The Relationship between Littoral Zone Fish Habitat Metrics and Species Assemblages in North Temperate Lakes
Orlando, Jessica L.
University of Wisconsin-Stevens Point, College of Natural Resources
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The complexity and diversity of littoral zone fish habitat can be highly variable. These nearshore habitats serve as important foraging, refuge, spawning, and nursery sites for most north temperate fish species. For many lakes, alterations of the adjacent riparian zone, lake basins, and watersheds may affect fish habitat directly and indirectly. Previous studies indicate that lakeshore development can be associated with loss of nearshore littoral zone habitat and can negatively affect fish assemblages. A proactive approach may be necessary to prevent negative effects on fish assemblages and indexing fish habitat could assist lake managers estimate functionality of physical habitat, predict habitat limitations, and develop management strategies. However, few studies have looked at fish assemblage relationships with multiple habitat dimensions. The objective of this study was to determine if there is a relationship between nearshore littoral zone habitat metrics and fish assemblages within and among north temperate lakes. Clear lakes in northern Wisconsin less than 210 ha with highly diverse littoral zones were selected for this study. Habitat was quantified at 25-30 sites in each lake by measuring depth, macrophyte bed structure, substrate, and woody structure; fish assemblage was quantified at these same sites using electrofishing. Physical habitat metrics used for analyses were mean depth, mean coarse substrate cover, mean floating leaf macrophyte cover, mean macrophyte cover, mean macrophyte height, mean medium woody structure (MWS) cover, sum of coarse woody structure (CWS) branching complexity within site, and sum of CWS diameter within site. The relationships between these habitat metrics and fish species diversity, richness, relative abundance, mean length, cyprinid abundance, centrarchid abundance, and percid abundance were tested using univariate linear regression, multiple linear regression, canonical correspondence analysis (CCA), and hierarchical cluster analysis. Overall, 26 fish species were collected, representing 9 families, and fish assemblage metrics were correlated with many of the nearshore littoral zone habitat metrics tested within lakes. These fish-habitat relationships tended to be lake-specific or lake-type specific due to among-lake differences in habitat availability, habitat sampled, and fish assemblage structure. However, there were some general trends in fish-habitat relationships among lakes that were specific to a particular fish assemblage metric. Increased depth was related to centrarchid abundance in 7 of 11 lakes. Macrophyte height was found to be related to species richness in 5 of 11 of lakes. Macrophyte cover was predictive of relative fish abundance in 6 of 11 lakes and species diversity in 5 of 11 lakes. While the majority of lakes had positive fish assemblage relationships with macrophyte cover, there were three lakes with negative relationships. Hierarchical cluster analysis revealed that these lakes were included in one group, along with two additional lakes, which may represent a lake type with limited or less suitable macrophyte habitat, or comprised of fish assemblages that do not relate to macrophyte habitat. Metrics relating to woody structure (MWS and CWS) were consistently predictive of fish assemblage. CWS diameter was predictive of mean length in 6 of 11 lakes. While fish abundance has been shown to increase with CWS branching complexity, high branching complexity CWS was rare in the nearshore littoral zones sampled and was frequently detached from the bole (thus categorized as MWS). However, centrarchid abundance was correlated with MWS in 6 of 11 lakes and species richness was correlated with MWS in 5 of 11 lakes. Generally, MWS was observed as tree branching that had fallen in from the riparian zone, pieces of CWS crown that had broken off from the bole, and beaver lodges; these were also the only habitats where certain fish species were sampled, including walleye (Sander vitreus). Fish assemblages were not sensitive to all habitat metrics tested. Coarse substrate was less related to fish assemblage than other habitat metrics tested, but may be more predictive of fish assemblage at the guild level (e.g., benthic fish). Floating leaf macrophyte cover was also less predictive of fish assemblage and may represent a habitat where electrofishing gear was inefficient, as visibility into the water column can be obscured. While relationships varied among metrics tested, fish assemblages were generally related to habitat complexity, within and among lakes. While diverse and complex habitats may support greater fish abundance and more diverse fish assemblages, simple habitats may also be important foraging habitats for many fish species and metrics relating to openness may be important for indexing nearshore littoral zone habitat. Coexisting fish species and size classes utilize habitat and resources differently and even closely related species are known to partition resources among habitats. In general, complex habitats provide refuge from predation, whereas open habitats allow more efficient feeding. Intermediate complexity or edge habitats may maximize energetic gains and reduce predation risk for optimal foraging. The goal of this research was to develop and test multiple nearshore littoral zone habitat metrics that could be incorporated into a multimetric, habitat quality index for north temperate lakes. The development of littoral zone habitat metrics that are sensitive to fish assemblage could also guide protocols for more efficient habitat sampling through an inexpensive, rapid assessment approach that lake managers could use to measure habitat complexity across habitat types. Efficient and cost-effective assessment of habitat, based on sensitive and predictive metrics, could be a useful tool to measure habitat relative to fish assemblage in order to identify, protect, restore, or enhance habitats in littoral zones of north temperate lakes.