The Frome and Piddle both drain into Poole Harbour, and I can recommend the middle beats of the Piddle, in particular - in general, the further up the Frome you go, the better the day ticket fishing and that may be a bit of a hike from Poole in holiday traffic. Both rivers are less manicured than many beats of the Hampshire chalkstreams, and many anglers think are the better for it.
You may have just missed the cream of the sport.
Richard Slocock sells day tickets for both rivers, and the Dorchester club has limited day tickets on their splendid beats of the Frome.
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Day Tickets | Dorchester Fishing Club