, Saginaw Bay had the highest CPUE of yellow perch. The yellow perch that we captured in Saginaw Bay were mostly age-0, indicating that these wetlands serve as important nursery areas. The population genetic structure of yellow perch in the Great Lakes suggest distinct populations in southern Lake Michigan, northern Lakes Michigan and Huron, and Saginaw Bay (Miller 2003; Parker et al. 2009a). Thus, inferring differences among wetland types or regions based on chemical/ physical characteristics may be confounded with local population dynamics (e.g. low or high recruitment) of yellow perch in the adjacent Great Lake. We suspect that laboratory studies and field investigations conducted at appropriate spatial scales could provide useful approaches for testing our hypotheses related to age-0 yellow perch distribution in wetlands and chemical/physical variables. Among Lake Michigan DRM systems, we found that yellow perch tended to be more prevalent in the downstream lake macrohabitats relative to upstream wetlands. No differences were found between the chemical/physical characteristics of the lakes and wetlands, which may mean that other variables, not measured in this study (such as organic sediment depth and water movement), may govern yellow perch distribution in DRM systems. Our results show that some wetlands, such as those in Saginaw Bay, provide important nursery habitats during summer for juvenile yellow perch and should be recognized as such when drafting and implementing fisheries management plans. Acknowledgments Funding for the various studies that generated this dataset came from the Great Lakes Wetlands Ecol Manage (2012) 20:137–150 Commission, Great Lakes Protection Fund, Michigan Department of Environmental Quality, Michigan Department of Natural Resources, U.S. Environmental Protection Agency, and U.S. Fish and Wildlife Service. ADP was funded by a research assistantship from Grand Valley State University’s Annis Water Resources Institute. 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