Chronic and intensive bottom trawling impairs deep-sea biodiversity and ecosystem functioning

Abstract

Bottom trawling has many impacts on marine ecosystems, including seafood stock impoverishment, benthos mortality, and sediment resuspension. Historical records of this fishing practice date back to the mid-1300s. Trawling became a widespread practice in the late 19th century, and it is now progressively expanding to greater depths, with the concerns about its sustainability that emerged during the first half of the 20th century now increasing. We show here that compared with untrawled areas, chronically trawled sediments along the continental slope of the north-western Mediterranean Sea are characterized by significant decreases in organic matter content (up to 52%), slower organic carbon turnover (ca. 37%), and reduced meiofauna abundance (80%), biodiversity (50%), and nematode species richness (25%). We estimate that the organic carbon removed daily by trawling in the region under scrutiny represents as much as 60-100% of the input flux. We anticipate that such an impact is causing the degradation of deepsea sedimentary habitats and an infaunal depauperation. With deep-sea trawling currently conducted along most continental margins, we conclude that trawling represents a major threat to the deep seafloor ecosystem at the global scale.

Indicator-Based Assessment of Marine Biological Diversity–Lessons from 10 Case Studies across the European Seas

“…The habitat richness is reflected by a high biodiversity (Coll et al, 2012;Micheli et al, 2013b), with approximately 49% of the species described for the Mediterranean Sea (Boudouresque et al, 2009;UNEP, 2015) and a variety of endemic species (e.g., 18% of the endemic fish species of the Mediterranean; UNEP/MAP-RAC/SPA, 2015). Human activities and multiple stressors, and in particular bottom trawling, hydraulic dredging and habitat loss, are certainly still impacting the Adriatic Sea (Micheli et al, 2013a;Pusceddu et al, 2014). However, the overall environmental condition is not worsening with respect to the past decade.…”

Is the meiofauna a good indicator for climate change and anthropogenic impacts?

“…This may explain why in the South Pacific, the relationship between diversity and ecosystem functioning appears weaker, whereas in other regions, such as the Atlantic Ocean and Mediterranean Sea, the relationship is stronger (Leduc et al 2013). On the continental slope of the northwestern Mediterranean Sea, deep-sea fisheries have caused the collapse of benthic meiofaunal biodiversity and ecosystem functioning, with potential consequences on biogeochemical cycles (Pusceddu et al 2014a). Moreover, a recent study assessed the influence of meiofauna on the relationship between macrofaunal biodiversity and ecosystem functioning (Piot et al 2014), highlighting the fact that meiofauna may have a strong direct impact on benthic properties, by modifying the interactions between macrofaunal species, and therefore their impacts on ecosystem properties.…”

Mediation of macronutrients and carbon by post-disturbance shelf sea sediment communities

“…Associated changes in grain size distribution, increased sediment sorting and alterations to porosity (Trimmer et al 2005) can disrupt nitrification and denitrification processes (Rysgaard et al 1994) through changes in oxygen penetration (Warnken et al 2003), which, in turn, may also affect faunal and microbial activity (Sciberras et al in review). Over extended periods of time these physical and biogeochemical changes reduce habitat complexity (Kaiser et al 2002) and alter community structure by reconfiguring species and functional trait dominance (Kaiser et al 2006;Pusceddu et al 2014;Sciberras et al 2016), causing a shift from sessile emergent species with high biomass to smaller bodied infaunal species . Importantly, such selective forcing may skew trophic structure (Duffy 2003;Wohlgemuth et al 2016) and lead to the loss of species interactions that influence nutrient generation and dynamics (Gilbertson et al 2012); the active redistribution of particles and fluids by infaunal invertebrates, for example, directly contributes to the spatial and temporal heterogeneity of oxic and anoxic zones (Bertics and Ziebis 2009), the availability of organic matter (Levin et al 1997), and the distribution of metabolic electron acceptors (Aller 1982;Fanjul et al 2007) that are important in controlling microbial process rates and benthic-pelagic coupling linked to primary productivity (Lohrer et al 2004).…”