Quantitative analysis of the behaviour of the flatheads (Platycephalidae) during the trawl capture process as determined by real-time multiple observations

The species of flatheads, including Platycephalus bassensis (Cuvier, 1829) and Neoplatycephalus richardsoni (Castelnau, 1872), are common shallow-water (30–160 m depth) teleosts and a valuable component of Australia's largest commercial fishing sector. In this study the behavioural responses of these species to a demersal trawl towed at 1.29 m s−1 (2.5 kt) were observed by the use of a 4-camera video system attached to the trawl. This system enabled simultaneous observations of the fish to be made at several locations between the trawl-mouth and the codend. One of the camera housings was equipped with an optical set-up capable of emitting four laser beams 25 cm apart. These beams were shone onto the seabed or fish body to provide a standard length scale. P. bassensis and N. richardsoni were observed cruising in front of the trawl-mouth for an average of 126 s and 67 s, respectively, while using intermittent rapid swimming and coasting manoeuvres before entering the trawl. Eighty-five percent (85%) of the observed flatheads entered the trawl. The remainder escaped through the gap between the footrope and ground gear below the bottom panel of the trawl or allowed the ground gear to run over them. Only a few individuals of P. bassensis that entered the trawl were observed to orient themselves towards the trawl-mouth and swim in the towing direction. These fish reached the codend in 18 s on average. However, the great majority of the flatheads that were observed in the trawl showed little response. Some of these individuals stayed on the bottom net of the trawl ahead of the codend until the trawl was hauled. The others simply drifted towards the codend. On average, this took 7 s for each of the species. This was significantly shorter than the average time for those fish that actively oriented themselves towards the trawl-mouth (P < 0.05). These behavioural characteristics suggested that in order to reduce catches of the undersized flathead species, or of those in excess of the quota, greater efforts are required to stimulate the flatheads to swim actively and attempt to escape as they pass through the trawl. The implications of these results for current Australian fishing practices are discussed.