Q. Why does Jewish law prohibit eating fish and meat together?
A. It is not a prohibition like a mixture of milk and meat, which may not be cooked together, eaten at the same meal or used to derive any benefit, and requiring an interval between them – a brief interval after milk and several hours after meat. Fish and meat are not eaten together or served on the same plate, but people often have a fish course before a meat course.
The sages stated that combining fish and meat could cause health problems. There are various opinions as to the nature of the health risk. It is certainly true that fish spoils easily.