Here is a letter I received from state biologist Shawn Good. Have caught 2 so far this year. Not sure if this could count a 5th Vermont esox member...you decide.
These northern pike – chain pickerel crosses are known to accidentally hybridize in Lake Champlain and other waters where they both occur, like Lake Memphramagog. It’s also been reported in the Great Lakes area. It is easy to confuse this natural hybrid with muskie, particularly for anglers who aren’t used to seeing and identifying muskie often.
We collected tissue samples from some of these hybrids on Lake Champlain a couple years ago, and had some genetic analysis done on them. The results show that they are male pickerel fertilizing female pike eggs. I don't think it's something they're doing on purpose (ie "pairing up"). I think it's a result of their spawning timing and locations overlapping, so that the two species are spawning in the same spots at the same time, and some male pickerel milt "drifts" in the water and ends up fertilizing some pike eggs. Odd weather patterns in the spring may be the result of the timing of the two species spawning closer together. A recent study suggested that pike and pickerel spawning might overlap more, and consequently we’ll see more hybrids, in the future as climate change changes spring warming patterns.