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FEATURE ARTICLE - Trawl and temperature pressure on Barents benthos

The third feature article of the year heads north to the Barents Sea, looking at monitoring the effects of bottom trawling – and the influence of temperature  – on the seabed and its inhabitants.
 
Published: 11 July 2017

​​​​​​​​Lis Lindal Jørgensen, Institute of Marine Research (IMR), Norway ​

The Barents Sea, being a transition from a boreal to an Arctic zone, has four main megafaunal (seabed organisms larger than 4 mm) regions: the southwest, the banks/slopes in southeast and west, the northwest, and the northeast. These regions significantly relate to depth, temperature, salinity, and the number of days when the sea is covered with ice. The southwest region is dominated by filter-feeders (sponges) in the inflow area of warm Atlantic water, while the deeper trenches have detritivorous fauna (echinoderms​ such as sea stars and cucumbers). In the southeast and west region, predators (sea stars, anemones, and snow crabs) prevail together with filtrating species such as sea cucumber and bivalves​ ​within a mosaic of banks and slopes. Plankton-feeding brittle stars are common in the northwest and northeast regions, but with an increasing snow crab population in northeast. ​

Trawling on the seafloor

One of the most widespread, yet manageable, pressures on the seabed is the disturbance of layers (substrate) by towed demersal fishing gear. Fisheries research and management have traditionally focused on economically important fish stocks, while resources of minor economic significance have received less attention. Fisheries policy objectives have gradually shifted towards a stronger emphasis on both ecological and economic sustainability, and a complex set of regulatory measures have been developed and put into force.

The effects of trawling on structurally complex seabed habitats and fauna have been compared with the effects of forest clear-cutting. Bottom trawling can remove benthic species that characterize the benthic habitats, which are used as refuge from predation or provide food for a wide variety of fish and invertebrates. Species removal can result in a significant reduction in the abundance of large invertebrates because of their slow recovery time and high catchability, which can then shift the size spectrum to favor an abundance of small invertebrates.

Asking questions and evaluating status and trends

Because of this it is necessary to monitor the status and trends of benthic biota, but it is also critical to take a larger view and understand the structure and dynamics of the habitats, how they contribute to marine ecosystem functioning, how fishing can alter ecosystems, and ultimately how to manage human activity while minimizing the risks of serious or irreversible harm to them. Asking specific questions including "what defines a bottom species with high risk of being hit by a bottom trawl?" and "where are these species and communities distributed" can lead to the use of trait analysis, which focuses on form and function of the organisms. Species coding and ranking for adult "height", "mean-weight", and "mobility" gives the opportunity to achieve a mean vulnerability to the seabed community being caught or hit by a trawl. A dominance of tall, large species that are anchored or attach themselves to substrates (known as sessile species) will be more vulnerable than communities with small, fast-moving species. Combined with species' sensitivity to global warming, benthos hot-spots of large, sessile, Arctic species will be subjected to multiple stressors if they are in areas previously inaccessible due to sea-ice but that have now opened up to potential bottom trawling due to fish stocks migrating northwards.     

Building a monitoring programme

With effects from climate change, trawling activity, new predatory species, and oil exploration, the seabed – and its organisms (benthos) – is changing fast. This has created the need for a time and cost efficient long-term monitoring plan. To enable this, benthos experts were included in the annual cruises that already existed for assessing commercial fish stocks. Benthos, already taken as bycatch by the scientific fish trawl was identified and measured on-board instead of being dumped back into the sea as usual. This means that the mega-benthos investigation uses no extra ship, ship-time, or equipment –​ and that it doesn't cause more damage to the seabed.

From between the start of the survey in 2009 and 2016 we had identified 70 tonnes of megafaunal biomass, comprising 4.4 million individuals and 467 taxa from 2280 sampling stations. Encouraged by building up an international monitoring plan within the Arctic Council (Circumpolar Biodiversity Assessment Program, CBMP_marine), Norway, Russia, Greenland, Iceland, Canada and the USA took the first step towards a pan-Arctic map with trawl-stations included and prese​nted this in the newly released "State of the Arctic Marine Biodiversity Report (SAMBR). Furthermore,  the Nordic Council (AG-Fisk) has financed the implementation of the Greenland long-term monitoring plan of the national scientific ground fish surveys (2015-2018), and will continue to finance a working group looking at how to use the growing international datasets in a simple, transparent, and standardized way.

Barents Sea Benthos

The results from the survey show that 23 species with large body weight and that are raised from the seabed (such as ​sponges and sea​ pens) are "easily caught" by a bottom trawl. In the southwestern ​​part of the Barents Sea, Geodia sponges may have functions like those of coral reefs due to an increase in the richness of associated species. Awareness of this region is, therefore, recommended. Intense trawling is recorded here, but the commercial fishing fleet may have generated trawling corridors to avoid filling trawls with sponges and may consequently have "protected" this area. This type of "trawling in corridors" should be applied northward to avoid possible reduction in number or local eradication of the basket star, sea pen, soft coral, and sea lily fields along with the fauna that use them as habitats, feeding area, or shelter in the northern Barents Sea. North of 80°, there needs to be a greater awareness of the highly vulnerable sea pen due to the lack of mapping and knowledge of its vulnerability.

To better understand the consequences of the effect of trawling on the seabed, vulnerability studies are recommended on species such as the sea spiders (such as the gigantic Colossendeis sp), stalked sponges (such as the giant carnivorous club sponge Cladohriza sp., and Asbestopluma spp.​), cephalopods (such as the Arctic Cirroteuthis moelleri and Benthoctopus spp.), the solitary coral, the solitary hydroid (Corymorpha spp.), the delicate soft-sediment sea anemones (Liponema multicornis and Cerianthidae indet), and the sea pen (Radicipes spp.).

This list of possible vulnerable species is just a sub-set of what is taken with the fish trawl in the Barents Sea. But action is needed to get an overview of vulnerable species. This could happen in the field, by understanding what happens to the organisms on the seabed when a trawl is passing, and how long it will take to get back to the original community structure again. Status and trend of the species composition, the community function and structure in time and space also need to be studied and followed. This is considerably more important, as climate change, invasive predatory species (such as the snow- and king crab) and oil exploration impact the benthic community and might obscure the effects of trawling.​

 

Lis lindal Jørgensen was a presenter in the ‘Climate change impact on Barents Sea ecosystem functioning and vulnerability’ session at the recent ICES co-sponsored ESSAS Arctic conference​ in Tromsø, Norway​.​

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Upraised, large-bodied species such as the mobile echinoderm crinoid, which prefers areas of the Barents Sea with low temperatures,​​ are both vulnerable to being caught or damaged by bottom trawling  photo: MAREANO.no​​

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FEATURE ARTICLE - Trawl and temperature pressure on Barents benthos

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