The role of fanning behavior in water exchange by a nest-guarding benthic fish before spawning

Displacement fanning (fanning during courtship before eggs are deposited) by fish may be an indicator of parental ability. Fanning may be used for ventilation or for the dispersal of chemical messages. We used particle image velocimetry to visualize and characterize flow fields generated by courting male Neogobius melanostomus. Once flow fields were characterized, we conducted a laboratory experiment to determine if caudal and pectoral fanning behavior was modulated by the presence of a female near the nest. There were significant differences between caudal and pectoral fanning (F_1,11 = 54.696, p < 0.001) as was the interaction term, fanning type x distance (F_{2, 22} = 9.870, p < 0.001), on water entering and leaving the nest. The presence of a single female near the nest entrance had no effect on fanning rates. The velocity of water exiting the nest induced by caudal fanning (with the tail at the nest entrance) was 6.0 ± 0.96 (mean ± SD), 5.6 ± 1.18, and 3.1 ± 1.05 mm s⁻¹ at 5, 10, and 15 cm away from the nest opening, respectively. In contrast, the mean velocity of water entering the nest via pectoral fanning (with the head at the nest entrance) was −3.9 ± 0.69, −0.2 ± 0.45 and −0.1 ± 0.28 mm s⁻¹ at the respective distances. Caudal fanning pushes water out of the nest and pectoral fanning pulls water into the nest. We speculate that these are specializations of locomotive structures that lead to water stirring (pectoral fins) and pumping (caudal fin) to disperse odors.