SHAWANO, Wis.—Jim Van Eperen came prepared for his overnight shift last week as a volunteer "Sturgeon Guard" protecting the ancient fish from poachers during their frenzied annual spawning season.

While other guards shivered on lonely banks of the Wolf River, he and a buddy played cribbage all night in the back of his heated van, pausing to check on each car that came into the parking lot of a city park—and prime spawning-viewing site.

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