From their FAQ (http://surlybrewing.com/contact/#faqs pulled June 2015):
"Yes, our beers are vegan friendly. We stopped performing animal sacrifices years ago. But seriously, we don’t use any animal products in the processing of our beer."
From a video interview, December 2011 (http://vimeo.com/33664147) 12:45:
"We just purchased a centrifuge... we're no longer fining our beer, so Surly is officially vegan"
Company email (2008):
yup,
we use the fish.
Email 2 (2008):
Our beers are made with isinglass.
Surly Mild
Vegan
This product is listed as vegan friendly.
What the company told us
Correspondence on record, with the date of each entry.