ESTIMATING THE SWEPT AREA OF DEMERSAL LONGLINES BASED ON IN-SITU VIDEO FOOTAGE

During a longline survey on BANZARE Bank (Division 58.4.3b) in May 2008, an in-line video camera recorded the interactions between the main line and the benthos during setting and hauling. Footage revealed that after setting the line moved very little, despite cross currents. During hauling, the section of line visible to the camera swept sideways, interacting with sessile benthic organisms before leaving the seafloor. Based on an estimate of the rate of sideways movement, the total time sideways movement occurred before the line lifted off the seafloor, the total length of the line and the total time to retrieve the line, we estimate that up to 0.122 km2 of seafloor was swept by the line while it was being retrieved. Consequently the area of seafloor affected by demersal longlines is shown to be comparable with that of demersal trawls. We also provide evidence that benthic fauna that are vulnerable to longline gear may not be retained at the surface, but can fall off the longline hooks before reaching the surface. Hence surface observations of some benthic bycatch are likely to underestimate of the actual level of interaction.