This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
The right skills and equipment for a given food. Freezing is best
where a person does not have skills, equipment and time. Pressure
canning is not complex but it is often done incorectly. Myth #1
in home pressure canning is that a little bit of leakage from jars is
normal - it is not. Leakage is a contaminated seal resulting from
Canning food is preserving food by: 1) placing it in an
hermetically sealable container, then 2) applying a heat treatment that
will destroy microorganisms and inactivate enzymes that would spoil the
product or render it unsafe. (from Jean Bergeron, foodchemist, ). A partial
vacuum is created by a change in pressure caused by heating, then cooling
said sealable cans and jars--Boyle's Law in action. The heat is
generally created by either a boiling waterbath or a pressure canner
(Boyle's Law again).
What you absolutely need to know is whether your product is highly acidic
(low pH) or not. High acid foods, like fruits and pickles, can be canned in
a boiling waterbath; relatively low acid foods, like vegetables and meats,
need to be pressure canned. You also need to know what your altitude is,
because the higher you are, the lower the boiling temperature of water.
Since you are creating an anerobic state, you need to be concerned about
_C. botulinum_ toxin.
Fish falls into the most absolute category of all. Frozen is simplest and
in most instances is next best to fresh. Dried fish from a salt/brine state
is next less toxic. Then we have smoked fish. The touchy end on the fish
preserving scale is occupied by canned fish. You will need excellent skills
in pressure canning before attempting fish. Use a recipe from a reputable
source like Putting Food By.