The dominant species of flat fish from Poole are plaice which can be caught from April to October, with the spring and autumn being the most productive. We normally fish for plaice by drifting close inshore over sand and mussel beds, ragworm being the favoured bait best fished running ledger stile behind a spoon and multiple beads.

Dabs don't seem to be as abundant as they once were but are normally caught in the same way as plaice. The plaice grounds also hold dover sole, but being nocturnal must be fished for at night at anchor with ultra light tackle. Brill and turbot are rarer and are normally only caught by accident when fishing for rays and some times when plaice fishing.  
Poole is also famous for its flounders, they can be fished for in the harbour from the end of October to January and then again March and April this can be a good standby for the light line enthusiast. Flat fish are another fish I wouldn't put back so I don't expect you to, except maybe flounders. 

Have a look below at some of the flat fish caught from my previous boat Seajay, hover over the image to see a larger picture.