Tuna

The Tuna (マグロ, Maguro) is a rare fish that can be found in the sea all day between November and March. The fish sells for an expensive 7,000 Bells at Tom Nook's store. The fish has a huge shadow size.

In
Blathers will say this when the Tuna is donated to the museum:

In
Upon being donated to the museum, the Tuna can be found in the back tank with other sea fish. When given the fish, Blathers will say this:

In
The information board in front of the tank that holds the Tuna says the following:

In
When donated, the Tuna can be found in the north-east room of the fish exhibit, in the large central tank with the porthole windows. It shares this tank with the Anchovy, the Blue Marlin, and Horse Mackerel.

Real-world information
The tuna, or tunny, is a saltwater fish related to the mackerel. There are a number of different types of tuna that exist, with the Atlantic bluefin (also known as the Northern bluefin) being one of the largest. It preys on squid, crustaceans, horse mackerel, sardines, and herring. Tunas are warm-blooded, allowing them to regulate body temperature. This allows them to hunt in the chilly Northern waters. The Atlantic bluefin is highly prized for consumption, which has led to a decline in population.