A simple regression model to assess environmental effects on fish growth

Multiple regression was used to assess relationships between annular growth and environmental variables. This approach (1) tests for the significance of environmental changes on fish growth and (2) can be successfully used to predict fish growth using different environmental conditions. Age usually explains a large proportion of the variation in growth increments measured in length, and these two variables are inversely related. Since length increments decline as age increases, environmental impacts on growth will be reduced as fish grow older. To account for within-age growth variation and incorporate this age-dependent factor, the inverse of age (1/age in years) is multiplied by the value of an environmental variable (ENV) that may be related to growth. This interaction term and age are regressed against mean annular growth increments in length (TLINC; l _{t +1}— l ₁):
TLINC = b₀—b₁AGE±b₂(1/AGE)*ENV
where b₀, b₁, and b₂ are the regression coefficients for intercept and slope, respectively. Additional variables that measure environmental factors can be added to the model. Environmental effects associated with growth rates of black crappie, Pomoxis nigromaculatus (LeSueur), are presented as an example.