It’s that time again — and the cool thing is that it’ll be that time through mid-June. There’s a mysterious grunion spawn going on off the beaches of La Jolla, and the Birch Aquarium wants to make sure you experience all it can offer. On certain spring nights following high tides, hundreds of the small silvery fish ride the waves onto shore for this remarkable mating ritual, which happens only in Southern California. While on the beach, female grunion bury themselves halfway in the sand to lay eggs, while males wrap themselves around the females to fertilize their eggs. Then, both catch a wave to return to sea. Birch naturalists will guide participants through an unforgettable night, starting with an interactive presentation on grunion, followed by a rare opportunity to witness grunion eggs hatch before their eyes. The experience continues with a moonlight beach exploration to witness the grunion run, nature willing. Bring a flashlight and warm jacket. The runs take place outside Sumner Auditorium at Scripps Institution of Oceanography; the season’s first is Sunday, March 22, beginning at 10:30 p.m. Admission is $14 for members and $16 for the public. Children ages 6 to 13 must be accompanied by an adult. Reservations must be made online at aquarium,ucsd.edu or at (858) 534-7336.