Impact of bottom trawling on deep-sea sediment properties along the flanks of a submarine canyon.

Jacobo Martín,Pere Puig,Albert Palanques,Pere Masqué,Anabel Sánchez-Gómez,Kay C Vopel

doi:10.1371/journal.pone.0104536

Abstract

The offshore displacement of commercial bottom trawling has raised concerns about the impact of this destructive fishing practice on the deep seafloor, which is in general characterized by lower resilience than shallow water regions. This study focuses on the flanks of La Fonera (or Palamós) submarine canyon in the Northwestern Mediterranean, where an intensive bottom trawl fishery has been active during several decades in the 400–800 m depth range. To explore the degree of alteration of surface sediments (0–50 cm depth) caused by this industrial activity, fishing grounds and control (untrawled) sites were sampled along the canyon flanks with an interface multicorer. Sediment cores were analyzed to obtain vertical profiles of sediment grain-size, dry bulk density, organic carbon content and concentration of the radionuclide 210Pb. At control sites, surface sediments presented sedimentological characteristics typical of slope depositional systems, including a topmost unit of unconsolidated and bioturbated material overlying sediments progressively compacted with depth, with consistently high 210Pb inventories and exponential decaying profiles of 210Pb concentrations. Sediment accumulation rates at these untrawled sites ranged from 0.3 to 1.0 cm y−1. Sediment properties at most trawled sites departed from control sites and the sampled cores were characterized by denser sediments with lower 210Pb surface concentrations and inventories that indicate widespread erosion of recent sediments caused by trawling gears. Other alterations of the physical sediment properties, including thorough mixing or grain-size sorting, as well as organic carbon impoverishment, were also visible at trawled sites. This work contributes to the growing realization of the capacity of bottom trawling to alter the physical properties of surface sediments and affect the seafloor integrity over large spatial scales of the deep-sea.

Highlights

Bottom trawling is a fishing technique that consists in pulling nets in contact with the seafloor to capture bottom-dwelling animals of commercial interest
The vertical profiles of sediment grain-size, dry bulk density, organic carbon content and excess 210Pb concentration for each sediment core, together with corresponding X-radiographs are shown in Figures 2 and 3 according to the location of coring sites on the north or south canyon flanks, respectively
Cores from untrawled sites (SF-3 to SF-6) showed a fairly continuous transition from liquefied to denser mud, while at the more landproximal trawled sites NF-1, NF-2, SF-1, SF-2, this transition was more abrupt with a limit between 5 and 10 cm depth, depending on the particular core, separating two sedimentary units of contrasting densities

Summary

Introduction

Bottom trawling is a fishing technique that consists in pulling nets in contact with the seafloor to capture bottom-dwelling animals of commercial interest. To make the fishing gear operative, heavy devices such as lead rolls, bobbins, sweeplines, tickler chains or otter boards are used which cause widespread mortality of benthos, stirring and resuspension of sediments, changes in oxygen penetration depths and sediment geochemistry and a general destructuration of benthic habitats [1,2,3,4] This practice has been extended to new and deeper grounds during the last few decades due to the exhaustion of shallow water fish stocks and taking advantage of technological improvements and subsidies that have encouraged the exploitation of previously inaccessible deep-sea resources [5,6]. The deep (.200 m) seafloor is in general subjected to a limited degree of physical disturbance and sediment remobilization by natural processes In such a context, the relative weight of anthropogenic disturbances can be overwhelming and long-lasting [7,8]. Human activities such as deep-sea mining, ocean dredge spoil dumping, laying of undersea cables, shipwrecks or warfare can occasionally lead to acute disturbances on the seafloor, but at present commercial bottom trawling largely leads the list of mandriven impacts on bottom sediments, due to the combination of its recurrence, intensity, mobility and wide geographical extent [9,10]

Methods

Results

Discussion

Conclusion