A group of gudgeons is called a troubling, a small freshwater fish widely used as live bait and often overlooked despite being genuinely widespread across European rivers, lakes, and ponds. Gudgeons are bottom-feeders, using a pair of sensitive barbels near their mouth to sniff out insect larvae and small invertebrates buried in silt and gravel.
“Troubling” belongs to a small family of old English collective nouns, “troubling of goldfish” is another, that describe a fish group in a slightly mischievous or unruly light, though there’s no strong documented reason gudgeons specifically earned it. It’s most likely simply borrowed vocabulary rather than a term coined for gudgeon-specific behavior.
Bonus fact: gudgeons were historically a popular target for novice anglers in the UK, easy to catch and abundant, they were often a child’s first fish.
Gudgeons are also occasionally grouped under the general terms shoal or school, shared with most other small freshwater fish.
Related: Collective Nouns for Fish