A light intensity threshold for schooling in the Atlantic mackerel, <i>Scomber scombrus</i>

Abstract

The schooling behaviour of Atlantic mackerel was studied in a large tank at different light intensities in the range 12.6–1.8 × 10 −10 μEs −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 −6 μEs −1 m −2 ) mackerel always formed a single school, whereas at lower levels (1.8 × 10 −8 μEs −1 m −2 ) they swam as individuals. At light levels down to 1.0 × 10 −6 μEs −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 −7 μEs −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 −9 μEs −1 m −2 . The results are discussed in terms of ambient light conditions in the sea.