He hadn't realized Cho was there. Often, he has good instincts for where people are and whether somebody's coming up behind him, and he has ever since he came out of the arena for the first time, instincts that were only heightened by spending so long in the Capitol. But he's so focused on the fish he'd seen, following its movements until
-- Yes, there's a school there, good -- that he doesn't notice Cho until he hears her call out in response to him.

"Yeah!"

He trusts Cho enough that he doesn't give in to the instinct to turn and stare, assess the intentions of the unexpected person behind him.

"There's a school of them, that's a good sign."