Development of schooling behavior in Spanish mackerel Scomberomorus niphonius during early ontogeny

Development of schooling behavior was studied in hatchery-reared Spanish mackerel Scomberomorus niphonius. Behavior of larvae and juveniles was video recorded from above in a 500-L rearing tank from day 7 up to day 23 at 1-2-day intervals. For the video image analysis, separation angle (SA), nearest neighbor distance (NND), separation swimming angle (SSA) and separation swimming index (SSI) were defined and measured. Schooling behavior developed from day 17 (15.9mm in standard length (SL)) to day 19 (19.6mm in SL). During this period, SA and NND decreased significantly, suggesting that they started aggregation forming parallel orientation. Separation swimming angle and SSI also decreased dramatically in this period. After day 17 and up to day 23 (26.6mm in SL), these parameters of schooling behavior did not show much change, suggesting that schooling behavior of Spanish mackerel juveniles was completed by day 19. Relatively high values of NND (1.2-1.5-fold SL compared to <1.0 in most other species) and SSI (0.6-0.8 compared to 0.15-0.29 in chub mackerel Scomber japonicus) reflected a loose school as pelagic fish juveniles, and in this way they might have been minimizing the risk of cannibalism in this piscivorous species.