Observation of seabed trenches left by a beam trawl in Tokyo Bay

Seabed disturbance by mobile fishing gears is of concern because they may destroy or substantially alter habitats or benthic communities. Information regarding seabed impacts by mobile fishing gears is limited, particularly for Japanese coastal waters. In this study, we documented a trench created by a beam trawl fishing in Tokyo Bay, Japan. Maps of the mud substrate in Tokyo Bay were created using side scan sonar before and after trawling operations. These maps were subsequently compared to assess operation-related disturbances. The conventional beam trawl used for this research measured 2.75 x 0.3 m at the opening and was 9 m in length. The beam (including tickler chain) weighed 130 kg. Towing speeds were either 4 knots (normal towing speed) or 2.5 knots. Towing locations were continuously recorded using differential GPS. Seabed images produced by side scan sonar were not significantly different between 'pre-trawling' and 'post- trawling' maps for tows made at 4 knots. On the other hand, one continuous trawl trench was observed for 113 m (6.5% of towed distance) for the 2.5 knots experiment. This result suggests that only extreme changes to the seabed can be detected by side scan sonar (e.g., depths > 6.5cm) and that impacts are greatest at slower towing speeds. Even though we demonstrated only small impacts to bottom topography using side scan sonar, it is likely that this gear type adversely impacts epifauna and possibly infauna. Other tools are required to assess these potential impacts.