Fondo is stock made with bones and some vegetables. In the case of fish it is called fish stock or fumet. However, blue fish are generally not usued.

Darker-fleshed and oily fish, like salmon, tuna, and mackerel, are not usually used in a classic fumet, since they impart a strong fish flavor. When it comes to fumet, the aim is generally to lean toward freshness and delicacy, not pungent fishiness.
https://www.seriouseats.com/2015/10/how-to-make-fish-stock-f...
https://es.wikipedia.org/wiki/Fondo_(caldo)