[Seasonally modified forms of the revised Janoschek function]

Early attempts for modifying growth functions to annual variations dating back up to 2 decades are recalled together with examples for their application showing rather different degrees of approximation. In order to secure independence from special functions, Sager (1982) has proposed a general concept for the modification of growth functions. In this way, examples were treated with the Pütter-Bertalanffy's for crustaceans and clams, with the Gompertz' for the goby, and with the Richards' for the pollack - a near relative of the cod. In continuation of these endevours, the revised Janoschek's is presented in 2 forms, namely one adapted to birth data or in an unbound variant. Special attention is given to the evaluation of the parameters used in nonlinear regressions for both forms. The new equation for bound growth opens a chance for giving real birth values even in seasonally changing growth if one deciding parameter will surmount 1 as has been the case in many applications heretofore. As an example the mussel Mytilus edulis taken from marine "farms" in the Menai-Straits of North Wales is treated with the seasonally modified Janoschek function. Although the special case cannot be realized in this case, the curves for length growth and growth increase are rather instructive compared with the basic behaviour with lacking annual variations. Approximations from 0.5 to 2.7 a show very close agreement with natural values of length as can be verified from numerical and graphical displays.