An imker is a beekeeper.
It is rare to find imker as an occupation in Dutch records since few people kept bees as their main income stream. I have found beehives (bijenkorven) tallied in estate inventories of some of my ancestors who owned farms, which told me they kept bees. The number of hives suggested they were not just for personal use, so they probably sold honey on local markets. You can find estate inventories in notarial records.