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Impacts of bottom trawling


Otter board on a bottom trawl. Photo: M. Aschendorf
Otter board on a bottom trawl. Photo: M. Aschendorf

The main type of fishing gear used in the German North Sea consists of bottom trawls and beam trawls, primarily to catch North Sea shrimp (mostly inshore), flatfish such as plaice and sole, and other demersal fish such as cod and sandeel. Adverse impacts of trawling include severe damage to the sea floor and large bycatch rates, notably of juvenile fish of target species but also large quantities of invertebrates such as crustaceans, starfish and sea urchins.

Bycatch

Bycatch is the unintentional catch of undersize specimens of commercially exploited fish species (in other words, juvenile fish) and the unintentional catch of non-target species. Bycatch thus includes fish, small marine animals such as crustaceans, starfish and jellyfish, and also large marine animals such as sharks, marine mammals and seabirds.

The composition and quantity of bycatch varies considerably according to fishing method, net type and size, target species and fishing zone. Until recently, most bycatch was thrown overboard as ‘discards’. Most animals do not survive discarding or, severely injured from being caught, become easy prey for seabirds and other marine animals. Under the EU Common Fisheries Policy, discards of quota species are gradually to be banned from 2015. The discard ban therefore does not apply for species for which no fishing quota is stipulated. This is questionable from a nature conservation standpoint because in its current form the discard ban does not reduce bycatch in general. ‘Unregulated’ fish species such as solonette and dragonet, crustaceans, starfish and snails, which in bycatch-intensive bottom trawling account for a large proportion of the catch, can still be discarded. For a discard ban to be really successful in making fisheries more ecosystem-friendly, operators need to be given an incentive to use fishing practices that avoid bycatch.

For seabirds, marine mammals and sharks especially, bycatch is a serious problem that calls for a precautionary approach, such as the use of alternative fishing gear and ecosystem-friendly fishing methods. Table 1 shows various fishing methods used in the North Sea together with their negative ecological impacts.

Fishing methods with negative ecological impacts in the North Sea
Method
Ecological impacts
Active fishing methods:
Flatfish-
beam trawl
  • Tickler chains and beam shoes cause severe physical damage to the sea floor and to benthic communities
  • Large bycatch of fish (non-target species) and inverte-brates
  • Little size selectivity
  • Mixed flatfish fishery only Hardly any selection between plaice and sole (mostly resulting in large bycatch of plaice in sole fishery).
Shrimp-
beam trawl
  • Bobbins cause physical damage to the sea floor and to benthic communities
  • Large bycatch of juvenile fish and invertebrates
  • Little size selectivity
Otter trawls
  • Foot ropes and otter boards cause physical damage to the sea floor and to benthic communities
  • Large bycatch of non-target species and invertebrates
  • Little size selectivity
  • Little species selectivity
Passive fishing methods:
Bottom set nets
  • Bycatch of seabirds and marine mammals

Table 1: Fishing methods with negative ecological impacts in the North Sea (source: DUH leaflet, Lebendige Nordsee, p. 34 – see PDF):
Lebendige Nordsee. Beispiele für vorbildliche Fangmethoden und ihre Anwendbarkeit auf den Nordseeraum, 2014 (PDF 3.8 Mb)

Damage to benthic habitats

Fishing with mobile, bottom-contact fishing gear (such as beam trawls, dredges and bottom-contact otter trawls) has major adverse impacts on benthic habitats such as sandbanks, reefs and biogenic structures (Sabellaria reefs, mussel beds, etc.) with their characteristic ecological communities and sensitive species. The degree of harm done depends on the fishing gear, its weight, the tow speed, the habitat and the species involved.

The foot rope of nets dragged along the seabed feature bobbins (on shrimp beam trawls or bottom trawls) or tickler chains (on flatfish beam trawls) that keep the net on the bottom and drive fish and shrimp out of the seabed. These devices thus also do physical damage to the sea floor. The heavy otter boards that hold a trawl open penetrate even deeper into the sea floor. They leave grooves up to 30 cm deep. The fishing gear and otter boards also churn up sediment, creating large turbidity plumes and moving sediment around.


Illustration of a bottom trawl. Otter boards (trawl doors, pictured right) can weigh several tons and penetrate up to 30cm deep into the sea floor. Graphics: Source Wikimedia Commons
Illustration of a bottom trawl.
In particular, the otter boards, but also the heavy foot rope have many negative impacts on benthic species and habitats. Photo: K. Wollny-Goerke

Varying responses in species and ecological communities

Some parts of the southern North Sea are trawled with mobile, bottom-contact fishing gear up to 20 times a year. Studies have shown a 39 percent decline in biomass for benthic organisms in the southern and central North Sea compared with the unfished state. Sensitive, long-lived species with low reproduction rates (such as some mussels and sea urchins) suffer greater harm from bottom trawl gear than fast-growing, opportunistic species (such as bristle worms and star-fish).

Species that do not reach maturity until a later age and that only have few off-spring are especially sensitive to the impacts of fishing and can be wiped out completely if fishing intensity is too excessive.

The greatest damage is done when fishing with dredges around biogenic habitats with biogenic hard substrates (see above), as research has shown. How long the affected organisms take to recover heavily depends on their life and reproduction cycle. Short-lived species are correspondingly quick to recover, whereas slow-growing species (such as sponges) take many years.

 
 

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