The Nashua River Watershed Association is holding a free presentation, “River Otters and Fishers in Massachusetts: A Tale of Two Weasels,’’ at 7 p.m. Tuesday at the group’s River Resource Center, 592 Main St. in Groton. Seating is limited, so preregistration is recommended. To register, contact Pam Gilfillan at 978-448-0299 or e-mail pamg@nashuariverwatershed.org.