A good fish wearing the wrong coat
Herring deserves its healthy reputation: an oily fish full of the fats that support skin, coat and joints, with protein a dog's body uses well. None of that is the problem at a family buffet. The problem is the jar. Pickled herring has been sitting in a sharp bath of vinegar, salt and usually sugar, and buffet versions frequently include sliced onion in the brine. By the time a piece reaches your plate it is seasoned far beyond anything a dog should eat as more than a token taste, and the fish's own virtues are buried under the pickling.
What to do at the buffet itself
If he is joining the occasion and you want him to have his piece of the tradition, choose one small piece, rinse it briefly under a tap to strip the surface brine, pat it dry, and check it thoroughly for bones before it goes anywhere near him. That rinsed piece, offered on its own or over a little of his own food if you brought some, is the whole portion — not a return trip to the jar each time you pass it. Keep him away from the jar itself and from dropped pieces on the floor, because floor herring comes with brine, onion and whatever else the buffet has shed, and a dog hoovering unattended can eat in a minute what you would have portioned across a week.
Plain herring at home is the real share
If herring is going to feature in his life, serve it the dog's way at home: plain cooked herring, grilled, baked or poached with nothing added, cooled, and carefully deboned. Herring bones are fine and flexible, which makes them easy to miss and easy for a dog to swallow unnoticed — work through the fillet with your fingers and a fork under good light. A few bites flaked over his kibble once in a while delivers everything good about the fish with none of the brine. The oily richness still argues for modesty: too much oily fish at once loosens stools, and it should complement his balanced food, not displace it.
Salt, bones and the dogs who should skip it
The salt load is the main reason pickled herring stays a token food. Dogs with heart or kidney concerns, and any dog whose vet has him on a controlled diet, should skip the buffet version entirely — for them even the rinsed piece is an avoidable risk, and plain fish under vet guidance is the only route. Any dog, healthy or not, who eats a large amount of pickled herring or drinks brine should be watched for excessive thirst and sickness, and a vet called if he seems unwell, because salt in quantity is a genuine poisoning risk rather than a tummy ache. Coughing, gagging or pawing at the mouth after fish means a bone may be stuck, and that is a same-day vet visit.
Buffet food reaches a dog from many hands, so his total is never what one person intended. Logging his fish portion in KinsLe through Care Teams during family gatherings gives everyone the same picture — one rinsed piece, already given — and stops the second and third kind offers before they happen.