The Scots word for lark is larick/lairick, shorter versions of laverock. It's quite possible that local (mis)spellings might have included larock/larok, so lark-catcher for laroker is most probably right.
Another vague possibility may be connected with fishing. Is the town concerned on the coast? Seine fishing often employed three boats in a team, the seiner, the vollier and the larker. I suppose it might be that the man in the last of these was also called a larker, which might have been (mis)spelt laroker, too.