Some canned fish is available in oil or brine (e.g. tuna, salmon, saury), while other types of canned fish are only available with oil (e.g. sardines, mackerel). If I were to home can some of the latter, would it be completely necessary to add vegetable oil?
The reason sardines and mackerel are generally packed in oil is due to the naturally high amounts of fat/oil in the fish. If you cook a raw mackerel thoroughly you will soon notice the large amounts of oil left in the tray.
On the other hand when cooking Tuna you will notice the opposite and almost no matter how much oil you put in your frying pan the tuna will just keep sucking it up.
For this reason, as canning involves cooking, the oily fish are either packed in their own oil (more could be added) or something like a tomato sauce which, with it's acidity will emulsify with the fat.
If canned in brine/water you would still be left with a layer of fat floating.
In response to the comment below.
- Mackerel - 100g - Raw - Fat Content: 13.9g
- Sardine - 100g - Raw - Fat Content: 4.2g
- Tuna - 100g - Raw - Fat Content: 1g
- Saury/Pike - 100g - Raw - Fat Content: 0.7g