Singular shops have singular prices, singular shops give singular change
I was looking at old British car 'ridealongs' or road tests. Morris, Austin, etc. Youtube decided to ADHD into this thing:
Marguerite Patten cooks HerringS in Oats.
Yaaarrrgggghhhh!
Well, it's not quite as bad as it sounds. The oats are apparently used for breading. It's not herringS boiled in oatmeal, though you can't outguess the absolute Yaaaarrrrggghhhness of Brit food.
Marmite. Nuff said.
What really caught my attention was herringS. We also say sardineS but we don't say troutS or tunaS or salmonS.
Conclusion: The rule for fish is the same as the rule for mammals. Within each category, the small critters have plurals and the big ones don't. Birds almost fit but not quite.
I suspect the real rule is more like: Upper-class food is singular. Lower-class food is plural.