Special Sessions at AAG 2008 - Marine Geomorphology
Marine Geomorphology as a Determinant for Essential Life Habitat: An Ecosystem Management Approach to Planning for Marine Reserve Networks
Association of American Geographers Annual Meeting
Boston, Massachusetts, USA, April 16, 2008
Empire Room #6, Westin Copley Place Hotel, 7th Floor
- co-organized by W. Heyman (Texas A&M) and D. Wright (Oregon State)
- co-sponsored by the Coastal and Marine, Geographic Information Science and Systems, and Biogeography Specialty Groups
Marine resources are in decline throughout most of the world's oceans and traditional, species-specific and/or catch based means to manage these resources are failing and are instead turning to ecosystem-based approaches. As attractive as it sounds, the concept has not yet been effectively translated into action. The reauthorization of the Magnuson-Stevens Fishery Conservation and Management as the Sustainable Fisheries Act in 2006 provides renewed incentive for innovative ways to implement ecosystem-based management. The goal of this session was to examine critically the growing body of data suggesting that the underlying geology and geomorphology of marine environments dictates the location of critical life habitat for a variety marine species. For example, it is becoming clearer that spawning aggregations of many species of commercially important reef fishes commonly occur at the windward edge of reef promontories that jut into deep water. As another example, seamounts serve as attractors for pelagic fishes. The broad implications of these findings suggest that geomorphology might be used as a proxy for (or at least help to identify) critical life habitat for marine species, and thus serve to advance the application of the ecosystem-based management and the design of marine reserve networks.
Presentations focused on:
- essential benthic habitat and geomorphology
- marine GIS and/or remote sensing for the purposes of integrating geomorphology and biology
- applications for marine reserve network design
- related policy issues and implications
Two special journal issues have resulted from these sessions:
Marine Geomorphology I, 8:00-9:40 a.m.
Session Chair: Will Heyman, Texas A&M
Presenter: Will Heyman, Texas A&M, Building a marine reserve network in Belize based on spatio-temporal patterns of reef fish spawning aggregations
Presentation (pdf, 3.3 Mb)
- Most large and commercially important reef fishes migrate and spawn in aggregations at specific times and places. They are valuable targets for the fishermen who have recognized these patterns. Many aggregations have therefore been fished to local extinction and heavily targeted species have seen regional declines. Nassau grouper, the best known example, was once the Caribbean's most important food fish, but is now listed as endangered by the World Conservation Union.
Recent concern over the decline of Nassau grouper in Belize led to a collaborative, national evaluation of the status of their breeding sites, starting in 2001. In addition, these studies evaluated the use of traditional Nassau grouper spawning aggregation sites by other transient spawning species. Analysis of nine traditional Nassau grouper spawning aggregation sites in Belize illustrated that only two were still viable and that the sites and the species were in need of immediate protection. Field data collected between 1998 and 2001 indicated that all of the sites served as multi-species spawning aggregation sites and appeared to exhibit similar geomorphological features. Gladden Spit, the best documented example, harbors aggregations of 21 species at various times of year. These data, in part, lead to the declaration of a closed season for Nassau grouper and a network of 11 new marine reserves in 2003. The present study synthesizes all presently existing data on the timing and location of multi-species spawning aggregations in Belize. The synthesis may be relevant to the design of marine reserve networks in other locations.
- The most well-studied reef fish species, Nassau grouper, has been declining and been recognized as a critically endangered species. The primary reason for the Nassau grouper’s decline has been attributed to selective over-fishing of its highly synchronized and site specific spawning aggregations. Nassau grouper has observed to share their spawning aggregation sites with other species in the western Caribbean. Observations and data suggest these transient reef fish spawning aggregations occurring on the outer reef edge, on promontories or near drop-offs. A promontory is defined as a distinct turning point, or bend, in the shelf break. The objective of this paper is to analyze the geomorphic features of existing and historical Nassau grouper spawning aggregation sites in Belize and the Cayman Islands. The spawning aggregation sites in Belize were collected directly in the field while the locations of five sites in the Cayman Islands were provided by the Cayman Islands Department of Environment. Bathymetric data in two of the sites in Belize and all sites in the Cayman Islands were collected with an echosounder with GPS in order to identify and compare geomorphic characters: the bottom depth, slope and curvature. Though it requires further study for the other spawning aggregation sites in Belize, the reef structure surrounding all spawning aggregation sites are steeply inclined profiles; they jut into deep water in a convex shape. These spawning aggregations occurred at reef promontories and drop-offs.
- In 1998, SCUBA divers in the Florida Keys documented black grouper (Mycteroperca bonaci) aggregations just seaward of a no-take marine protected area (MPA) that had been established around Carysfort Reef less than a year earlier. This ironic discovery highlighted the need to, first, determine the environmental characteristics associated with such aggregations, and, second, develop mapping tools to facilitate the rapid prioritization of areas for conservation.
A study conducted at Carysfort Reef in 2003 used the Quester Tangent Series V (QTCV) commercial acoustic seabed classification system to map bathymetry and surficial geology of the seabed. Analysis of this dataset in conjunction with diver-based visual census of reef fish abundance revealed that, overall, groupers were preferentially found over patchy areas of the seabed as measured both by substrate type (i.e. rock vs. sediment) as well as with a new index of acoustic variability derived directly from the QTCV echoes. The aggregation site, in particular, had high acoustic variability, steep slope, and was located directly inshore of a series of shore-parallel linear ridges in deeper water.
Interviews with local fishers revealed five historical, but currently inactive, spawning aggregation sites in the Upper Keys. These historical sites were surveyed in 2007 in order to test the hypothesis that their geomorphology would be similar to that at Carysfort Reef. Preliminary analysis indicates that locally steep slopes are a common characteristic across all aggregation sites surveyed. Low cost, single beam, bathymetric mapping may therefore prove to be a powerful tool for identifying potential MPAs.
- One approach to marine conservation is establishing networks of marine reserves. Designing effective networks is complicated by the need for connectivity among reserves. Connectivity is required because many marine organisms reproduce with planktonic larvae that drift significant distances from their parents. Further, many sessile organisms are distributed across patchy habitat in metapopulations of many small local populations. Each local population must receive sufficient numbers of larval settlers to persist, whether those larvae are produced locally or drift in from elsewhere.
A key difficulty in evaluating effectiveness of a proposed reserve network is anticipating the rates of larval flow between different patches of habitat. Larval flow / population connectivity is estimated from several information sources: genetics, natural or artificial marks in otoliths, floats in circulation modeling, monitoring settlement and physical conditions. Ultimately we need a larval connectivity matrix from all habitat patches to all habitat patches. We may eventually get that from circulation models (but they are expensive and still in early development). Whatever method we use, we end up modifying it based on basic principles (e.g., flow is greater along bathymetry) and information from all these data sources.
Current projects establishing reserve networks require estimates of dispersal patterns NOW that are at least qualitatively reasonable. We propose a simple method for modeling larval dispersion based on bathymetry. We then show an application of this model to California's central coast and illustrate the insights that it provides into potential source-sink metapopulation dynamics of sessile marine organisms with different dispersal distances.
Presenter: Scott Nichol, Geoscience Australia, Using 'seascapes' to help predict Australia's benthic marine habitat diversity in the development of a national system of marine protected areas
Presentation (pdf, 2.7 Mb)
- One of the biggest challenges facing marine scientists is to develop a robust and defensible way to represent potential seabed habitats and ecosystems based on easily mapped and spatially-abundant biophysical properties. Geoscience Australia is currently developing a statistical approach to integrate multiple spatial biophysical data layers into a single map of seabed habitats or “seascapes” for Australia’s marine region. In an application of the methodology for the SW Australian margin, a total of six ecologically-meaningful seascapes were defined on the shelf and eight defined for the off-shelf areas. On the shelf and off-shelf regions, the seascapes were delineated most strongly by variations in seabed sedimentology and geomorphology. A key feature is the delineation of separate seascapes between the western and southern margins, which broadly corresponds with a regional change in sediment facies from tropical-temperate carbonates on a N-S trending margin to cool-water carbonates on a E-W trending margin. Areas where a network of marine protected areas could maximise biodiversity coverage by protecting the maximum seascape heterogeneity are identified using a focal variety analysis in ARCGIS. Most spatial heterogeneity in the seascapes occurs on the geomorphologically complex slope environments, characterised by ecologically important submarine canyons and deep-sea ridges. Future research includes working with Australia’s marine biologists to correlate the seascapes with high-resolution biological data.
Marine Geomorphology II, 10:10 - 11:50 a.m.
Session Chair: Dawn Wright, Oregon State
Presenter: Dawn Wright, Oregon State, The Quest for a High-resolution Map of the Oregon Territorial Seafloor in the Context of Current West Coast Marine Reserve Initiatives
Presentation (pdf, 6 Mb)
- Bathymetry and geomorphic classification are crucial data layerd for the planning of marine reserve areas or networks, as they, in combination with water column observations, enable the mapping of nearshore habitats. These extremely versatile layers can also enable estimates of wave run-up and inundation associated with a major tsunami, model erosion from storm waves, mitigate nearshore oil spills, or guide commercial and recreational bottom fishing. In Oregon, only 5% of its territorial sea (measuring 3 miles out from the coast, constituting ~950 square miles) has been mapped in sufficient detail to aid in all of these issues so critical to coastal communities. With the fragmented and piecemeal approach currently underway, it will take decades to completely map this essential area without a more focused effort. Therefore, in the spring of 2006, a group of 22 academic and government agency scientific experts, with the support of several scientists and resource managers from regional conservation organizations, developed, signed and circulated a "Scientific Consensus Statement for Mapping the Oregon Territorial Seafloor." The statement lays out a rationale and strategy for a high-resolution multibeam bathymetric mapping effort over the course of 2 years, an effort that stands to contribute greatly to the Oregon Ocean Policy Advisory Council (OPAC) Marine Reserve Working Group, and to the West Coast Governor's Agreement on Ocean Health. This talk reviews the current Oregon mapping effort, the related science and policy issues and hurdles, and the interesting connections to the Marine Life Protection Act mapping efforts in California.
- The California Marine Life Protection Act (MLPA) calls for the establishment of a managed network of marine protected areas (MPAs) for the state of California. The MLPA Initiative, a collaboration between the Resources Legacy Fund Foundation and the California Department of Fish and Game, has organized a group of scientists and specialists in Geographic Information Systems (GIS) to enable the MLPA process. Here we detail the MLPA GIS decision support system designed and implemented by our group, including (1) the data gathered and synthesized by our technicians and scientists, (2) a geodatabase, (3) web maps, (4) a custom web-based decision support tool, and (5) terminal services for running ArcGIS and custom Model Builder tools. As California will be the first state in the union to implement a managed network of MPAs, our GIS based decision support system may serve as a model system for similar efforts in other state or federal processes.
- The Massachusetts Office of Coastal Zone Management is leading an effort to map the Commonwealth's seafloor environment. In conjunction with the United States Geological Survey, high resolution acoustic imagery, including interferometric sidescan sonar and multibeam bathymetry, were collected to precisely map relative seafloor hardness and water depth for approximately 1,300 square kilometers in Massachusetts waters. These acoustic data are the foundation to better understand the terrain, geologic structure, and ecology of the seafloor. We are currently investigating several GIS/remote sensing-driven semi-automated classification techniques to create a seafloor map showing surficial geological habitat (patches of uniform bottom type). Maps will be based on a combination of physiographic data (bathymetry and derivatives including slope; orientation; curvature and relative position including Bathymetric Position Index; and terrain variability including rugosity, Terrain Ruggedness Index, and fractal dimension) and lithologic data (sidescan sonar interpretations and derivatives). We will intersect these parameters using map algebra, multivariate statistical, and supervised classification routines, all common to landscape ecology and terrestrial image classification. Resultant geologic habitat maps will be compared to groundtruth data in an error matrix to quantify the accuracy of select elements. The final surficial geologic habitat map will serve as basemap that will guide future research, inform decision makers facing increased coastal zone development pressures, and serve as the basis for species-specific habitat suitability modeling.
- We report upon seafloor mapping efforts in Alaska where geology plays a major role in
Essential Fish Habitat formation. We have used a combination of sidescan sonar, multibeam, and in-situ observations from an occupied submersible to investigate marine benthic habitats in the Eastern Gulf of Alaska. Eroded volcanic edifices, lava fields, and a pit crater, as well as shutter ridges, deformed and differentially eroded sedimentary bedrock, and highly fractured and faulted plutonic rock outcrops are features that attract adult rockfish (Sebastes spp.) and lingcod (Ophiodon elongatus). In addition, the characteristic columnar basalt exposed along the sides of eroded lava lakes at the summits of volcanic cones provide rugged, steep habitat including walls and overhangs utilized by rockfishes and, blocky platforms used by lingcod. These volcanic edifices are significant in that they interrupt currents in turn producing upward eddies that bring nutrients to all species residing on the rocks. The diversity and density of fish species inhabiting these volcanic features is greater than that of the adjacent benthic habitat. One area, The Edgecumbe Pinnacles has been classified as an Habitat Area of Particular Concern and closed to all fishery removals. These volcanic features lie along strike of the Fairweather transform fault zone, (a leaky transform fault system where oblique convergence produces shudder ridges of varying relief and complexity) and juxtapose hard, rugged bedrock with flat, soft sediment. We are investigating these features in consideration of using the resultant seafloor deformation and volcanic construction as surrogates for essential fish habitats.
- The European Union Habitats Directive obliges member states to protect marine species and habitats in their territorial waters and exclusive economic zones through a coherent network of marine protected areas by 2008. Among the listed habitats, which need protection, are reefs, whether they are biogenic or geogenic. A better understanding of the extent and character of rocky reefs in the central English Channel is required to inform policy and create a profound scientific basis for the selection of marine protected areas. Therefore, we acquired bathymetric data and processed the datasets with Benthic Terrain Modeler to derive benthic zones, structures and rugosity.
In this tide-swept environment, which is characterised by a thin and discontinuous cover of coarse lag deposits, a close correlation between bedrock geology and physical habitat exists. Where soft and uniform Upper Cretaceous chalk is present at the seafloor, slope and rugosity are low and the seabed is flat and smooth, with a thin gravel lag on top. Conversely, Upper Jurassic and Lower Cretaceous bedrock comprises a varied and partly cyclic lithology, displaying a differential resistance to erosion. Consequently, slope and rugosity are highly variable where these bedrock types are found at the seafloor. Such areas are dominated by series of bedrock ridges of up to several tens of kilometres length. Seabed photography showed distinct biological communities associated with these differing physical habitats, supporting a growing evidence base that underlying geology has the potential to be used as a proxy when mapping benthic habitats in support of marine management.
Marine Geomorphology III, 1:00 -2:40 p.m.
Session Chair: Dawn Wright, Oregon State
Presenter: Brian Andrews, USGS, The Role of Marine Geophysical Mapping in Characterizing and Managing Marine Ecosystems: A Federal/State Partnership for Massachusetts Coastal Waters
Presentation file (pdf, 2.2 Mb)
- The U.S. Geological Survey and the Massachusetts Office of Coastal Zone Management formed a partnership in 2003 to conduct geophysical mapping of the inner continental shelf off the coast of Massachusetts. The goal of this cooperative is to improve our understanding of the regional geologic framework of the coastal waters on a broad scale, and also characterize the seafloor at finer scale. The data and maps produced from this cooperative illustrate the distribution of surficial bottom types on the seafloor, and will support new ecosystem-based approaches to managing ocean resources. The geologic characteristics of the seabed largely determine the species of flora and fauna that inhabit a particular area. Determining the surficial geology and associated biota on the seafloor are important for protecting essential fish habitat, delineating marine reserves, and assessing changes in habitat due to natural or human impacts. The products and knowledge developed by this project have broad applicability and help address other important issues such as coastal erosion, storm impacts, sea-level change, and water resources. Here an overview of the cooperative program is provided, illustrating the methods used to establish the geologic framework, and how this information contributes to an ecosystem based management of the inner continental shelf.
Presenter: Pat Iampietro, CSU-Monterey Bay, Multivariate prediction of rockfish abundance and distribution in Cordell Bank National Marine Sanctuary and Del Monte shalebeds, California, USA
- There is a great need for accurate, comprehensive maps of seafloor habitat for use in fish stock assessments, marine protected area design, and other resource management pursuits. We used high-resolution seafloor geomorphology data along with submersible and ROV video data at Cordell Bank National Marine Sanctuary (CBNMS) and the Del Monte shale beds of Monterey Bay, CA, USA, to produce preliminary species-specific habitat suitability models for eight rockfish (Sebastes) species. These results were compared to those relying on video observation alone. General Linear Models (GLMs) were created using habitat classification analyses of bathymetric digital elevation models and supervised texture classification from backscatter mosaics along with observation data for various species of rockfish. These GLMs proved efficient at predicting the distribution of S. rosaceus, S. flavidus, and S. elongatus in CBNMS with average accuracies of 81%(±10.5%), 76%(±17.8%), and 62%(±23.3%) respectively. When compared to the video results, GLMs were better at predicting the abundance of S. flavidus and S. rosaceus with percent errors of 16% and 13% respectively, compared to percent errors of 68% and 44% from the video analysis. Both methods yielded equivalent results for S. elongatus. These results indicate that the use of algorithmic habitat classification applied to high-resolution acoustic remote sensing data combined with in situ video observation of species habitat relationships can produce significantly more accurate models of species distributions and stock size than video observations alone. The application of these techniques holds great promise for more efficient and successful marine protected area design and management.
Presenter: Sophie De Beukelaer, Monterey Bay National Marine Sanctuary, Using Geographical Information Systems to Evaluate the Need for Marine Protected Areas in the Monterey Bay National Marine Sanctuary
Presentation (pdf, 1.8 Mb)
- Carefully planned marine protected areas can be an effective tool for conserving biological diversity and seafloor habitats. In order to address the need for marine protected areas within the Monterey Bay National Marine Sanctuary, we integrated relevant data layers within a Geographical Information System project. This rich compilation of physical, biological, geological and socioeconomic data provides stakeholders and managers with a more thorough understanding of the spatial distribution of marine resources, processes and human uses within the Monterey Bay National Marine Sanctuary. However, because this area is so large, over 5000 square miles, the biological data is not congruent over time and space and we have to rely on using benthic habitat as a proxy for biology, especially in water deeper than 200 m. For example, many rockfish species are associated with rocky substrates. To support the ecosystem-based planning and analysis for marine protected areas, the Geographical Information System project was supplemented with the development of Doris. Doris is a web-based decision support tool that allows stakeholders to use basic viewing and querying tools on public data layers and complete detailed data analyses such as habitat area summaries for a network of marine protected areas. These two systems complement each other to help managers and stakeholders make decisions on marine protected areas with the best available information in formats that are easy to interpret.
- A recent focus of NOAA's National Marine Sanctuary Program has included integrated assessments for characterizing resources and assessing the condition of marine sanctuaries. Gray's Reef National Marine Sanctuary (GRNMS) is located off the coast of Georgia on the inner continental shelf. It is an important habitat for the North Atlantic right whale, pelagic fishes and many reef-related fishes. As part of the effort to characterize, manage, and protect these marine resources, detailed benthic maps have been produced making it possible to examine relationships between biota and geomorphology. Benthic habitats within the Sanctuary include flat and rippled sand, sparsely colonized live bottom, and densely colonized live bottom with rocky ledges. Species richness and abundance of fish is strongly associated with percent live cover and high ledges.
Recently, hydroacoustic fisheries surveys have been conducted to provide additional information on fish size and abundance in the water column over a broad extent and at a fine spatial resolution. This paper describes the use of GIS and underwater acoustics as a non-intrusive method for assessing patterns of fish biomass in relation to bottom habitat. This approach is useful for sanctuary managers and researchers interested in quantifiable biological metrics for integrated assessments of important marine ecosystems. Specific examples using acoustic data collected at GRNMS are incorporated, with an emphasis on relating fish biomass to specific benthic habitats. These surveys confirm the association of fishes with complex benthic habitats, provide quantitative information for the entire water column, and cover a broad area efficiently and without extraction.
Presenter: Lisa Wedding, U. of Hawaii, Integrating remote sensing, ecology, and GIS to examine the relationship between habitat complexity and fish assemblage structure for the optimal design of marine reserves in Hawaii
Presentation (html) | Movie (188.4 Mb avi)
- Habitat complexity plays an important role in structuring fish assemblages in the marine environment. Remotely sensed Light Detection and Ranging (LIDAR) data has recently been utilized in coral reef ecosystems to derive rugosity (a measure of habitat complexity) at scales that are commensurate with the function of these systems. We used LIDAR-derived rugosity to examine the relationship between habitat complexity and various fish assemblage metrics (numerical abundance, diversity, richness, and biomass) in Hawaii. Fish assemblages were assessed using standard underwater belt transect surveys at 283 sites in a wide range of habitat types across the Main Hawaiian Islands. We established significant positive associations between LIDAR-derived rugosity, in hard bottom habitat, and these measures of fish assemblage structure. We also demonstrated that LIDAR-derived rugosity was a good predictor of fish biomass across a range of spatial scales and components of the assemblage responded to different spatial scales of rugosity depending on their size and mobility. Habitat complexity derived from remotely sensed data can be used to predict the fish assemblage of an area and can therefore aid in the optimal location and design of marine protected areas by identifying specific areas that offer great natural protection. The results of our study suggest that LIDAR data has the potential to assist in prioritizing areas for conservation and management in the Main Hawaiian Islands and similar insular tropical ecosystems.
Marine Geomorphology Discussion Panel. 3:10 - 4:50 p.m.
Session Chair: Will Heyman, Texas A&M
Will Heyman, Scott Nichol, Doug Fischer, Gary Greene, Dawn Wright
Sappington et al., 2007, on Quantifying Landscape Ruggedness (alternative rugosity calculation, pdf - thanks to Dan Sampson)
Sappington, J.M., Longshore, K.M., Thompson. D.B., 2007, Quantifying landscape ruggedness for animal habitat analysis: A case study using bighorn sheep in the Mojave Desert. J. of Wildlife Management, 71(5): 1419-1427.
EUNIS Biodiversity Database, including seabed habitat classifications for Europe
Special issue of Applied Acoustics, October 2009, The Application of Underwater Acoustics for Seabed Habitat Mapping, edited by Craig J. Brown, Philippe Blondel and Michiel Postema
Contributed by Markus Diesing:
A major EU-funded initiative to harmonise mapping approaches and collate habitat maps in NW-Europe. Several products like a guide to habitat mapping, a scoping tool, standards & protocols and recommended operating guidelines are available.
See also this link for related projects in Europe and beyond.
Here is a list of reports and publications covering work done in my organisation:
Tech Reps. can be downloaded from http://www.cefas.co.uk/publications/scientific-series/technical-reports.aspx
Boyd, S.E., Coggan, R.A., Birchenough, S.N.R., Limpenny, D.S., Eastwood, P., Foster-Smith, R.L., Philpott, S., Meadows, W.J., James, J.W.C., Vanstaen, K., Soussi, S., Rogers, S., 2006. Role of seabed mapping techniques in environmental monitoring and management. Sci. Ser. Tech. Rep., Cefas Lowestoft, 127: 166 pp.
Brown, C.J., Hewer, A.J., Meadows, W.J., Limpenny, D.S., Cooper, K.M., Rees, H.L., Vivian, C.M.G., 2001.Mapping of gravel biotopes and an examination of the factors controlling the distribution, type and diversity of their biological communities. Sci. Ser. Tech. Rep., Cefas Lowestoft, 114: 43 pp.
James, J.W.C., Coggan, R.A., Blyth-Skyrme, V.J., Morando, A., Birchenough, S.N.R., Bee, E., Limpenny, D.S., Verling, E., Vanstaen, K., Pearce, B., Johnston, C.M., Rocks, K.F., Philpott, S.L., Rees, H.L. 2007. Eastern English Channel Marine Habitat Map. Sci. Ser. Tech Rep., Cefas Lowestoft, 139: 191pp.
Brown, C.J.,Cooper,K.M.,Meadows,W.J., Limpenny,D.S., Rees,H.L., 2002. Small-scale mapping of seabed assemblages in the eastern English Channel using sidescan sonar and remote sampling techniques. Estuar. Coast. Shelf Sci., 54: 263–278.
Brown, C.J., Hewer, A.J., Meadows, W.J., Limpenny, D.S., Cooper, K. M., Rees, H.L., 2004. Mapping seabed biotopes at Hastings Shingle Bank, eastern English Channel. Part 1. Assessment using sidescan sonar. J. Mar. Biol. Assoc. UK, 84: 481–488.
Eastwood, P.D., Souissi, S., Rogers, S.I., Coggan, R.A., Brown, C.J., 2006. Mapping seabed assemblages using comparative top-down and bottom-up classification approaches. Can. J. Fish. Aquat. Sci., 63: 1536–1548.
Foster-Smith, R.L., Brown, C.J., Meadows, W.J., White, W.H., Limpenny, D.S., 2004. Mapping seabed biotopes at two spatial scales in the eastern English Channel. Part 2. Comparison of two acoustic ground discrimination systems. J. Mar. Biol. Assoc. UK, 84: 489-500.