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Quantifying interaction strength between species is of interest in food webstudies for understanding population dynamics. Theory has run ahead ofexperiment in solving equations describing ecological systems (theLotka-Volterra equations, for example). Modeling approaches depend uponquantitative knowledge of interaction strengths among species, yet there arefew methods available for estimating strengths of interactions between species,especially in the field. Competition and predation in fishes is traditionallystudied with a variety of methods, most requiring extensive sampling todetermine stock densities or extensive stomach content or isotope analysis.Average relative weight of fish species in an ecosystem can usually beaccurately determined with much smaller sample sizes than required for accuratedetermination of stock densities. Correlations of mean annual relative weightsmay be used to test hypotheses and establish interaction strengths regardingcompetition and predation among fishes. The trends are sufficiently suggestiveto use a strong positive correlation as supporting evidence of competition andpossibly indicative of the magnitude of competition if other evidence such asanalysis of stomach contents is also present. Likewise, a strong negativecorrelation might be interpreted as relevant when forming a hypothesis ofpredation and as supporting evidence of the magnitude of predation if otherevidence is also present. In systems where regular weight and length surveydata are available, this method can suggest the likely strength of competitiveand predatory relationships and may be more cost effective than stomach contentor isotope analysis.

Estimating Interaction Strengths with Correlations in Annual Relative  Weight: Interspecific Competition and Predation in Fishes, Pueblo Reservoir,  Colorado