Daily variation in the shoaling behavior of zebrafish Danio rerio

Abstract Shoaling behavior provides numerous fitness benefits for fish, including enhanced access to mates, increased success in foraging and protection from predators. We were interested in determining whether shoaling intensity differed throughout the day. To do this we kept adult zebrafish Danio rerio in different lighting conditions for 10 days: “Normal” (12:12LD, lights on at 0800 hrs), “Reverse” (12:12LD, lights on at 2000 hrs), DD, or LL, and then observed the shoaling behavior at different times during the day. Our findings suggest that daily variations exist in shoaling behavior, with mean shoaling times for fish from the ‘normal’ group being the lowest at the mid–point of the dark phase in the fish’s subjective day (00:00 hrs), then rising significantly throughout the day, reaching their highest intensity at 20:00 hrs (lights out). Fish from the “reverse” LD cycle (lights on at 20:00 hrs) showed differences in the mean shoaling times at different times of day, but did not show a gradual increase in shoaling throughout their subjective day. Fish from the DD and LL groups did not show significant differences in the mean shoaling values at different times of day, suggesting that the differences observed in LD fish may not represent circadian rhythms. Therefore, these results demonstrate the existence of daily variations in the shoaling behavior of fish and suggest that environmental cues in the form of light/dark cycles play an important role in regulating these variations.