Why are there large circles in which there is no apparent fishing?

The large circles without fishing are almost always exclusive economic zones (EEZs) around islands that heavily restrict or prohibit fishing (if you toggle on the EEZs Layer in the Global Fishing Watch map, you will see that they often match with these blank areas). The circles could also be marine protected areas (MPAs) that prohibit fishing – you can toggle on the MPA layer(s) to see if that’s the case.