I have made bottarga with success using trevally roe. The thin lipped female grey mullet having escaped me in recent times. The process is not difficult, follow the steps on p. 274 of A Charcuterie Diary.
Start with a nice large fresh roe (trevally, flathead or tuna; use what you can get).
Salt them in a container, draining the liquid regularly.
They will darken in colour somewhat.
After drying, they are quite firm.
Consider smoking them. It is very good.