heya, if you can, try to avoid canned tuna (or for that matter, any species that tends to bioaccumulate mercury).
here's a list of good oily/fatty fish:http://www.sacn.gov.uk/pdfs/fics_03_01_annex_01.pdf
Salmon, Trout, Mackerel, Herring, Sardines, Pilchards, Kipper, Eel, Whitebate, Tuna (Fresh only), Anchovies, Swordfish, Bloater, Cacha, Carp, Hilsa, Jack fish, Katla, Orange roughy, Pangas, Sprats
i haven't even heard of some of those species, and others listed here are endangered (example: bluefin tuna), but anyway there's somewhere to start, if it's feasible for you to eat better fish.
here's another list you can compare with the above. it is an american resource, but provides a list of best choices, and flags fish with concerns related to mercury.http://www.montereybayaquarium.org/cr/c ... lGuide.pdf
Arctic Char (farmed), Barramundi (US farmed), Catfish (US farmed), Clams (farmed), Cobia (US farmed), Cod: Pacific (US non-trawled), Crab: Dungeness, Stone, Halibut: Pacific (US), Lobster: California Spiny (US), Mussels (farmed), Oysters (farmed), Sablefish/Black Cod (Alaska & Canada), Salmon (Alaska wild), Sardines: Pacific (US), Scallops (farmed), Shrimp: Pink (OR), Striped Bass (farmed & wild*), Tilapia (US farmed), Trout: Rainbow (US farmed), Tuna: Albacore (Canada & US Pacific, troll/pole), Tuna: Skipjack, Yellowfin (US troll/pole)
if you can manage to eat oily fish 1-2 times per week that could be good. if you were to eat 2 out of 3 meals a day using vegetarian protein sources, and 1 meal per day with meat/poultry/fish protein, do you think that your knees would be okay?