Fish-trawling impacts in the recent sedimentary record on the Barcelona continental margin and their consequences on organic carbon fluxes

Abstract

Bottom fish-trawling of crustaceans is one of the most important commercial fishing resource in the Catalan margin, while its fishing grounds have been extending to greater depths associated to the modernization of its fishing fleet. The impacts of this activity have been studied in detail in La Fonera Canyon, off Palamós, during the past decade, but no similar studies have been carried out elsewhere in the Catalan margin. For this study, two sediment cores were retrieved on the flanks of Morràs Canyon, off Barcelona, where trawling takes place, and two more coreswere obtained from the adjacent untrawled canyon axis. Different sediment characteristics (dry bulk density and grain size) were analyzed for all cores, as well as the radioisotopic concentrations of 210Pband 137Cs, and theorganic carbon fraction, which were used to assess the impacts on fishing grounds and on the presumed sediment depocenter. Results indicate that trawled areas have superficial over consolidated sediments depleted of 210Pb and organic carbon as a consequence of scraping superficial sediments by trawling, or reworked sediments piled up on eroded sub-layers, attributed to remobilization of sediment from plowing the sea-floor. In the canyon’s axis, an almost tripling of sedimentation rates was quantified in one of the sediment cores from 0.12 ± 0.02cm·y-1to 0.32 ± 0.01cm·y-1, dated to the late sixties. This increase was attributed to trawling activities due to their proximity and the simultaneous industrialization of fishing fleet, causing a doubling of total vessel horsepower. Only one sedimentation rate was quantified for the other sediment core, although the resemblance of 210Pb and 137Cs concentration profiles and the proximity of both cores in this wide canyon axis suggest that both cores should have similar inputs of sediments. Further analyses need to be done to quantify changes in sedimentation rates in this second core