It's a species of killifish. If you caught it in NH it would have to be a banded killifish (Fundulus diaphanus). or a mummichog (Fundulus heteroclitus) as those are the only killifish found in your state. Looks to me like a banded killifish.
They're in with a pike? What species (grass, redfin, northern)? Grass and redfin pike are both peaceful fish, and the same can't really be said for most killifish. I may be wrong about these, but IME, killies are aggressive. Your pike will likely grow big enough to eat them fast if it isn't big enough already.