That's because once someone has signed up they then use a robot to automatically make multiple posts simulteaneously.
As Williams said, forum spamming is now big business and teams of low paid children are employed to do it from places like china, thailand, Eastern Russia and malaysia. They simply sit in front of computers looking for places to spam a list of links. All day, every day.
If email activation is required they usually use free services like Yahoo, Hotmail or Googlemail or often dot-info adresses and sometimes simply hijacked peoples' legit email.
The purpose of the lists they post is so that the lists get indexed by search engins and so increase the ranking and search position of the listed sites. This is why it's important to remove the lists ASAP before they get indexed.
Over at my Wife's (non fish) forum we also have been troubled by this and now we have resorted to Moderator review of each new member.
A member that signs up with the name 'viagra4u' or 'phamacyest' is clearly not legit and gets deleted before they are even approved.
Other people reveal themselves by lieing about their location so we've made the location field on sign-up compulsory.
Sweat-shop spammers are fully aware that their location has a reputation for spamming & fraud so they put that they're elsewhere. But we then search their IP and find where they are and liers are deleted.
Obviously, checking each sign-up takes our time up but it's been fool-proof so far!
William, there are modification out there that deal with this sort of thing, a couple that spring to mind are one that will not let a member post an url untill they have a certain post count- that would stop people signing up just to spam of their first post.
Another modification simply makes it so that any post containing an URL has to be approved by a Moderator before it appears.