Mary:
> Where do you buy your mackerel?

Asian grocers. Often it is frozen/thawed--ask. Fresh is best for aging/drying.

>I was planning to dry mine in the
>dehydrator, put a little salt on it, and eat it.

It may not need salt at all. Such aged fish is a perfect food to eat
instincto--if it tastes good it is probably just what your body needs. If
it tastes like rotten fish, don't eat it. If you salt it you might end up
eating it when it is not what your body is after. Maybe its not such a big
deal, but raw seafood is a pretty powerful food and I would get a second
opinion (from your tastes buds w/o salt) before eating a lot of
it--especially at first.

>I tried drying snapper/rock
>cod and it was a little tough after drying.

Lower oil fish get very hard after completely dried, as you found. Try
eating it before it is completely dried--after the surface is crusty, but
the inside is still moist and melting. (Don't cut thin slices for
this--just put out the whole fillet or cut into 2cm x 2cm x ??cm
rectangular cubes--like big french fries.)

>How long would I need to dry it,
>to kill bacteria?

You won't kill all bacteria, but most of the ones causing an off-flavor
have died once the surface is dry...

Cheers,
Kirt