Tuna is available all year round, but at their best when in season, starting from May going through to early September (according to variety).
Choose the best - as tuna are oily, they go off quickly, so need to be very fresh. When buying tuna steaks, look for those that have been trimmed neatly, with firm, dense red or dark red flesh and a meaty aroma. Avoid those with strong discolouration around the bone, or which have a dull, brownish cast. Thicker-cut steaks will stay juicier during the cooking process.
Tuna is also available tinned. Albacore is one of the best types sold this way. Skipjack (which, like bonito, is a somewhere between tuna and mackerel, but classed as a tuna) is the most commonly tinned variety. Tinned tuna is available packed in water, brine, vegetable oil or olive oil (with the last being the best).
When preparing Tuna - the steaks should not be washed before cooking - just pat them dry with some kitchen paper. Tinned tuna should be drained before use.
With regard to storing Tuna, if it's fresh tuna, remove any packaging, wipe with kitchen paper, put on a plate, then cover with cling-film and put on the bottom shelf of the fridge. Consume within a day. Tinned tuna should be stored in a cool, dark place. After opening, it should be transferred to a non-metal, airtight container and kept in the fridge for up to 24 hours.
You may cook Tuna by Barbecue, grill or pan fry (up to 2 minutes each side), by Bake, wrapped in oiled foil (10-15 minutes), by Braise (10-15 minutes).
Some alternatives to Tuna are herring, tilapia or mackerel."
here are some