During the past decades, bottom trawling has spread around the globe and its spatial extent has now been estimated to be an order of magnitude greater than the total extent of all other anthropogenic activities. The direct impacts of this technique on benthic communities and fish populations are known but trawling can also modify the physical properties of seafloor sediments, water-sediment chemical exchanges and sediment fluxes. Using high-resolution seafloor relief maps, Puig et al. (2012) found that on upper continental slopes, sediment displacement and removal by trawling gradually causes the morphology of the deep sea floor to become smoother over time thus reducing its complexity and habitat heterogeneity. It is anticipated that many parts of the world’s oceans could be altered by intensive bottom trawling, producing comparable effects on the deep sea floor to intensive agricultural activities on land.—Evelyn Byer
Puig, P., Canals, M., Martín, J., Amblas, D., Lastras, G., Palanques, A., Calafat, A.M., 2012. Ploughing the deep sea floor. Nature 489, 286-289.
Puig and colleagues from the Marine Sciences Institute and University of Barcelona in Barcelona, Spain used a unique experimental strategy by combining monitoring of contemporary sediment transport processes, sediment coring, remote operated vehicle in situ observations and high-resolution multibeam bathymetric surveys coupled with the information provided by satellite-based navigation tracks of fishing vessels. Time series observations were carried out on the flank of La Fonera Canyon, a trawled submarine canyon in the Mediterranean, to measure sediment transport induced by fishing. An instrumented mooring was placed within a tributary valley to capture sediment gravity flow. In 2007 a high-resolution multibeam bathymetry survey was conducted in the canyon. Puig et al. plotted four years (2007-2010) of satellite-based navigation tracks from all large bottom trawlers operating in the area on top of the multibeam bathymetry of La Fonera Canyon.
Instrumented mooring observations included increases of near-bottom suspended sediment concentrations and current velocities during weekdays thus revealing a highly active scenario with the almost daily occurrence of sediment transport linked to trawling fleets upslope of the observation site. Analysis of the bathymetry data revealed a noticeable smoothing of bottom topography along the northern canyon flank at depths shallower than 800 meters, which had in the past been interpreted as a result of dense shelf water cascading flowing southward. Smoothing, however, was also observed in the southern canyon flank, away from the region potentially affected by cascading flows, challenging the previous hypotheses. The depth of 800 meters coincided with the maximum trawled depths which points to trawling as a potential shaping agent within this depth range. Overlaying satellite-based navigation tracks over the bathymetry data revealed that navigation tracks coincided with the smoothed canyon flanks. In contrast, untrawled canyon flanks are dominated by more complex topography. Therefore, a causative relationship can be established between trawling-induced erosion effects and the reduction of morphological complexity. Evidence from sediment cores and remote operated vehicle footage support the observed contrasts between untrawled and trawled seafloors. In conclusion, Puig et al. approximated that 2.4 X 10^(-4) km^3 of sediment has been exported from this fishing ground annually. This study raises questions over the possibility that today trawling is an important driver of seascape evolution. Along with alteration of topography, trawling also reduces habitat heterogeneity which may affect species diversity. The impacts of trawling have been compared to the impacts from forest clear-cutting, however the results from Puig et al. indicate a better comparison to be the impacts from intensive agricultural activities, although farmers usually plough their land a few times per year while sea trawling con occur on a nearly daily basis. Puig et al. argue for the inclusion of trawling in the list of major present and future human footprints in the ocean.