It is illegal to net walleye in Ohio waters of lake Erie if you suspect they are walleye nets call the ODNR and report it. They are probably trap nets used for perch. Your sonar is zoomed in to reflect slightly more than a square mile and the sonar indicates the water depth to be rather shallow to be to far offshore. There were nets just offshore of the beach at the west edge of Port Clinton earlier this year running SW to NE they were left there for about 3 weeks.