Not sure about how it effects fish but as a general rule of thumb out east(Southern shore of Nova Scotia) we never picked shell fish in months without an R in it maybe folk lore passed through the ages but strictly adhered to by all sort of an unwritten rule I was always told it had something to do with whatever they ate at the time was high in toxins and would make you very ill Just a thought since you mentioned they were full of Zebra mussels