You can't catch a shark on your first try. You need to throw out the hook a million times and get it back. We need a lot of feed before we can catch and throw the hook again. This is a very cumbersome thing, need to go to the beach to dig a lot of clams to make. To be honest, it's a huge waste of time. If you go to acbellsbuy.com and buy enough ACNH Items -- fishing rods or fish feed -- you'll get twice the result with half the effort. Donate the sharks to The Blathers museum and see them swimming happily in an aquarium, and you're well off the hook -- in many ways, it's hard to keep sharks in captivity.
In 1977, the Steinhart Aquarium built a two-story "circular" Aquarium, a tall circular Aquarium that was designed to give open Marine fish the illusion of continuous, nonstop swimming in circles. The team leader at the time, Michael McCormick, collected a seven-and-a-half-foot shark from fishermen, but three and a half days later, he removed the shark, named Sandy, from the tank, took it to the Faralen Islands and released it.
In 1981, SeaWorld San Diego imprisoned a shark sample for 16 days before releasing it. In 1984, the Monterey Bay Aquarium and others tried to breed great whites in captivity because they wouldn't eat, but released them back into the ocean just 11 days later.
In 2004, the Monterey Bay Aquarium had even greater success. The aquarium brought in a young female white shark and put her on display for 198 days before release -- the longest period ever recorded. In 2006, Japan's Okinawa Miramay Aquarium tried to showcase an 11-foot-long male great white shark that died after spending three days in the tank. Needless to say, a real aquarium wouldn't be able to keep a white shark alive.
In fact, because of Animal Crossing, we can easily achieve technology that is not possible in the real world, which is of course a feature of the game. The Blathers museum not only has a collection of creatures but also a very superficial look at them. That's enough for the kids.
The Blathers museum still has an inconvenient design and you have to go to a dedicated room to look at the fossils, while fossil models are for sale at https://www.acbellsbuy.com/. You can put the fossils on the floor and you won't need to pee frequently through the doors of the Blathers Museum. This site now offers a 10% discount, which is much cheaper than other sites.