Important indirect effects of fisheries on the ecosystem

Interactions, drivers and pressures 2016
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In order to conclude on the total impact of trawling, an extensive mapping of fishing effort and bottom habitat would be necessary. In general, the response of benthic organisms to disturbance differs with substratum, depth, gear, and type of organism. Seabed characteristics from the Barents Sea are only scarcely known and the lack of high-resolution (100 m) maps of benthic habitats and biota is currently the most serious impediment to effective protection of vulnerable habitats from fishing activities.

An assessment of fishing intensity on fine spatial scales is critically important in evaluating the overall impact of fishing gear on different habitats and may be achieved, for example, by satellite tracking of fishing vessels. The challenge for management is to determine levels of fishing that are sustainable and not degradable for benthic habitats in the long run.

The qualitative effects of trawling have been studied to some degree. The most serious effects of otter trawling have been demonstrated for hard-bottom habitats dominated by large sessile fauna, where erected organisms such as sponges, anthozoans and corals have been shown to decrease considerably in abundance in the pass of the groundgear. Barents Sea hard bottom substrata, with associated attached large epifauna should therefore be identified.

Effects on soft bottom have been less studied, and consequently there are large uncertainties associated with what any effects of fisheries on these habitats might be. Studies on impacts of shrimp trawling on clay-silt bottoms have not demonstrated clear and consistent effects, but potential changes may be masked by the more pronounced temporal variability of these habitats (Løkkeborg, 2005). The impacts of experimental trawling have been studied on a high seas fishing ground in the Barents Sea Trawling seems to affect the benthic assemblage mainly through resuspension of surface sediment and through relocation of shallow burrowing infaunal species to the surface of the seabed.

During 2009–2011 work between Norway and Russia was conducted to explore the possibility of using pelagic trawls when targeting demersal fish. The purpose with pelagic trawl is to avoid impact on bottom fauna and to reduce the mixture of other species. During the exploratory fishery it was mandatory to use sorting grids and/or a more stable four-panel trawl geometry with square mesh in the top panel of the codend to avoid catches of undersized fish. The efficiency of pelagic trawling was also tested compared with bottom trawling with regards to reduce the oil consumption per kilo of fish caught, i.e. to improve profitability and reduce NOx emissions.

After three years of exploratory fishing with pelagic trawls, pelagic trawling for cod, haddock and other demersal fish are still not allowed, mainly due to on average a smaller size of the fish and too big catches which are difficult to handle. The experiment has, however, led to a further development of the bottom trawls, including bigger trawl openings, better size selection and escapement windows to prevent too big catches.

Lost gears such as gillnets may continue to fish for a long time (ghost fishing). The catch efficiency of lost gillnets has been examined for some species and areas (e.g. Humborstad et al., 2003; Large et al., 2009), but at present no estimate of the total effect is available. Ghost fishing in depths shallower than 200 m is usually not a significant problem because lost, discarded, and abandoned nets have a limited fishing life owing to their high rate of biofouling and, in some areas, their tangling by tidal scouring. Investigations made by the Norwegian Institute of Marine Research of Bergen in 1999 and 2000 showed that the amount of gillnets lost increases with depth and out of all the Norwegian gillnet fisheries, the Greenland halibut fishery is the métier where most nets are lost. The effect of ghost fishing in deeper water, e.g. for Greenland halibut, may be greater since such nets may continue to “fish” for periods of at least 2–3 years, and perhaps even longer (D. M. Furevik and J. E. Fosseidengen, unpublished data), largely because of lesser rates of biofouling and tidal scouring in deep water. The Norwegian Directorate of Fisheries has organized retrieval surveys annually since 1980. All together 10 784 gillnets of 30 metres standard length (approximately 320 km) have been removed from Norwegian fishing grounds during the period from 1983 to 2003. During the retrieval survey in 2011 the following were retrieved and brought to land: more than 1100 gillnets, 54 red king crab traps, 13 km trawlwire, 12 km of ropes, 40 km longlines, trawl codends, 14 tonnes of fish and about 12000 crabs, mainly red king crab.

Other types of fishery-induced mortality include slipping (pelagic catch is released, but too late to survive), burst net, and mortality caused by contact with active fishing gear, such as escape mortality. Some small-scale effects are demonstrated, but the population effect is not known.

The harbour porpoise (Phocoena phocoena) is common in the Barents Sea region south of the polar front and is most abundant in coastal waters. The harbour porpoise is subject to bycatches in gillnet fisheries. Revised estimates of harbour porpoise bycatches in two Norwegian coastal gillnet fisheries suggest an annual bycatch of ~3000 harbour porpoises along the entire Norwegian coast (Bjørge and Moan, 2016).

Fisheries affect seabird populations in two different ways: 1) Directly through bycatch of seabirds in fishing equipment and 2) Indirectly through competition with fisheries for the same food sources.

Documentation of the scale of bycatch of seabirds in the Barents Sea is fragmentary. Special incidents like the bycatch of large numbers of guillemots during spring cod fisheries in Norwegian areas have been documented. Gillnet fishing affects primarily coastal and pelagic diving seabirds, while the surface-feeding species will be most affected by longline fishing. The population impact of direct mortality through bycatch will vary with the time of year, the status of the affected population, and the sex and age structure of the birds killed. Even a numerically low bycatch may be a threat to red-listed species such as Common guillemot, White-billed diver and Steller’s eider.

Several bird scaring devices has been tested for long-lining, and a simple one, the bird-scaring line (Løkkeborg, 2003), not only reduces significantly bird bycatch, but also increases fish catch, as bait loss is reduced. This way there is an economic incentive for the fishers to use it, and where bird bycatch is a problem, the bird-scaring line is used without any forced regulation.

In 2009, the Norwegian Institute for Nature Research (NINA) and the Institute of Marine Research (IMR) in Norway started a cooperation to develop methods for estimation of bird bycatch. Data on seabirds taken as bycatch from 2006 to 2009 in the coastal reference fleet programme that is managed by IMR were analysed. These estimates suggest that a total of 4000 to 6000 seabirds are killed by these fisheries. More detailed studies of seabird bycatch in the lumpsucker and Greenland halibut longline fisheries are in progress to provide more accurate data on bycatch and evaluate different measures to mitigate seabird bycatch.