Effects of fisheries on benthos

Trawling activity. Photo: Norwegian Polar Institute

Benthos
Typography
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Recent ocean warming has caused some commercial fish species to expand northward. This includes the Atlantic cod (Gadus morhua), which has recently been recorded north to 82ºN on the edge of the Barents Sea shelf to the Arctic Ocean (Johansen et al., 2013; Kjesbu et al., 2014).

As a consequece, the commercial trawling fleet may follow this stock to more northern parts of the Barents Sea, e.g. from the Hopen Deep, to formerly ice-covered areas previously too cold for predominantly boreal fish species such as Atlantic cod.

Bottom trawling, suggested to be the equivalent of forest clear-cutting on land (Watling and Norse, 1998), is known to have significant and potentially long-lasting impacts on seafloor communities. Impacts include: removal of habitat-forming organisms; homogenization of seafloor habitats; altered sediment structure; and reduced oxygen penetration into the sediments (e.g. Collie et al., 2000; Thrush and Dayton, 2002; Kaiser et al., 2006). It can affect benthic megafauna, particularly erect sessile forms which are fragile and easily damaged or destroyed by bottom trawls (Kaiser et al., 2002; Hiddink et al., 2006). Frequent disturbance of soft-sediment communities leads to the proliferation of smaller benthic species with faster life histories. Because the larger species are usually removed by bottom-fishing gears, both habitat complexity and the depth of bioturbation are often reduced, resulting in lower benthic production (Jennings et al., 2001; Kaiser et al., 2006).

Specific functional groups of benthic organisms have been shown to be particularly sensitive to bottom trawling. These include emergent epifauna, some bioturbating decapods, suspensions feeders, and long-lived high-biomass organisms (including sponges and corals) (Garcia et al., 2007; Olsgard et al., 2008; and Mangano et al., 2014). Changes affecting such groups could have far-reaching ecosystem impacts and alter ecosystem services (fish nursery habitat, feeding grounds, biogeochemical cycling) provided by such components of benthic communities, including food-web interactions which support commercial fish and shellfish stocks (Widdicombe et al., 2004; Olsgard et al., 2008). Habitat-generating species are represented by a wide range of taxonomic groups (e.g. Porifera, Polychaeta, Cnidaria, Mollusca, and Bryozoa (Kaiser and de Groot, 2000) provide shelter for diversely associated species, and are examples of entire communities which require protection.

Studies indicate that between the 1920s and 1960s, benthic biomass in the Barents Sea declined by up to 70% in some areas (Denisenko, 2001). It is uncertain how much of that reduction is attributable to bottom fishing, but the correlation between spatial patterns of biomass reduction and fishing pressure is reasonably high (Figure 4.3.21). Some systems may recover from fishing activity, as was the case for Svalbard Bank after intense scallop dredging in the 1980s (Kędra et al., 2013). Such a recovery, however, may depend upon local hydrodynamics (affecting grain size and food supply) and the intensity of fishing pressure (Collie et al., 2000). Identifying a set of indicators to determine the impacts of trawling would be a huge step forward for benthic habitat management.

Megafauna and areas vulnerable to trawling

Fisheries in the Barents Sea are generally conducted in certain areas (see Figure 4.3.21 and 4.3.22), but variations occur due to interannual fluctuations in commercial fish stock (see Figure 4.3.22). A slight northward expansion of fishing activity on the east side of Svalbard was observed during 2006-08 and 2010-12.

Figure 4.3.21a. Fishing activity in the 2006-2008 (Norwegian registered fishery). Figure 4.3.21a. Fishing activity in the 2006-2008 (Norwegian registered fishery).

Figure 4.3.21b. Fishing activity in the 2010-2012 (Norwegian registered fishery). Figure 4.3.21b. Fishing activity in the 2010-12 (Norwegian registered fishery).

Figure 4.3.21c. Fishing activity in the Barents Sea 2002-2005 (Russian registered fishery).Figure 4.3.21c. Figure 4.3.21c. Fishing activity in the Barents Sea 2002-2005 (Russian registered fishery).

Bottom trawling is expected to capture large bodied megafauna (Jørgensen et al. 2014). When “mean body weight” (i.e. weight/abundance) and “height” was used as a proxy for vulnerable species, possible vulnerable areas were observed along the continental slope of the Barents Sea, in the Arctic northern and eastern Barents Sea, and in a narrow band in the south (Figure 4.3.22, left). In the central Barents Sea, particularly outer Bear Island Channel and the Pechora Sea in the south-eastern region, the megafauna consisted of relatively small individuals.

Figure 4.3.22a. Particular vulnerable areas (red circles left side map) (Jørgensen et al 2015).Figure 4.3.22a. Particular vulnerable areas (red circles left side map) (Jørgensen et al 2015).

Figure 4.3.22b. Distribution of large/voluminous and/or large upraised body-size species easily taken by a trawl (Jørgensen et al 2015).Figure 4.3.22b. Distribution of large/voluminous and/or large upraised body-size species easily taken by a trawl (Jørgensen et al 2015).

Figure 4.3.22b shows species easily taken by a trawl due to large and/or upraised bodies, which include:

  • Geodia sponges distributed from the southwestern Barents Sea and northward along the continental shelf, round northern Svalbard and further east
  • King crab populations along the southern coastline
  • Sea cucumber populations on the Svalbard Bank and Banks in SE (Cucumaria frondosa), or the deep continental slope in SW (Parasticopus tremulus)
  • Sea lilies along the shelf area of the banks and in the northern part of the Barents Sea
  • Ssnow crab populations
  • Gorgonocephalus spp. with fields of basket stars covering vast areas in the north
  • Sea pen Umbellula encrinus north of 80ºN, making these into possible areas in need of protection against trawling (Jørgensen et al in 2015)

All of these represent species and areas in possible need of protection against trawling (Jørgensen et al., 2015).

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