Oceans Past V - The evolution of the impact of bottom-trawling on demersal fish populations and the benthic ecosystem

The fif​th chapter of the ICES Oceans Past conference ​series takes place this week in Tallinn,  Estonia, providing a forum where the latest research in marine environmental history and historical marine ecology can be presented and debated.
Published: 19 May 2015

​​​​​​​In his keynote lecture at Oceans Past V​Adriaan Rijnsdorp, IMARES, the Netherlands, shared his work on the historical development of the bottom trawling impact on demersal fish populations and the benthic ecosystem with Oceans Past participants.

​​​​​​​Benthic ecosystems provide important goods and services, such as fisheries products, and support regulation and cultural services. There is serious concern about the adverse impact of fisheries, in particular bottom trawling, on benthic ecosystems. The impact of bottom trawling is determined by the footprint of the fishery (spatial extent), the type of fishing gear used, and the sensitivity of the sea bed habitat and benthic ecosystem. Recent work has for the first time generated maps of the distribution and intensity of bottom trawling in the northeast Atlantic showing on one hand the intensive exploitation of particular parts of the sea bed, while other parts are not trawled or are trawled at an intensity of less than once a decade. This raises the question how we got to this situation.

In an upcoming paper, along with my co-authors, I will present a reconstruction of the historical development of intensity and spatial extent of bottom trawling. The reconstruction is based on a variety of data sources (archaeological, historical, fisheries technological, geological, fisheries) with particular focus on the North Sea. Commercial exploitation of marine fish resources dates back to at least the fourteenth century. The main species were targeted with passive gears such as drift nets (herring) and hook and lines (cod, ling, haddock). Although concern about bottom trawling is expressed in historical documents going back to the late fourteenth century, the available fishing technology restricted the use of bottom trawls to the flat sandy bottoms in shallow coastal waters.

The expansion of bottom trawling occurred in the second half of the nineteenth century with an increase in the number of sailing trawlers deploying a beam trawl. Substantial parts of the seabed were considered untrawlable because of the occurrence of stones, boulders or hard biogenic structures that obstructed the safe deployment of bottom trawls. However, with the transition from sailing trawlers to steam trawlers and motor trawlers, larger and stronger trawls could be deployed. The use of rollers on the groundrope allowed fishers to work on grounds that were previously considered untrawlable.

Based on information from the number of sailing smacks, the dimension of the fishing gear used, the towing speed, the gradual expansion from coastal waters to deeper offshore waters, and the relative distribution of fishing effort over the major fishing grounds, bottom-trawling footprints are estimated for different phases in the development of the bottom trawling fleet. The preliminary results suggest that the proportion of the North Sea floor that wa​s not trawled decreased from more than 90% at the first half of the nineteenth century to about 50% at the beginning of the twentieth century and less than 10% at the beginning of the twentifirst century. The proportion surface area trawled at least once a year increased from less than a few percent in 1850 to around 25% in 1910 and 60% at the beginning of the twentifirst century. The increase in footprint in recent decades is likely related to the technological innovations, in particular, the development of twin trawling.

Although the footprint results are preliminary, historic data sources provide a sound basis to quantify the development of the footprint of bottom trawl fisheries. Further research into the trends in the number of fishing vessels and the dimensions of the gears deployed will improve the footprint estimates.

This research was conducted under the FP7-BENTHIS​ project on integrating the benthic ecosystem in fisheries management.
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​Steam ​trawlers in Great Yarmouth, UK, 1930s. Photo: Cefas. 

Ole R. Eigaard, DTU Aqua, Denmark
Niels T. Hintzen, IMARES​, the Netherlands 
Georg H. Engelhard​​, Cefas, UK
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Oceans Past V - The evolution of the impact of bottom-trawling on demersal fish populations and the benthic ecosystem

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