Some authors discussed growth of cultured species of crayfish. Evans and Jussila
(1997) recommend the use of the specific growth rate (SGR) as a standard for reporting. In
our study, we found that a single overall rate to describe growth of juvenile of redclaw
crayfish was not straightforward. As Hopkins (1992) has clearly indicated, SGR is an
estimate of parameter G in the exponential equation with a transformation of this equation.
When we estimated G by nonlinear regression procedures, final weight was systematically
over-estimated. The use of the transformation to determine SGR, on the other hand, would
help to overcome over-estimation, but would result in sub-estimation of crayfish weight
observed during the trials. This study indicates that growth rate of juvenile crayfish was
variable during the nursery stage, and that using G in the exponential equation, or SGR,
did not suffice for adequate description and comparison of growth curves between
treatments. The use of Eq. (1) and the invariance test allowed us to characterize growth
more adequately and reliably determine the effect of density on growth parameters, rather
on a single growth rate. Eq. (1) does not over-estimate final weight, and it presents good
close-to-linear estimation properties, thus making inferences from the invariance test
reliable. The importance of desirable, close-to-linear estimation properties has been
extensively discussed by Ratkowsky (1990). Lower values of residual variance (i.e.,
residual error corrected for degrees of freedom-number of parameters) and significant
parameter estimates for Eq. (1) indicated that there was no over-parameterization, and that
our equation was preferable over the exponential curve.