Beam trawling causes physical disruption of the seabed through contact of the gear components with the sediment and the resuspension of sediment into the water column in the turbulent wake of the gear. To be able to measure and quantify these impacts is important so that gears of reduced impact can be developed. Here we assess the physical impact of both a conventional 4 m tickler-chain beam trawl and a “Delmeco” electric pulse beam trawl. We measure the changes in seabed bathymetry following the passage of these gears using a Kongsberg EM2040 multi-beam echosounder and use a LISST 100X particle size analyser to measure the concentration and particle size distribution of the sediment mobilized into the water column. We also estimate the penetration of the gears into the seabed using numerical models for the mechanical interaction between gears and seabed. Our results indicate that the seabed bathymetry changes between ∼1 and 2 cm and that it is further increased by higher trawling frequencies. Furthermore, our results suggest that the alteration following the passage of the conventional trawl is greater than that following the pulse trawl passage. There was no difference in the quantity of sediment mobilized in the wake of these two gears; however, the numerical model introduced in this study predicted that the tickler-chain trawl penetrates the seabed more deeply than the pulse gear. Hence, greater alteration to the seabed bathymetry by the tickler-chain beam trawling is likely to be a result of its greater penetration. The complimentary insights of the different techniques highlight the advantage of investigating multiple effects such as sediment penetration and resuspension simultaneously and using both field trials and numerical modelling approaches.

Tickler-chain SumWing and electrode-fitted PulseWing trawls were compared to assess seabed impacts. Multi-beam echo sounder (MBES) bathymetry confirmed that the SumWing trawl tracks were consistently and uniformly deepened to 1.5 cm depth in contrast to 0.7 cm following PulseWing trawling. MBES backscatter strength analysis showed that SumWing trawls (3.11 dB) flattened seabed roughness significantly more than PulseWing trawls (2.37 dB). Sediment Profile Imagery (SPI) showed that SumWing trawls (mean, SD) homogenised the sediment deeper (3.4 cm, 0.9 cm) and removed more of the oxidised layer than PulseWing trawls (1 cm, 0.8 cm). The reduced PulseWing trawling impacts allowed a faster re-establishment of the oxidised layer and micro-topography. Particle size analysis suggested that SumWing trawls injected finer particles into the deeper sediment layers (∼4 cm depth), while PulseWing trawling only caused coarsening of the top layers (winnowing effect). Total penetration depth (mean, SD) of the SumWing trawls (4.1 cm, 0.9 cm) and PulseWing trawls (1.8 cm, 0.8 cm) was estimated by the depth of the disturbance layer and the layer of mobilized sediment (SumWing = 0.7 cm; PulseWing trawl = 0.8 cm). PulseWing trawls reduced most of the mechanical seabed impacts compared to SumWing trawls for this substrate and area characteristics.

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The increased use of backscatter measurements in time series for environmental monitoring necessitates the comparability of individual results. With the current lack of pre-calibrated multibeam echosounder systems for absolute backscatter measurement, a pragmatic solution is the use of natural reference areas for ensuring regular assessment of the backscatter measurement repeatability. This method mainly relies on the assumption of a sufficiently stable reference area regarding its backscatter signature. The aptitude of a natural area to provide a stable and uniform backscatter response must be carefully considered and demonstrated by a sufficiently long time-series of measurements. Furthermore, this approach requires a strict control of the acquisition and processing parameters. If all these conditions are met, stability check and relative calibration of a system are possible by comparison with the averaged backscatter values for the area. Based on a common multibeam echosounder and sampling campaign completed by available bathymetric and backscatter time series, the suitability as a backscatter reference area of three different candidates was evaluated. Two among them, Carré Renard and Kwinte, prove to be excellent choices, while the third one, Western Solent, lacks sufficient data over time, but remains a valuable candidate. The case studies and the available backscatter data on these areas prove the applicability of this method. The expansion of the number of commonly used reference areas and the growth of the number of multibeam echosounder controlled thereon could greatly contribute to the further development of quantitative applications based on multibeam echosounder backscatter measurements.

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