Seabed Habitat Mapping
AFBI has been operational in seabed mapping for more than ten years.
Habitat maps aid the spatial planning of the seabed and inform the decision making processes involved in managing the demands placed on our marine environment; by fishing and the extraction of oil, gas and aggregates (gravel extraction), the planning of cable routes for electricity and telecommunications. There are also increasing pressures from offshore wind-farms/tidal turbine developments and leisure activities.
AFBI’s Marine Research Vessel Corystes, has enhanced its seabed mapping capability by taking delivery of a EM3002 high performance dual head multibeam system and Seapath 200 precise heading, attitude and positioning sensor to conduct high resolution benthic habitat mapping of the sea floor. Example bathymetry for
Torr Head,
Dundrum Bay and
South Down coast are provided here.
This state-of-the-art equipment significantly enhances AFBI’s capability in the field of seabed mapping to be carried out to the highest international standards.
Image of the wreck of the Rose II at the mouth of Belfast Lough gathered during the commissioning on the RV Corystes.
Multibeam bathymetry of the North Channel gathered on the RV Corystes.
Habitat Mapping and Fisheries
Habits maps may be used by fisheries managers to map and quantify resources to direct fishing effort in order to develop more effectively management regimes. More problematic for the same managers is the need to identify potential closed areas which may reduce the area available for fishing both on a spatial and a temporal basis. In the past the industry has regarded the use of habitat maps with suspicion regarding them simply as tool to restrict their activities.
The adoption of a spatial planning approach in which habitat maps are a central tool has had only a patchy uptake by fisheries managers although there a some notable success in the shellfish sector. However, as the management of fisheries continued to move to an ecosystem management approach it is to be expected that habitat maps will play a central role in policy making.
The fishing industry itself has long recognized the value of many of the techniques developed for habitat mapping and may well be regarded as a stimulus for the development of Acoustic Ground Discrimination Techniques.
The shellfish aquaculture sector with its tradition of licensing areas of seabed has had a history of realizing the value of maps in managing their industry. The use of acoustic techniques for locating and quantifying resources has gained widespread acceptance in this sector.
Survey Planning Tool: Optimum Allocation Analysis and Ground Truthing
Seabed habitat mapping is an increasingly popular activity worldwide, coinciding with rapid methodological development and technological advances such as multibeam echosounders, acoustic ground discrimination systems and sidescan sonar. The ultimate difficulty lies in the substantial survey effort required to physically confirm the acoustic readings and relate these to the biology of an area. Ground-truthing strategies have rarely been adequately addressed or even specified within published benthic mapping literature. Precisely how the first stage physical survey informs the ground-truthing is poorly defined and occasionally complex. Most rely on expert judgement, while in some advanced studies the ground-truthing strategy uses identified acoustic ground-types with sampling being related to ground-type area.
By integrating the areas and variances, a statistical method such as Optimum Allocation Analysis (Sukhatme & Sukhatme, 1970; Cochran, 1977) could calculate surrogate ground-type heterogeneity values to direct ground-truthing effort. Optimum Allocation Analysis (OAA) may be defined as a procedure used in stratified sampling to allocate numbers of sample units to different strata to either maximize precision at a fixed cost or minimize cost for a selected level of precision.
The parameters used to define the population can be any derived acoustic variable, e.g. bathymetry, slope angle and acoustic backscatter. Subpopulations, or ‘strata’, are the ground-types that the remotely sensed data are classified into prior to being confirmed with ground-truthing.
Actual values should be standardised through simple linear re-scaling on the 8-bit grey level image scale of 0-255: this allowed calculation and comparison of means and standard deviations for all variables. The summary statistics (mean and variance) can be entered into an Microsoft Excel Macro containing embedded calculations for OAA. The area of each predicted ground-type will be shown in m2.
The coefficient of variation (CV) can be adjusted for calculating the optimum sample numbers per ground-type. It must be stressed that although OAA recommends the area of seabed to be sampled, it does not advise where these should be placed within each ground-type, how they should be distributed amongst patches of the same ground-type or what sampling equipment should be used. Once the ground-truthing data are collected, it is possible to use the OAA in reverse.
It is hoped that this Macro will provide a dialogue and framework for future consideration of how the science of benthic habitat mapping can be significantly advanced. If used in a cited report, please reference:
Clements, A. J., Strong, J. A., Flanagan, C., and Service, M. 2010. Objective stratification and sampling-effort allocation of ground-truthing in benthic-mapping surveys. – ICES Journal of Marine Science, 67: 628–637.
References
Cochran, W.G. 1977. Sampling Techniques; Third Edition. Wiley Publications in Statistics, New York, United States of America.
Sukhatme, P.V., and Sukhatme, B.V. 1970. Sampling Theory of Surveys with Applications. Iowa State University Press.
