The short answer: It is not recommended.
The long answer: Similar to raw meat, raw fish is not recommended for cat consumption. This is because of the harmful bacteria that may be found in it such as salmonella, clostridium, listeria, and vibrio. These pathogens are known to cause food poisoning and digestive issues in cats. Additionally, the enzyme thiaminase found in raw fish destroys thiamine, an essential vitamin that cats require to maintain optimal health. Thiamine deficiency can even cause serious neurological problems and lead to convulsions and coma.
What to do if your cat accidentally eats raw fish: Monitor any changes in your cat’s behaviour as well as her litter box habits. Get in touch with your veterinarian if she exhibits any of the following: salivating, twitching and fitting, difficulty in breathing, vomiting, diarrhea, shock or collapse, inflammation or swelling of skin, depression or coma, and/or changes in drinking, urinating, and appetite.
In summary: Though cooked fish may occasionally be fed to your pet cat, feeding her raw fish is highly discouraged. The potential risks outweigh the benefits she may gain from it. If you plan on serving her raw fish, be sure to get your veterinarian’s advice and clearance beforehand.