Measuring and assessing the physical impact of beam trawling

Abstract

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.

Impacts of electrofishing for razor clams (Ensis spp.) on benthic fauna

“…The low by-catch for non-target species and undersized Ensis, and the minimal physical impact of electrofishing gear on the seabed in comparison to conventional dredge fisheries are consistent with results of sea-trials of electrofishing for other species (ICES, 2006;Polet et al, 2005b;Van Marlen et al, 2014;Depestele et al, 2015;Soetaert et al, 2015). Such findings have allowed Dutch and Belgian fleets to successfully apply for EU derogations to legally investigate the use electrofishing to harvest sole and Crangon respectively.…”

“…(Woolmer et al, 2011). Recent research examining the use of electrodes in electrotrawls in the dover sole fishery (Van Marlen et al, 2014) and the Crangon fishery (Polet et al, 2005a) suggest that such techniques can reduce the impact of fishing on non-target species and the benthic habitat (Depestele et al, 2015). Electrofishing for Ensis spp., where small inshore vessels fly-drag up to three pairs of electrodes slowly across the seabed, followed by divers who collect emerging razor clams, was first reported in Scotland in 2004 (Breen et al, 2011).…”

Towards a framework for the quantitative assessment of trawling impact on the seabed and benthic ecosystem

“…On soft sediments, heavy components of the gear, such as the doors of an otter trawl or the shoes of a beam trawl, will penetrate in the seabed and create a furrow by pushing aside the sediment (Schwinghamer et al, 1996;Smith et al, 2007;Buhl-Mortensen et al, 2013;Depestele et al, 2016;O'Neill and Ivanović, 2016). Rakes, or a series of tickler chains running in front of the groundrope, will penetrate and enhance the mixing in the impacted layer; this alters the sediment sorting and damages the tubes and burrows of infaunal species.…”

Reducing bycatch in beam trawls and electrotrawls with (electrified) benthos release panels

“…Demersal fishing techniques have direct physical contact with the seabed to ensure adequate capture rates of target species (Depestele et al, 2016), which can affect the marine environment and particularly the benthic communities, reducing their biomass, production and diversity (Lindeboom and de Groot, 1998;Kaiser et al, 2006;Depestele et al, 2014). Beam-trawl impact may result from the direct mortality caused by the trawl and the indirect effects of this mortality on species interactions (Ramsay et al, 1997;Jennings et al, 2002).…”