This is the 10051st most frequent Japanese word.
逃せ
"逃せ" means "let escape" or "miss (an opportunity)."
In this sentence, "逃して" (nogashite) is used to imply that someone did not take advantage of a specific opportunity, thereby letting it slip away.
彼はその機会を逃してしまった。
He missed that opportunity.
Here, "逃して" (nogashite) refers to letting go or failing to keep hold of a physical entity, in this case, the fish.
その魚を逃してはいけない。
Don't let that fish escape.
In this example, "逃さない" (nogasanai) is used to express the action of ensuring not to overlook or miss something important, such as a deadline.