Why the broth matters less than the solids
A spoonful of salmon soup broth is mostly water with trace amounts of fish oils and seasonings. For a 2 kg cat, this small volume won't deliver enough of any concerning substance to cause harm. The real risks in salmon dishes come from chunks of raw fish or bones that might be in the soup, neither of which are present in the broth alone. What you should do instead is check the spoon for clinging onion flakes or garlic bits, as those are toxic in tiny amounts.
Cooked salmon protein itself isn't dangerous to cats in these quantities. The broth likely contains less than 1% of the original salmon's mass, spread across water and fats. Your cat's stomach will process this like any other mild food exposure. Watch their litter box for the next 24 hours—completely normal stools mean their system handled it well.
When to worry about soup ingredients
The greater concern isn't the salmon but what else the broth simmered with. Onion and garlic powders dissolve into liquids, making them harder to spot than solid pieces. A single spoonful might contain under 0.1 gram of these seasonings, but cats can react to as little as 5 grams of onion per kilogram of body weight over time. Your cat would need to consume fifty times more broth to reach that threshold today.
Signs of allium toxicity (from onion/garlic) take 1-3 days to appear, starting with sluggishness and pale gums rather than immediate vomiting. Since you saw only broth contact, the risk is minimal, but knowing the soup's recipe helps. If it contained wine or excessive salt, offer extra water to dilute those compounds.
How a small cat processes fish oils
A 2 kg cat's digestive system can handle the omega-3 fatty acids in a spoonful of salmon broth without issue. These oils might even benefit their coat at this dosage. Problems arise with cups of broth daily, which could lead to pancreatitis over weeks—but that's not your situation. The liquid form means rapid absorption, so any reaction would occur within 8 hours if it were going to happen.
Fish broth sometimes causes mild stomach upset simply because it's new to your cat's diet. One loose stool isn't an emergency, but repeated diarrhea or vomiting warrants a vet call. For now, observe whether your cat seeks more broth—this indicates they tolerated it well, though you shouldn't make it a habit due to salt content.
The next 12 hours for a lightweight cat
With such a small body mass, your cat will metabolize the broth quickly. Expect them to drink slightly more water today as their kidneys process the sodium. The lack of immediate gagging or pawing at the mouth suggests no bone fragments were present, which is the main salmon-related danger in soups.
If the broth contained dairy (like cream), watch for lactose intolerance signs: small puddles of diarrhea or excessive licking of the lips. These would appear within 4-6 hours. For plain fish broth, the risk is so low that you only need to intervene if your cat stops eating their regular meals by tomorrow morning.
The full verdict, portion guidance and serving notes are collected on the Salmon (cooked) food facts page.