How a kitten's tiny size changes the math
At 3 kg, your kitten is about one-fifth the weight of an average cat, which means her body processes everything more intensely. But in this case, that works in your favor — the single bite she licked probably amounted to less than a teaspoon of fish once you account for what stayed on the plate. Her stomach acid is already handling rich mother's milk, so this small oily intrusion won't overwhelm her.
The baking process matters here too. Heat denatures the fish's proteins and renders some fat out, making it less challenging to digest than raw mackerel would be. She might lick her lips more than usual or seek extra water, but true distress from this quantity would be extraordinarily rare in a healthy kitten.
Timing clues that differentiate worry from normal
Since this happened just 20 minutes ago, you're in the prime window to observe her immediate reaction. Kittens often broadcast digestive upset clearly — if she's going to react, you'll likely see signs within the next 2-4 hours. Pawing at her mouth or exaggerated swallowing could suggest the oil is irritating her throat, while sudden zoomies might mean her body is trying to hurry the digestion along.
More concerning would be lethargy setting in after the initial energy burst. Young animals usually bounce back quickly from minor dietary indiscretions, so if she curls up and won't engage with toys or treats within the hour, that's your cue to check her gums for paleness and call your vet for guidance.
The one vomit rule for small kittens
Single vomits happen — kittens' systems are still learning what to tolerate. If she brings up a small amount once, withhold food (but not water) for 2-3 hours, then offer a teaspoon of her regular wet food. The problem comes if she vomits repeatedly or can't keep water down. At her size, three vomits in an hour or any dry heaving warrants an urgent call.
Diarrhea is less likely from this tiny amount but watch her litter box habits tonight. Kitten stools are normally soft, so don't mistake typical mush for trouble. Real concern starts if you see mucus, blood, or her straining without production. Her usual playful self between bathroom trips is a good sign.
Why tomorrow's breakfast matters more than tonight
The real test won't be tonight's antics but whether she attacks her morning meal with normal kitten enthusiasm. A skipped breakfast or hesitant nibbling could indicate lingering nausea, even if she seemed fine overnight. Kittens shouldn't fast longer than 12 hours, so if she snubs two meals in a row, it's vet time.
For now, resist the urge to "help" with treats or milk — her system needs to handle this solo. Keep her warm and her water fresh, and maybe stash future mackerel plates farther from curious paws. At this age, every stolen bite is a learning experience for both of you.
The full verdict, portion guidance and serving notes are collected on the Mackerel food facts page.