Yes, you have used "lock on" correctly and naturally. In the context of something like radar, to "lock on" means "to detect something, and keep tracking it continuously." For example, "the searchlight found the plane. The pilot conducted evasive action and tried to escape the searchlight beam, but it was locked on." When you press the "search" button on a car radio, it searches through the radio band, and when it finds a strong signal, it "locks on" to the station and stops searching.
A group of fish swimming together is "a school." In your sentence, "tuna" would be used in the singular as an uncountable noun. So, you would not say "a flock of tunas," you would say "a school of tuna."
"Flock" can be used to birds ("a flock of seagulls") or sheep ("the shepherd watched his flock.")
Finally, I have to ask: do you actually find tuna with radar? That doesn't sound right. Maybe you can find them with sonar. Maybe you can find them by using radar, but not to find the tuna directly, but to find flocks of birds flying above the tuna. I just don't know.
Warning: English has an extraordinary number of specialized "collective nouns," many of which are _very_ obscure words. Of late it has become a kind of game to discover and list them. For example, there is a kind of bird called a "starling." If you say "there was a flock of starlings in our yard" in a group of people who enjoy word games, someone is apt to say, gleefully, "That's not 'a flock of starlings,' that's 'a murmuration of starlings.'" That's just showing off, nobody uses the strange collective nouns except as a kind of word game.