A mathematical equation for describing growth of freshwater species

We introduced a new four-parameter growth equation and tested it with obser ved growth data sets for a variety of aquatic species. The equation is: W = W-f - (W-f - W-o)/{c . (1 - W-o/W-f) . [1 - exp(kt)] + exp(kt)} where W-f and W-o are the upper asymptotic and initial values respectively, and c and k are constants. The new equation is a modification of the logis tic and the Spillman equations with a special value of parameter c. Unlike the logistic and the Spillman functions, the new model has an unfixed value of the inflection point as dictated by the additional parameter c. We comp ared the model to the logistic, Spillman, Gompertz, and Bertalanffy equatio ns using 10 sets of reference growth data of freshwater species ranging fro m protozoans to crustaceans to fishes. The new equation yielded excellent f its to each data set, which suggests that it is worthy of being considered by freshwater growth data analysts.