Bottom trawling

Abstract: Fishers have been complaining about the effects of bottom trawl gear on the marine environment since at least the 14th century. Trawl gear affects the environment in both direct and indirect ways. Direct effects include scraping and ploughing of the substrate, sediment resuspension, destruction of benthos, and dumping of processing waste. Indirect effects include post‐fishing mortality and long‐term trawl‐induced changes to the benthos. There are few conclusive studies linking trawling to observed environmental changes since it is difficult to isolate the cause. However, permanent faunal changes brought about by trawling have been recorded. Research has established that the degree of environmental perturbation from bottom trawling activities is related to the weight of the gear on the seabed, the towing speed, the nature of the bottom sediments, and the strength of the tides and currents. The greater the frequency of gear impact on an area, the greater the likelihood of permanent change. In deepe...

Abstract: Bottom trawling causes widespread physical disturbance of sediments in seas and oceans and affects benthic communities by removing target and non-target species and altering habitats. One aspiration of the ecosystem approach to management is to conserve function as well as biodiversity, but trawling impacts on benthic community function need to be understood before they can be managed. Here we present the large scale and long term impact of chronic trawling on the functional composition of benthic invertebrate communities using a comprehensive set of functional traits. The effects of chronic trawling disturbance on the functional composition of faunal benthic invertebrate communities, as sampled with a small beam trawl, were investigated at 6 to 13 sites in each of 4 contrasting regions of the North Sea. Each site was subject to known levels of trawling disturbance. Information on the life history and ecological function traits of the taxa sampled was translated into fuzzy coding and used to analyse the relationship between life history and functional roles within the ecosystem. Multivariate analyses were used to examine changes in the distribution of traits over gradients of trawling intensity. Changes in the functional structure of the community due to the effects of long-term trawling were identified in 3 of the 4 areas sampled. Filter-feeding, attached and larger animals were relatively more abundant in lightly trawled areas, while areas with higher levels of trawling were characterised by a higher relative biomass of mobile animals and infaunal and scavenging invertebrates. Univariate analysis of selected traits confirmed the patterns observed in multivariate analysis. These results demonstrate that chronic bottom trawling can lead to large scale shifts in the functional composition of benthic communities, with likely effects on the functioning of coastal ecosystems.

Abstract: Deep-sea fisheries operate globally throughout the world's oceans, chiefly targeting stocks on the upper and mid-continental slope and offshore seamounts. Major commercial fisheries occur, or have occurred, for species such as orange roughy, oreos, cardinalfish, grenadiers and alfonsino. Few deep fisheries have, however, been sustainable, with most deep-sea stocks having undergone rapid and substantial declines. Fishing in the deep sea not only harvests target species but can also cause unintended environmental harm, mostly from operating heavy bottom trawls and, to a lesser extent, bottom longlines. Bottom trawling over hard seabed (common on seamounts) routinely removes most of the benthic fauna, resulting in declines in faunal biodiversity, cover and abundance. Functionally, these impacts translate into loss of biogenic habitat from potentially large areas. Recent studies on longline fisheries show that their impact is much less than from trawl gear, but can still be significant. Benthic taxa, especially the dominant mega-faunal components of deep-sea systems such as corals and sponges, can be highly vulnerable to fishing impacts. Some taxa have natural resilience due to their size, shape, and structure, and some can survive in natural refuges inaccessible to trawls. However, many deep-sea invertebrates are exceptionally long-lived and grow extremely slowly: these biological attributes mean that the recovery capacity of the benthos is highly limited and prolonged, predicted to take decades to centuries after fishing has ceased. The low tolerance and protracted recovery of many deep-sea benthic communities has implications for managing environmental performance of deep-sea fisheries, including that (i) expectations for recovery and restoration of impacted areas may be unrealistic in acceptable time frames, (ii) the high vulnerability of deep-sea fauna makes spatial management—that includes strong and consistent conservation closures—an important priority, and (iii) biodiversity conservation should be > balanced with options for open areas that support sustainable fisheries.; ;

Abstract: Bottom trawling is a fishing technique whereby heavy nets and gear scrape along the sea bed, and is shown here to disturb sediment fluxes and modify the sea floor morphology over large spatial scales. The direct impact of bottom trawling on local fish populations has received much attention, but trawling also affects other aspects of the ocean environment. This paper shows that bottom trawling — a commercial practice in which heavy nets and gear are dragged along the ocean floor — induces sediment reworking and erosion, causing the gradient of the sea floor to become smoother over time. This reduces the morphological complexity of deep-sea environments. The authors draw parallels between the effects of bottom trawling at sea and intensive agriculture on land, with the important difference that, on land, ploughing takes place once or twice a year, whereas, at sea, bottom trawling can be a frequent occurrence. Bottom trawling is a non-selective commercial fishing technique whereby heavy nets and gear are pulled along the sea floor. The direct impact of this technique on fish populations1,2 and benthic communities3,4 has received much attention, but trawling can also modify the physical properties of seafloor sediments, water–sediment chemical exchanges and sediment fluxes5,6. Most of the studies addressing the physical disturbances of trawl gear on the seabed have been undertaken in coastal and shelf environments7,8, however, where the capacity of trawling to modify the seafloor morphology coexists with high-energy natural processes driving sediment erosion, transport and deposition9. Here we show that on upper continental slopes, the reworking of the deep sea floor by trawling gradually modifies the shape of the submarine landscape over large spatial scales. We found that trawling-induced sediment displacement and removal from fishing grounds causes the morphology of the deep sea floor to become smoother over time, reducing its original complexity as shown by high-resolution seafloor relief maps. Our results suggest that in recent decades, following the industrialization of fishing fleets, bottom trawling has become an important driver of deep seascape evolution. Given the global dimension of this type of fishery, we anticipate that the morphology of the upper continental slope in many parts of the world’s oceans could be altered by intensive bottom trawling, producing comparable effects on the deep sea floor to those generated by agricultural ploughing on land.