Fish Sauce

You can use a light soy sauce.

Or else, if you look around there are vegetarian fish sauces available.

Or you could make your own. Here are 3 recipes I found with a quick search:
http://en.wikibooks.org/wiki/Cookbook:Fish_Sauce_(Vegan)
http://www.grouprecipes.com/49971/vegan-fish-sauce-substitute.html
http://bittergreens.typepad.com/bittergreens/2003/09/vegetarian_fish.html

Awesome, thanks for that. yeah I was thinking a soy sauce instead, but was just wondering if anyone had a fav substitute that they preferred.

Lamyong oyster sauce!

http://wholefoodcentral.com.au/index.php?main_page=product_info&cPath=8_239&products_id=83

I remember making a really salty vege soup and it tasted just like seafood chowder. So yea the fish taste is mostly salt I reckon.