It appears I was mistaken and you do need a licence, though it's free. You should also ask the landowner's permission. I had thought you only needed the licenc

It appears I was mistaken and you do need a licence, though it's free. You should also ask the landowner's permission. I had thought you only needed the licence to set up traps, which I'd rather not myself as there's a risk of catching/injuring/killing other species, particularly native crayfish or otters. Sorry for the inaccurate post. Sadly it's too late to edit it or I'd put strikethrough all over it... More details about catching (mostly with traps) and licence details below. http://foragedfoods.co.uk/how-to-catch-signal-crayfish/ http://foragedfoods.co.uk/how-to-catch-signal-crayfish/ https://www.gov.uk/guidance/permission-to-trap-crayfish-eels-elvers-salmon-and-sea-trout#crayfish-trap-authorisation https://www.gov.uk/guidance/permission-to-trap-crayfish-eels... To answer your questions, I'd wait until at least spring; I've only been in summer. Not sure about speed or depth (I've seen them in fast shallow streams and in a slower metre-deep river) but they like lots of hiding places like rocks or overhangs.