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My 3.5 kg cat ate a whole small oily mackerel fillet 2 hours ago — what symptoms should I watch for and when will they start?

Cats · Food safety

Last reviewed: 2026-09-05 · Reference ranges: ASPCA, Pet Poison Helpline & Merck Veterinary Manual

Short answer

Watch for vomiting or diarrhea, which could start within the next 2 to 6 hours. The high oil content is hard for her to digest and may cause pancreatitis, especially in a small cat. Signs of pancreatitis include repeated vomiting, a hunched posture, and obvious abdominal pain. She might also just have a single bout of soft stool. Make sure she has access to water, as diarrhea can lead to dehydration. If she vomits more than once, seems lethargic, or refuses food by tomorrow morning, call your vet.

Mackerel — Food safety facts

Safe to share · Cats · full breakdown on the linked food page

Safe cooked, but it is oily — small occasional portions only.

Dogs and cats carry the same rating (Safe to share), so the notes below hold for either pet.

Mackerel is not toxic to cats. The real limit is calories, fat and salt, so treat it as a small extra alongside the normal ration rather than a meal in itself.

There is no poisoning risk here, and the usual problem is simply eating too much at once: a heavy fat or salt load can bring on loose stools or a single vomit. Offer water, keep an eye on the pet for 24 hours, and do not induce vomiting.

Why these cautions matter

Rich, fatty food is the classic trigger for pancreatitis, a painful inflammation of the pancreas that usually needs veterinary treatment rather than rest alone.

Mackerel can stay on the rotation, but only as an extra mouthful: keep the rest of the day's food unchanged and watch the litter box and how quickly they finish the next meal.

The full verdict, portion guidance and serving notes are collected on the Mackerel food facts page.

Open the full food page: Mackerel food facts →