Light level thresholds for visual reaction of mackerel, Scomber scombrus L., to coloured monofilament nylon gillnet materials

Abstract A continuously swimming school of 12 dark-adapted mackerel was challenged to show visual reaction to barriers made from horizontal parallel lines (5 cm apart) of either monofilament nylon (0.40 mm diameter) gillnet material or, for comparison, the more visible twisted multifilament nylon (1.73 mm diameter). Six colours of each material were tested. The barriers were placed across the path of the approaching mackerel school while their behaviour was observed at low intensities of green light arranged to mimic undersea conditions. At very low light levels, the mackerel swam without hesitation through the barrier, whereas at higher light levels they turned or paused, indicating that they visually aware of the presence of the lines. The light level thresholds for visual reaction for the six colours of monofilament nylon were between −0.06 and −1.0 log lx, and for the same colours of twisted multifilament nylon were between −1.8 and −3.6 log lx. These findings are discussed in relation to the natural intensities of light in the sea, and the prediction of the visibility of drift and gillnet materials.