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Can fishing make marine ecosystems more susceptible to environmental change?

Published: 16 August 2017

Image courtesy of Leigh HowarthMarine ecosystems are threatened by a range of human impacts, many of which are increasing in severity and occurrence. Over the last century, the use of bottom trawls and other mobile fishing gears have greatly increased in intensity and geographic extent. By coming into contact with the seafloor, these fishing gears can disturb and re-suspend sediments, damage seafloor habitats, reduce the abundance of target and non-target species, and truncate the age and size structures of populations. All of which can negatively impact reproduction and reduce the capacity of populations to buffer environmental change. In contrast, rising ocean temperatures, acidity and nutrient loads can alter global levels of primary production, which can alter marine food webs, fishery yields and ocean chemistry. As primary production and fishing pressure can have such strong influences on marine ecosystems, attempts to better understand their combined effects could help to guide management efforts and predict future drivers of ecological change.

Dr Leigh Howarth and Prof Jan Hiddink of Bangor University conducted a large empirical study into the interactive effects of bottom trawling and primary production on seafloor ecosystems. Using Bangor University’s Research Vessel, The Prince Madog, they spent two months sampling the seabed in the western English Channel, and the Irish and Celtic Sea. By deploying a series of grabs, dredges and nets they successfully collected data on the full community of animals living on the seabed, ranging in size from the smallest microscopic worm, buried deep within the sediment, to the largest fish swimming in the water column.

Fishing pressure had a negative relationship with community biomass, a trend which strengthened with increasing primary productionOverall, they found trawling truncated the biomass and size structure of the entire seafloor community. The study also highlighted that trawling affected some species more than others. For instance, trawling reduced the abundance of species attached to or living on the surface of the seabed; animals such as corals, hydroids and bryozoans. However, these organisms play an important role within marine ecosystems as they provide an element of 3-dimensional structure to the seafloor. In doing so, they supply important refuges for small and juvenile fish, provide important feeding sites and provide habitat for the settlement of scallops and other invertebrates. Consequently, the damage sustained by nursery habitats from trawling may reduce an area's capacity to support biodiversity and negatively impact commercially important species. Trawling also negatively impacted burrowing organisms and animals that feed on detritus suspended in the water column. Hence, by affecting these kinds of organisms, bottom trawling is likely to alter levels of nutrient cycling, water quality and energy flow.

The negative effects trawling had on seafloor communities were all found to be stronger in areas of high primary production. Similar results have frequently been reported in studies of freshwater lakes and estuarine systems. However, the implications of such an interaction could be profound. As climate change, acidification and eutrophication are predicted to alter global levels of primary  production, this could make marine ecosystems less resilient to the impacts of fishing, particularly in areas where primary production increases. This study therefore supports the notion that fishing can make marine ecosystems more susceptible to environmental change.

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