Unless you make miso soup yourself, or are eating in a veggie restaurant, there’s a chance that your soup, or meal, has a fish-based dashi in it.
Dashi is a Japanese stock, which is commonly made from dried sardines (niboshi), dried and smoked bonito tuna (katsuo-bushi), dried kelp (kombu) or shiitake mushrooms (hoshi-shiitake).
Apart from miso, dishes like agedashi tofu, Japanese curry and a variety of dipping sauces often contain a fish dashi. And even if a veggie dashi has been used in a dish, there may be bonito flakes added at the end as garnish/flavour. In the friendly Japanese food store near me, I noticed bonito added to packets of instant miso soup, instant noodles and curry sauces.
Like most Asian cuisines, true veggie food is a rarity in mainstream Japanand various meat/fish based stocks or ingredients are used in what seem like veggie meals. If avoiding fish is important to you, ask questions and at the restaurants you eat in, tell them you don’t want fish dashi or bonito flakes used in your food.
Buying from a Japanese food store or eating at a restaurant in the language you know is easy. But when travelling in Japan, it’s near impossible to guarantee your food is veggie unless you eat at a veggie or Buddhist restaurant. Once again, the wonderful Happy Cow saves the veggie day or check out Herwin’s Vegan Japan or Tokyo Vegetarian Cuisine.
Just an aside: on my trip to the Japanese food store I also discovered that a lot of products have monosodium glutamate (MSG) in them. I’ve always associated MSG with Chinese cooking so was surprised to see how many products in the Japanese food store contained it. If you’re sensitive to it, it’s another thing to be aware of in regards to Japanese cuisine…