The heat and pressure involved in the canning process basically destroys the omega 3 content (can't think of a better word!) which is why fresh fish is one of the best sources.
Edit: I knew this was the case for tuna, but here's some info for other fish.
Salmon, sardines, mackerel, trout and pilchards all count as oily fish when they're canned and when they're fresh. This is because the canning process doesn't significantly reduce the fat content of the fish.
http://www.eatwell.gov.uk/asksam/health ... q/#A221141
Fresh tuna is an oily fish, high in fatty acids. But when it's canned, these fatty acids are reduced to levels similar to white fish. This is because tuna is cooked before it’s canned and most of the oil is lost during this process. So, although canned tuna is a healthy choice for most people, it doesn't count as oily fish.