No sea trade?

[Marcus Gallicus]

In my dark age campaign, i'm playing as the byzantines, but i noticed there's no sea trade at all, even if i have a port.
Is this intended, or do i need to construct another building before they start trading? or is it just a bug?

[Zom]

You need to build a protected anchorage.

[Marcus Gallicus]

You need to build a protected anchorage.

Ah, ok. Glad it's not a bug. Thanks friend!

[SerbianInfantry]

I thought you needed to build werehouses and stuff for export/import, and shipwrights for increesed income and building ships...