A light intensity threshold for schooling in the Atlantic mackerel, Scomber scombrus

The schooling behaviour of Atlantic mackerel was studied in a large tank at different light intensities in the range 12.6–1.8 × 10−10μEs−1 m−2. Variable light intensity was produced by accurately controlling the current to a green light‐emitting diode (LED) 3 m above the experimental tank. Under high light levels (1.8 × 10−6μEs−1 m−2) mackerel always formed a single school, whereas at lower levels (1.8 × 10−8μEs−1 m−2) they swam as individuals. At light levels down to 1.0 × 10−6μEs−1 m−2 the mean nearest neighbour distance in a school remained relatively constant (0.3–0.9 body lengths), and individual mackerel swam along a path which deviated from the position of their nearest neighbours by less than 14°. As light dropped below 1.8 × 10−7μEs−1 m−2, both nearest neighbour distance and heading angle between nearest neighbours increased, with mean values of 1–1.8 body lengths and 23–92°, respectively, at 1.8 × 10−9μEs−1 m−2. The results are discussed in terms of ambient light conditions in the sea.