Tracking spatial patterns and daily modulation of behavior in a natural population of the pulse-type weakly electric fish, Gymnotus omarorum

, we show that (1) there are more fish in core than in edge zones; (2) transitions into and out of the recording sites were more frequent at night, and (3) resident fish show robust nocturnal increases of EOD rate likely associated with daily variations of water temperature. This experimental approach can be extended to other species to improve our understanding of the behavior, ecology, and well-being of electric fish in natural environments.