My bad. I was only thinking of Tsujita’s main shop and totally forgot about Annex. Annex serves a twist on Jiro-style ramen and it is damn good. If you have had Tsujita Annex’s ramen, this is on a similar level. However, Mendoki is more true to the Jiro style. If I recall correctly, Tsujita adds chili powder?
Yeah, fish stock is common. It is also common to add dry fish to other stocks. See the Nagi thread for further discussion on niboshi (baby sardine) ramen stocks.