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AWAITING REVIEW

Indicator summary

Summary of indicator structure and function

IndicatorAttributePurposeIf restricted to taxa, list which onesEcosystem applicabilityIdentified capabilityBiological classification levelResponse variableDriversRobustness
Bottom roughness - Acoustic signal strengthHabitat structure and conditionFisheries, benthic habitatsBenthicApplicable to all ecosystemsDemonstrableCommunity

Environmental

Anthropogenic, EnvironmentalMedium to high

Examples of how the indicators is used for ecosystem management and ecosystem status and trends

Indicator examplesCurrent status and trendsManagement objective/directionStakeholder/Public acceptability
A study of trawl impacts on the benthos of a Scottish Loch   

Definition and/or background

The following is from Fulton et al 2004a -

Recent advances in acoustic instrumentation are making it possible to conduct controlled experiments on the impacts of mobile fishing gear on benthic habitats, as well as investigate impacts of existing fishing operations (Schwinghamer et al. 1998). The biological basis for using acoustic indicators is that trawling and other mobile fishing gear changes the topography of the seabed, in particular, smoothing and compacting sediments especially in habitats at depths not usually subject to natural disturbance. Signals from acoustic equipment can be tested for statistical significance regressed against trawl sets for a given treatment area to track changes in sea floor topography (Schwinghamer et al. 1998) and provide an indicator of habitat destruction. There are now several approaches using side-scan sonar and acoustic signals, which generate signal levels that provide indicators of bottom roughness and hardness, that have been demonstrated to capture changes that occur under the impacts of mobile fishing gear on the sea floor. For example, the Rox-Anntm acoustic bottom classification system processes echo-sounder signals from the sea floor, and has been used in a number of North Sea studies to indicate changes in sea floor characteristics after trawling (Schwinghamer et al. 1998, Gordon et al. 1998). The Rox-Anntm produces two return signals that are proxies for sediment disturbance. The E1 signal records bottom texture or hardness (e.g. gravel, sand, mud) whilst the E2 signal records bottom roughness (e.g. presence of shells, ripples, trawl tracks, cobbles etc.). The indicators for both are the strength of the return signal, and an increasing signal (E1 or E2) indicates increasing bottom disturbance. 

Researchers have found that it is important to use video imaging in association with acoustic measures, because acoustic results alone can sometimes be misleading. For example, in a 3-year otter trawling experiment on a sandy-bottom ecosystem of the Grand Banks off Newfoundland, three 13km long corridors were trawled 12 times and measurements of changes to grain sediments and bottom topography made before and after trawling. Results showed that repeated trawling did not affect bottom texture, but increased surface relief or roughness (i.e. the mean E2 index climbed from 50-60 when regressed against consecutive trawl sets over 3 years, Schwinghamer et al. 1998). However, concurrent video imaging showed that in contradiction to the increasing roughness detected by Rox Ann, trawling smoothed the seabed topography and removed biogenic structures. When this discrepancy was investigated, it was shown that the Rox Ann was detecting the furrows and berms created by the trawl doors because it has a wider footprint (200m2) than the video grab. So, whilst Rox Ann is a robust and reliable method of acoustic measure, problems of spatial scale need to be accounted for in interpreting results, and as with many indicators, it is important to use a complementary combination of methods.

Attribute

Habitat structure and condition

Purpose

Benthic habitat, Fisheries

Taxa

Benthic taxa

Data required

The following is from Fulton et al 2004a -

  • Acoustic returns of seafloor from side-scan sonar or echo-sounder.
  • Video imaging of seafloor

Ecosystem applicability

The following is from Fulton et al 2004a -

Applicable to all ecosystems, but especially slope and deep sea habitats.

Robustness

The following is from Fulton et al 2004a -

Medium to high: Issues to do with spatial scale considered require it to be used in conjunction with video-imaging where possible and side-scan sonar. Reference areas are required to provide baseline levels of bottom topography and habitat complexity.

Current status and trends

The following is from Fulton et al 2004a -

In a study of trawl impacts on the benthos of a Scottish Loch (Tuck et al.1998) the Rox Ann E1 index given below (Figure 1) showed that trawling smoothed the seabed, and that prior to experimental trawling the seabed was rougher. These changes were not detected by side-scan sonar, again indicating the need to use a range of measures when monitoring benthic environments.


 

Figure 1: Transect of E1 (roughness) along Loch Gareloch for the treated (trawled) areas (dashed line) and reference areas (solid line). The relative difference in roughness between the treatment and reference areas increased during the experimental trawling program (After Tuck et al. 1998). (Figure provided by Fulton et al 2004a).

References

Fulton, E.A., Smith, A.D.M., Webb, H., and Slater, J. (2004a) Ecological indicators for the impacts of fishing on non-target species, communities and ecosystems: Review of potential indicators. AFMA Final Research Report, report Number R99/1546.

References that Fulton et al uses for this indicator:

Gordon, D. C. Jr., P. Schwinghamer, T. W. Rowell, J. Prena, K. D. Gilkinson, W. P. Vass, and D. L. McKeown. 1998. Studies in eastern Canada on the impact of mobile fishing gear on benthic habitat and communities. In: Effects of fishing gear on the sea floor of New England. Editors E. M. Dorsey, and J. Pederson, pp 63-67. MIT Sea Grant Publication, 98-4. Boston, Mass.: Conservation Law Foundation.

Schwinghamer, P., D. C. Jr. Gordon, T. W. Rowell, J. Prena, D. L. McKeown, G. Sonnichsen, and J. Y. Guigne. 1998. Effects of experimental otter trawling on surficial sediment properties of a sandy-bottom ecosystem on the Grand Banks of Newfoundland. Conservation Biology 12, no. 6: pp 1215-22.

Tuck, I. D., S. J. Hall, M. R. Roberston, E. Armstrong, and D. J. Basford. 1998. Effects of physical trawling disturbance in a previously unfished sheltered Scottish sea loch. Marine Ecology Progress Series 162: pp 227-42.

Background reading

Fulton, E.A., Fuller,M., Smith, A.D.M., and Punt, A. (2004) Ecological indicators of the ecosystem effects of fishing: Final report. AFMA Final Research Report, report Number R99/1546.

 

Citation

Please cite this page as:
SOKI Wiki (2014) Friday 11 Apr 2014.

Page created by:Shavawn Donoghue

Last modified on: Apr 11, 2014 15:11

Versions: 6, 5, 4, 3, 2, 1

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