(Photo from Ezu Restaurant)
It is true that the majority of sushi is fish - tuna, crab, salmon, shrimp, yellowtail and fresh water eel.
However, there are plenty of non-fish sushi out there event a vegetarian person can have a complete meal without ever eating one fish sushi.
Here is a list of non-fish sushi.
Traditional Japanese Non-Fish Sushi
Kappa - Cucumber roll. Japanese maki standard. Refreshing, so it's good to eat between fish nigiri to cleanse your palate.
Oshinko Maki- Pickled daikon radish roll. Another maki standard and favorite. One of the finishing pieces before dessert.
Kanpyo Maki -Dried shavings of calabash, cooked in soy sauce and sugar. Nothing like this in Western dishes. Chewy texture with sweet soy flavor.
Natto Maki- Cut or hand roll. Natto is fermented soybeans. It has a very strong "foul" smell, yet loved by many Japanese. (though it is also true that there are many Japanese who are unable to eat natto.)
Shitake Mushroom - Mostly cooked in soy sauce and sugar, sometimes grilled makes a nice nigiri sushi.
Wasabi Maki - cut or hand roll. Yes, fresh wasabi has wonderful aroma and sweet flavor. Cut into small matchsticks, wasabi is rolled into hosomaki - a thin roll or temaki, hand roll.
Tamago - egg custard. Some eat it as a finishing dessert piece place for, say ice cream. It's sweet, juicy and has fluffy texture. It is said that you should order tamago to find out the sushi chef's skill.
There are of course many many other vegetable sushi out there including red peppers, asparagus, zucchini, eggplant - virtually any vegetables you can find (and goes well with sushi rice.)
My personal discovery is to use wine vinegar and fruit vinegar instead of rice vinegar. Champagne vinegar also makes nice sushi rice which goes well with vegetables.