That is for eels.

I met a fella whose income came from going up coastal streams (canoe? Dingy with outboard?) to catch eels to sell. He definitely left the largest (queens?).

Good article: https://ourwayoflife.co.nz/from-vermin-to-icon-new-zealands-...

Also interesting: https://www.1news.co.nz/2025/04/05/thousands-of-eels-found-d...