What is the halachic basis for treating poultry as fleishik? The torah prohibition "Lo tevashel gedi bechalev imo" seems to apply specificially to animals and not to poultry or fish. If chazal decided that poultry was also covered by the same prohibition then why not fish also?

A decree of Chazal because of Marit Ayin. However, fish was not included in the prohibition as it was not considered Marit Ayin (i.e. fish meat is not as similar to meat as poultry is). ESW