[S]cientists were having a hard time telling delta smelt apart from a fish species from Japan called wakasagi. … Wakasagi were introduced by the government in the 1950s. There’s no shortage of them here or in Japan. Especially when they’re young, to the naked eye they look virtually identical to deltas. They’re so similar, in fact, to the nearly extinct fish that scientists were worried about hybridization — that this plentiful species and the delta smelt would start hooking up, making mixed-species fish babies.