RANDOLPH — Jacob Kubel is a man on a mission. The 36-year-old conservation scientist with the Massachusetts Division of Fisheries and Wildlife’s Natural Heritage and Endangered Species Program has been crisscrossing the state since late August in search of the Commonwealth’s rarest salamander — Ambystoma opacum, also known as the marbled salamander — to document where it is breeding and to get a feel for how the population is doing. And he only has a couple more weeks to do so before the salamanders head back underground.