The impacts of deep-sea fisheries on benthic communities: a review

Abstract

Deep-sea fisheries operate globally throughout the world's oceans, chiefly targeting stocks on the upper and mid-continental slope and offshore seamounts. Major commercial fisheries occur, or have occurred, for species such as orange roughy, oreos, cardinalfish, grenadiers and alfonsino. Few deep fisheries have, however, been sustainable, with most deep-sea stocks having undergone rapid and substantial declines. Fishing in the deep sea not only harvests target species but can also cause unintended environmental harm, mostly from operating heavy bottom trawls and, to a lesser extent, bottom longlines. Bottom trawling over hard seabed (common on seamounts) routinely removes most of the benthic fauna, resulting in declines in faunal biodiversity, cover and abundance. Functionally, these impacts translate into loss of biogenic habitat from potentially large areas. Recent studies on longline fisheries show that their impact is much less than from trawl gear, but can still be significant. Benthic taxa, especially the dominant mega-faunal components of deep-sea systems such as corals and sponges, can be highly vulnerable to fishing impacts. Some taxa have natural resilience due to their size, shape, and structure, and some can survive in natural refuges inaccessible to trawls. However, many deep-sea invertebrates are exceptionally long-lived and grow extremely slowly: these biological attributes mean that the recovery capacity of the benthos is highly limited and prolonged, predicted to take decades to centuries after fishing has ceased. The low tolerance and protracted recovery of many deep-sea benthic communities has implications for managing environmental performance of deep-sea fisheries, including that (i) expectations for recovery and restoration of impacted areas may be unrealistic in acceptable time frames, (ii) the high vulnerability of deep-sea fauna makes spatial management—that includes strong and consistent conservation closures—an important priority, and (iii) biodiversity conservation should be > balanced with options for open areas that support sustainable fisheries.

Cold-Water Coral Habitats in Submarine Canyons of the Bay of Biscay

“…Trawl gear can damage CWC communities by reducing or changing coral abundances, diversity and community or the removal of structuring species (reviewed by Clark et al, 2016), as shown for coral reefs of the NE Atlantic (Hall-Spencer et al, 2002) and Solenosmilia thickets on Australian seamounts (Althaus et al, 2009). In addition to damages caused by physical contact with trawl gear, trawling can have an indirect impact on CWCs.…”

Deep-Sea Mega-Epibenthic Assemblages from the SW Portuguese Margin (NE Atlantic) Subjected to Bottom-Trawling Fisheries

“…It is postulated that trawling practices may have stronger effects on the deep-sea mega-epibenthic fauna in comparison with shallower areas (Clark et al, 2015). This arises from the typical characteristics of deep-sea species, particular lifehistory traits (k-selected; e.g., slow growth, high longevity), metabolic rates (low productivity) and reproductive strategies (e.g., intermittent spawning events), which make them more vulnerable and less resilient to the effects of trawling practices (Thrush and Dayton, 2002).…”

“…This arises from the typical characteristics of deep-sea species, particular lifehistory traits (k-selected; e.g., slow growth, high longevity), metabolic rates (low productivity) and reproductive strategies (e.g., intermittent spawning events), which make them more vulnerable and less resilient to the effects of trawling practices (Thrush and Dayton, 2002). Heavier trawl gears and more localized practices can also exert a stronger pressure on deepsea habitats (Clark et al, 2015). Yet, the effects on deepsea benthic habitats and mega-epibenthic assemblages are still debated and geographically dependent, since in most cases we lack either background knowledge of the biodiversity on the long-term targeted areas, or an obligatory legislation that requires impact assessment and monitoring programmes at recent fishing grounds (Clark et al, 2015).…”

“…Heavier trawl gears and more localized practices can also exert a stronger pressure on deepsea habitats (Clark et al, 2015). Yet, the effects on deepsea benthic habitats and mega-epibenthic assemblages are still debated and geographically dependent, since in most cases we lack either background knowledge of the biodiversity on the long-term targeted areas, or an obligatory legislation that requires impact assessment and monitoring programmes at recent fishing grounds (Clark et al, 2015). The most obvious effects identified so far include the large-scale changes of the seabed topography and sediment dynamics (e.g., fishing grounds at the upper flank of La Fonera canyon, Catalan margin; Puig et al, 2012).…”

“…With each trawling haul, the seafloor is flattened and large amounts of sediment are re-suspended, often resulting in alterations of both surface and sub-surface sediment properties, namely organic matter concentrations, grain size composition, and porosity (Martín et al, 2014;Oberle et al, 2016). These high turbidity periods often extend beyond the fishing grounds, indirectly impacting non-target areas by smothering filterfeeding organisms and increasing mortality rates of their faunal assemblages (Greathead et al, 2007;Leys, 2013;Clark et al, 2015). Effects on mega-epibenthic fauna include the decline of both standing stocks (abundance and biomass) and species richness, and changes in community composition (Clark et al, 2015 and references therein).…”