Mentaiko is a new obsession. The ones I can find in New York come from Japanese or Korean speciality food stores and are whiting roes cured in salt, Korean red chilli flakes and a little ground sesame seed. I believe you can also find cod or pollock roes cured in the same way but I'm not sure if you can taste any difference in the type of fish used.
I like to eat them as a snack, membrane and all, cut into chunks and seasoned with a little yuzu or lemon juice and a few grind of fine white pepper. They are very salty though so not everyone would agree.
For the less salt-addicted among us these mildly spicy, briny eggs make a fantastic sauce for pasta, creating a Japanese spin on spaghetti alla bottarga.