Mr. Robert Van Dam,

If you are trying to fit VonBer. curve to data such as length at capture
, length at REcapture and time at large I would suggest to use FISAT
software fits either a seasonal with sin function or a strait forward
VB curve. FISAT is a combination of ELEFAN and LFSA distributed by FAO
in Rome .
you can search the address through internet addresses.

Good luck