Dredging

About: Dredging is a(n) research topic. Over the lifetime, 3300 publication(s) have been published within this topic receiving 28325 citation(s).

A quantitative analysis of fishing impacts on shelf-sea benthos

Abstract: 1. The effects of towed bottom-fishing gear on benthic communities is the subject of heated debate, but the generality of trawl effects with respect to gear and habitat types is poorly understood. To address this deficiency we undertook a meta-analysis of 39 published fishing impact studies. 2. Our analysis shows that inter-tidal dredging and scallop dredging have the greatest initial effects on benthic biota, while trawling has less effect. Fauna in stable gravel, mud and biogenic habitats are more adversely affected than those in less consolidated coarse sediments. 3. Recovery rate appears most rapid in these less physically stable habitats, which are generally inhabited by more opportunistic species. However, defined areas that are fished in excess of three times per year (as occurs in parts of the North Sea and Georges Bank) are likely to be maintained in a permanently altered state. 4. We conclude that intuition about how fishing ought to affect benthic communities is generally supported, but that there are substantial gaps in the available data, which urgently need to be filled. In particular, data on impacts and recovery of epifaunal structure-forming benthic communities are badly needed.

Environmental impacts of dredging on seagrasses: a review.

Abstract: Main potential impacts on seagrasses from dredging and sand mining include physical removal and/or burial of vegetation and effects of increased turbidity and sedimentation. For seagrasses, the critical threshold for turbidity and sedimentation, as well as the duration that seagrasses can survive periods of high turbidity or excessive sedimentation vary greatly among species. Larger, slow-growing climax species with substantial carbohydrate reserves show greater resilience to such events than smaller opportunistic species, but the latter display much faster post-dredging recovery when water quality conditions return to their original state. A review of 45 case studies worldwide, accounting for a total loss of 21,023 ha of seagrass vegetation due to dredging, is indicative of the scale of the impact of dredging on seagrasses. In recent years, tighter control in the form of strict regulations, proper enforcement and monitoring, and mitigating measures together with proper impact assessment and development of new environmental dredging techniques help to prevent or minimize adverse impacts on seagrasses. Costs of such measures are difficult to estimate, but seem negligible in comparison with costs of seagrass restoration programmes, which are typically small-scale in approach and often have limited success. Copying of dredging criteria used in one geographic area to a dredging operation in another may in some cases lead to exaggerated limitations resulting in unnecessary costs and delays in dredging operations, or in other cases could prove damaging to seagrass ecosystems. Meaningful criteria to limit the extent and turbidity of dredging plumes and their effects will always require site-specific evaluations and should take into account the natural variability of local background turbidity. 2006 Elsevier Ltd. All rights reserved.

Physical properties of marine sediments

Abstract: : The unconsolidated sediments that blanket the ocean floor are of widely varying thickness but seismic observations indicate that 200 to 400 meters in the Pacific and one kilometer in the Atlantic are fairly typical values for deep water. At present direct observation of these sediments is limited to such samples as may be recovered by dredging or coring operations, for drilling has been carried out only in the shallow waters of the coastal shelves. Knowledge of the physical properties of the great bulk of the sediments deeper than the few tens of feet reached by coring equipment is thus necessarily derived from geophysical observations.

The impact of dredging works in coastal waters : A review of the sensitivity to disturbance and subsequent recovery of biological resources on the sea bed

Abstract: The present review provides a framework within which the impact of dredging on biological resources that live on the sea bed ("Benthic" communities) can be understood, and places in perspective some of the recent studies that have been carried out in relation to aggregates dredging in European coastal waters. The impact of dredging works on fisheries and fish themselves, and on their spawning grounds is outside the scope of this review. We have, however, shown that empirical models for shelf waters such as the North Sea indicate that as much as 30% of total fisheries yield to man is derived from benthic resources, and that these become an increasingly important component of the food web in near-shore waters where primary production by seaweeds (macrophytes) and seagrasses living on the sea bed largely replaces that by the phytoplankton in the water column. Because dredging works are mainly carried out in near-shore coastal deposits, and these are the ones where benthic production processes are of importance in supporting demersal fish production, our review concentrates on the nature of ben thic communities, their sensitivity to disturbance by dredging and land reclamation works, and on the recovery times that are likely to be required for the re-establishment of community structure following cessation of dredging or spoils disposal. Essentially, the impact of dredging activities mainly relates to the physical removal of substratum and associated organisms from the seabed along the path of the dredge head, and partly on the impact of subsequent deposition of material rejected by screening and overspill from the hopper. Because sediment disturbance by wave action is limited to depths of less than 30m, it follows that pits and furrows from dredging activities are likely to be persistent features of the sea bed except in shallow waters where sands are mobile. Recent studies using Acoustic Doppler Current Profiling (ADCP) techniques suggest that the initial sedimentation of material discharged during outwash from dredgers does not, as had been widely assumed, disperse according to the Gaussian diffusion principles used in most simulation models, but behaves more like a density current where particles are held together during the initial phase of the sedimentation process. As a result, the principal area likely to be affected by sediment deposition is mainly confined to a zone of a few hundred metres from the discharge chute. Our review suggests that marine communities conform to well-established principles of ecological succession, and that these allow some realistic predictions on the likely recovery of benthic communities following cessation of dredging. In general, communities living in fine mobile deposits, such as occur in estuaries, are characterized by large populations of a restricted variety of species that are well adapted to rapid recolonization of deposits that are subject to frequent disturbance. Recolonization of dredged deposits is initially by these "opportunistic" species and the community is subsequently supplemented by an increased species variety of long-lived and slow-growing "equilibrium" species that characterize stable undisturbed deposits such as coarse gravels and reefs. Rates of recovery reported in the literature suggest that a recovery time of 6-8 months is charactristic

The effects of fishing on non-target species and habitats: biological, conservation and socio-economic issues

Abstract: Introduction. Acknowledgements. Contributors. PART 1: Distribution of Fishing Effort and Physical Interaction with the Seabed. 1. Spatial and temporal patterns in North Sea fishing effort (S. Jennings, K.J. Warr, S.P.R. Greenstreet and A.J. R. Cotter). 2. Physical impact of beam trawls on seabed sediments (R. Fonteyne). 3. Is bottom traw3ling partly responsible for the regression of Posidonia oceanica meadows in the Mediterranean Sea (G.D. Ardizzone, P. Tucci, A. Somaschini and A. Belluscio). PART 2: Effects of Fishing on Benthic Fauna and Habitats. 4. Fishing mortality of populations of megafauna in sandy sediments (M.J.N. Bergman and J.W. van Santbrink). 5. Effects of otter trawling on the benthos and environment in muddy sediments (B.Ball, B. Munday and I. Tuck). 6. The effects of scallop dredging on gravelly seabed communities (C. Bradsaw, L.O. Veale, A.S. Hill and A.R. Brand). 7. Impact of scallop dredging on maerl grounds (J.M. Hall--Spencer and P.G. Moore). PART 3: Fishing As A Source of Energy Subsidies. 8. The behavioural response of benthic scavengers to otter--trawling disturbance in the Mediterranean (M. Demestre, P. Sanchez and M.J. Kaiser). 9. Food subsidies generated by the beam--trawl fisher in the southern North Sea (M. Fonds and S. Groenewold). 10. Impact of trawling on populations of the invertebrate scavenger Asterias rubens (K. Ramsay, M.J. Kaiser, A.D. Rijnsdorp, J.A. Craeymeersch and J. Ellis). 11. Seabirds and commercial fisheries: population trends of piscivorous seabirds explained (C.J. Camphysen and S. Garthe). PART 4: Long--Term Changes Associated with Fishing. 12. Distribution of macrofauna in relation to the micro--distribution of trawling effort (J.A. Craeymeersch, G.J. Piet, A.D. Rijnsdorp and J. Buijs). 13. Long--term changes in North Sea Benthos: Discerning the role of fisheries (C.L. J. Frid and R.A. Clark). 14. Effects of fishing on non--target fish species (S.P.R. Greenstreet and S.I. Rogers). 15. Impacts of fishing on diversity: from pattern to process (S. Jennings and J.D. Reynolds). PART 5: Conservation Methods, Issues and Implications for Biodiversity. 16. Technical modifications to reduce the by--catches and impacts of bottom--fishing gears (B.van Marlen). 17. Fishing and cetacean by--catches (N.J.C. Tregenza). 18. Effects of fishing on non--targeted species and habitats: identifying key nature conservation issues (M.L. Tasker, P.A. Knapman and D. Laffoley). 19. The need for closed areas as conservation tools (H.J. Lindeboom). 20. No--take zones: a management context (J.W. Horwood). PART 6: Socio--Economic Implications and Mechanisms for Reducing the Impacts of Fisheries. 21. Economic incentives to discard by--catch in unregulated and individual transferable quotas fisheries (S. Pascoe). 22. Options for the reduction of by--catches of harbour porpoises (Phocoena phocoena) in the North Sea (J.M. McGlade and K.I. Metuzals). 23. Economic and sociocultural priorities for marine conservation (P.J. S. Jones). 24. Integrated management: the implications of an ecosystem approach to fisheries management (D. Dymes). PART 7: Workshop Conclusions. 25. The implications of the effects of fishing on non--targeted species and habitats (M.J. Kaiser). Glossary. Index.