u/ComprehensiveStep487

I’m measuring the population of sea urchins in 3-4 beaches/coves. Each beach is 300-500ft wide and maybe 500 ft long. I need help how do I get an semi-accurate measurement so I can compare population sizes. I have no official scientific instruments because I’m a highschool senior doing this for my senior project
The beaches are specifically:
-fort brag
-Caspar beach
-Russian gulch beach
-Gerstle cove

All beaches are in Sonoma and Mendocino county California