Diel variation in trawl catch rates of Pagrus auratus (Sparidae)

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An experiment was conducted to determine whether trawl catch rates of Pagrus auratus show any diel variation. Two study sites were each sampled at seven time periods (four during the day and three at night) with three replicate tows per period. Fish were grouped into six size classes. The smallest (< 6 cm, 0+) and largest (≥ 30 cm) size classes had significantly greater catch rates at night than during the day; the 7–14.9 cm (1+) and 15–21.9 cm (2+) size classes had significantly greater catch rates during the day, and the 22–24.9 (3+) and 25–29.9 size classes showed no diel variation. For size classes other than the smallest (0+), the differing responses may reflect a balance between two opposing processes: (a) visual herding by the trawl gear that tends to increase the catch rates of all size classes during the day, and (b) net avoidance that is greater by day and increases with fish size. Because of high escapement through the meshes and small sample size, the greater night catch rates for the 0+ size class require confirmation.