The Walker Scale: Details of a Method for Assessing Beach-Spawning Runs of California Grunion Leuresthes tenuis (Atheriniformes: Atherinopsidae)

The California Grunion Leuresthes tenuis, a marine atherinopsid, makes spawning runs onto some of the busiest urban beaches of southern California. Eggs remain buried under moist sand, tidally emerged from water throughout incubation. Concern over the effects of human activities on beaches holding incubating fertilized eggs of L. tenuis prompted a need to assess spawning runs consistently through time. A new metric, the Walker Scale, focused on the peak of the runs, was developed to train citizen scientists, also known as community scientists. Reports of sightings were verified by scientists by locating buried clutches and collecting eggs at locations identified by volunteers. Reports between multiple observers from the same run were highly correlated, and also positively correlated with the density of clutches of eggs. Over the past two decades, the Walker Scale method has been used by thousands of volunteers as well as resource managers, environmental consultants, coastal projects, the US Army Corps of Engineers, and the California Department of Fish and Wildlife. This paper provides details about the method and its correct use by citizen scientists and professional biological monitors. The Walker Scale method is reliable, easy to understand, and the most effective method for assessing this elusive, charismatic fish.