Tree regression analysis on the nesting habitat of smallmouth bass

Ecology, Jan, 1999 by Cynthia Rejwan, Nicholas C. Collins, L. Jerry Brunner, Brian J. Shuter, Mark S. Ridgway

INTRODUCTION

Smallmouth bass are among numerous species that reproduce in the nearshore environment of lakes, where physical and biological conditions are typically very heterogeneous. Goff (1985) and Shuter et al. (1980) have documented the fragility of this species during the first few weeks of life, when they are virtually immobile. These two circumstances suggest that the habitat requirements for smallmouth bass reproduction are probably not satisfied throughout the littoral zone. Therefore, identifying littoral conditions where nest aggregations are common could provide insight into the mechanisms influencing the production of young. Survival rates of smallmouth bass early in life are an important influence on year-class abundance in adulthood (MacLean et al. 1981). Consequently, predicting high-density spawning locations would be prudent for their protection from angling activities during the spawning/rearing period (which are known to severely compromise smallmouth bass reproductive success (Phillipp et al. 1997)), and from anthropogenic development in and around littoral areas (Christensen et al. 1996).

Smallmouth bass reproduction involves the creation of a nest (a depression in the sediment excavated by the male, containing offspring). High nest-site fidelity of returning adults (Ridgway et al. 1991a), suspected phylopatry (Gross et al. 1994), and stationary locations of nest aggregations from year to year (Rejwan et al. 1997) all suggest that habitat conditions influence the nest distributions of lake-dwelling smallmouth bass. Despite this evidence, no specific relationships between nest patch locations and habitat variables have been quantified (Rejwan et al. 1997). Of the numerous combinations of habitat conditions that may influence nest density, water temperature during the nest-guarding period, shoreline complexity, littoral-zone rugosity, and prevailing wind/wave exposure of study sites were measured in this study as a consequence of their importance to other aspects of smallmouth bass reproduction. Temperature, prevailing wind/wave exposure, and shoreline reticulation all affect the early survival (Shuter et al. 1980, MacLean et al. 1981, Goff 1985) and growth (Shuter et al. 1980, Ridgway et al. 1991b) of the young. Temperature is also an important influence on the year-class abundance of northern smallmouth bass populations (Shuter et al. 1980, MacLean et al. 1981, Serns 1982). Littoral benthic rugosity was measured since nest locations of riverine smallmouth bass populations are located in rugged river-bottom terrain (Sabo and Orth 1994).

Standard multiple-regression procedures have been commonly used to study relations between an organism and various aspects of its environment. However, nonlinear relationships (Philippi 1993) and heteroscedasticity (Dutilleul and Legendre 1993), which are typical of ecological data, limit the effectiveness of standard multiple regression. Furthermore, interactions among environmental variables that are also typical of ecological data (Philippi 1993) are difficult to detect using this method of analysis. Tree regression analysis is a relatively new (Breiman et al. 1984) and useful alternative statistical approach (Efron and Tibshirani 1991) since it does not require the existence of linear relationships among the variables or homoscedasticity in variances, and because interactions among habitat variables in relation to the organism are detected automatically in the analysis. Tree regression results present the relations among physical habitat conditions and nest density in an easily interpretable way by expressing nest-site selection as a hierarchy of decision and/or selection processes, based on physical habitat criteria. Since both tree and standard multiple regression are statistical analyses that quantify the significance and the relative importance of independent (habitat) variables on a dependent variable (nest density), the usefulness of tree regression analysis was compared with the better known standard regression analysis.

FIELD SAMPLING METHODS AND PRELIMINARY ANALYSES

Detection of smallmouth bass nests

Sampling was conducted in Lake Opeongo, Ontario (5780 ha., 45[degrees]42[minutes] N, 78[degrees]22[minutes] W), where smallmouth bass nest depths range between 0.4 and 2.5 m (most frequently from 0.5 to 1.5 m). By snorkelling along the 1-m depth contour, exact nest locations were recorded throughout the 155-km perimeter of Lake Opeongo in four years (1984, 1992, 1993, 1994), and throughout Jones Bay (a 6.3-km particularly high-density nesting area in Lake Opeongo) in eleven years (1977-1979, 1984, 1988-1994). These surveys of the smallmouth bass nesting population are among the most comprehensive records known for the breeding locations of a fish population (Rejwan et al. 1997).

Patchiness in nest distributions has been identified throughout a large range of spatial scales in Lake Opeongo (100-m to 10-km quadrat sizes) (Rejwan et al. 1997). Thirty-six 1-km-long littoral sites throughout Lake Opeongo and thirty-one 100-m-long littoral sites throughout Jones Bay, Lake Opeongo, were selected for measuring habitat conditions in such a way that they encompassed the entire ranges in nest densities in approximately uniform proportions.