Direct and deferred sediment-transport events and seafloor disturbance induced by trawling in submarine canyons.

Bottom trawling on marine environments can drastically modify seafloor geomorphology and sedimentary dynamics not only on the fishing grounds but also in adjacent downslope regions, particularly in submarine canyons environments, which are hotspots of benthic biomass and productivity in the deep sea. When this type of fishery occurs along submarine canyon flanks, it can induce sediment gravity flows that descend along tributary gullies towards the main canyon axis. However, these flows had only been clearly identified in the Palamós Canyon, where they could be recorded synchronously with the passage of the trawling fleet. In this study we also recorded trawl-induced sediment gravity flows in the Blanes Canyon, both synchronously and asynchronously with the passage of trawlers. Increases in particulate matter fluxes in other trawled submarine canyons occurring in absence of natural triggering mechanisms, were not directly associated with bottom trawling because of the lack of direct synchronicity of these events with this human activity. Here we show, however, that the practice of bottom trawling along canyon flanks can not only resuspend and directly trigger sediment gravity flows, but they can also pile up disturbed sediment on steep areas, which can become unstable and collapse afterwards, asynchronically with the passage of trawlers. Our study provides evidence that sediment gravity flows in submarine canyons affected by bottom trawling, where the causal mechanisms are presently unidentified, may potentially be linked to instabilities in sediment originating from recurrent bottom trawling, which can precondition these events.

Metal concentrations and detoxification mechanisms in Solea solea and Solea senegalensis from NW Mediterranean fishing grounds.

The common sole, Solea solea and the Senegalese sole, Solea senegalensis are two important commercial species that coexist in the NW Mediterranean Sea. The present field survey was designed to assess the role of kidney in metal handling and detoxification in the two sole species collected at six fishing grounds along the Catalan coast. Metallothionein (MT) and selenium (Se) were analysed in relation to toxic metal loads in kidney as potential protective mechanisms. Acetylcholinesterase (AChE) and lactate dehydrogenase (LDH) activities as well as lipid peroxidation (LP) levels were measured in several tissues as general markers of toxicity. AChE was measured in brain muscle and gills, LDH in plasma and LP in muscle and gills. The protective role of MT and Se was indicated by the positive correlations with Hg and Cd levels as well as with the high Se:Hg ratio, in a species-dependent way. Principal Component Analysis (PCA) considering all chemical and biomarker variables discriminated individuals collected at the different fishing grounds.

Distribution and heavy metal pollution of the suspended particulate matter on the barcelona continental shelf (North-Western Mediterranean).

The distribution of heavy metal pollution associated with suspended particulate matter on the Barcelona continental shelf has been studied to evaluate the environment l impact of anthropogenic metals in this Mediterranean area. The main sources of heavy metal pollution on this continental shelf are the Besós River and the sewage sludge produced in the Barcelona-Besós wastewater treatment plant. The levels of Pb, Cr, Cu, Cd and Ni are very high around the mouths of the river and the pipeline of the wastewater treatment plant. The highest suspended sediment and heavy metal concentrations are along the inner and mid-shelf due to aggregation processes and the low energy of the dominant currents flowing in the study area. However, a significant amount of polluted suspended sediment is transferred to the slope by advective processes. This amount may be increased significantly by the action of wave-induced currents during strong storms. Trawl fishing may also contribute to the shelf-slope transfer of contaminated particles.