Multibeam Sonar Mapping and Scallop Stock Assessment: GIS Data Integration in Support of Sustainable Fisheries Management

G. D. Sutton (1), O. Tully (2), A. Hervas (2), J. Hickey (2)

(1) Coastal and Marine Resources Centre , Environmental Research Institute, University College Cork (IE)
(2) Irish Sea Fisheries Board (IE)

This study describes the integration of biological and geophysical data in a GIS environment to support the development a strategic plan for the management of scallop stocks off the south east coast of Ireland. Multibeam sonar (MBES) maps (bathymetry and acoustic backscatter) and other seabed data (sediment samples and video imagery) are overlain and analysed in combination with standard scallop catch rate data in order to assist in determining the relationship between sediment type and scallop stock density. Initial positive results indicate a sharp correlation between scallop stock density and sediment type, supporting the potential for accurate targeting of fishing effort within the context of a sustainable fisheries management plan, as well as the potential for identifying new scallop fishing grounds.

Background & Introduction
The south east coastal shelf is the location for Ireland's most important national King Scallop (Pecten maximus) fishery. Maintenance of CPUE (catch per unit effort) and controlling total effort are specific management objectives within the current development of a long-term plan to ensure the future viability of this fishery (Tully, 2002). A detailed scientific understanding of the mechanisms governing abundance and distribution of both adult and larval scallop are important in order to ultimately identify and delineate management zones within the fishery e.g. for protection of spawning stock.
Existing knowledge of scallop ecology indicates that population distributions are patchy (Robert and Butler, 1998), and that high scallop abundance correlates with coarser sediments such as sands and gravels (Bousfield, 1960; Robert, 1997; Kostylev, 2001).
Multibeam sonar systems have become the tools of choice in the mapping of seabed topography, morphology, and sediment characteristics (Mitchell & Hughes-Clarke, 1994, Courtney & Shaw 2000). When used in conjunction with optical imagery (still and video recordings) and sediment samples the complete seabed coverage generated by MBES facilitates very detailed spatial characterisation of substrates and habitats.
This paper highlights the seabed mapping and GIS aspects of research that began in 2001, and which is due for completion in 2004. The work is being conducted within the framework of a multidisciplinary research program funded under the Irish National Development Plan. The project participants are drawn from three academic institutions: Trinity College Dublin and National Universities of Cork and Galway, the Irish Sea Fisheries Board (BIM), and the South East Shellfishermen's Association.

Methods

Biological Stock Assessment
Initial research surveys were carried out in 2001 using scallop dredges from a commercial fishing vessel. The first objective was to map the general distribution of Scallop stock on a regular grid throughout the entire known extent of the fishing grounds. A follow-up survey was undertaken during Sept 2002 in which scallop stock sampling was stratified according to the two dominant acoustic facies (identified from backscatter data) and within three broad depth zones. For each thirty-minute tow the mean number of scallops within specific shell size ranges were recorded along with start and end co-ordinates and stored in MS Excel Tables.

Seabed Mapping
MBES sonar data were collected by the project team using the RV Celtic Voyager of the National Marine Institute equipped with a Simrad EM 1002. Coverage was prioritised in order to coincide with areas of high scallop density determined from stock assessments. All MBES data were processed using CARIS (Universal Systems) Hydrographic Information Processing System (HIPS). The following GeoTiff format imagery was generated: relative backscatter values gridded at five and ten metres; sun illuminated bathymetry gridded at three and five metres. Bathymetric data were also output in ASCII "x,y,z" format. These numerical soundings were generated at various grid intervals for subsequent use as both a GIS data layer and in the construction of a digital terrain model (DTM) as a fundamental step in the development of a high-resolution hydrodynamic model. Statistical classification of backscatter data is planned in the near future using a combination of both proprietary (QTC Multiview) software and conventional GIS based image analytical techniques. Sediment facies/habitat types thus identified will then be targeted in the final ground truthing survey. Surficial seabed sediments were sampled using a Shipek grab and subjected to laboratory particle size analysis (standard sieves and laser granulometry). Ground truthing of inferred acoustic facies will be undertaken as the final stage of field studies during the first week of October 2003. (It is anticipated that preliminary results including seabed video and still imagery and co-located sediment sample data will be available for incorporation into the oral presentation).

GIS
GIS is an essential element of study, and ArcView3.3 was used to provide a common platform in which all spatial data operations were undertaken from initial operational planning for survey coverage through to data integration, analysis and presentation. All data were converted from geographic co-ordinates to a common reference frame in UTM. Tabulated point data (sediment samples, photographic locations, scallop sample tow locations) were imported via SQL, whilst MBES data products were imported directly.

Results
The initial broad-scale stock density surveys detected commercial sized scallop densities of up to 300 individuals per tow. Densities in excess of 80 were encountered over a large semi-continuous area extending seaward to the 80m isobath directly south of the major estuary of the River Suir.
Coherent MBES data covering approximately 65% of the total south coast scallop bed area was generated during two annual field (2001/2) campaigns. The seabed in the study area slopes gradually from approximately the 40m isobath in the north (landward) to +/- 90m water depth in the south, over a distance of 50km. Initial visual and qualitative interpretations reveal, in the north of the area, a narrow band of outcropping rock bisected by a single large incised sediment channel (palaeochannel) with other conspicuous glaciofluvial features. This distinctive terrain gives way to a rather flat submarine plain dominated by two main acoustically distinct sedimentary facies extending southward to the seaward limit of the scallop grounds. This area is characterised by a presumed gravel lag over which are migrating extensive elongate trains of large low relief arcuate dune-like structures. These dunes and other mobile sedimentary features with distinct topographic expressions are also clearly discernable in the backscatter imagery, typically possessing a distinctive "dark" appearance. Intervening areas have a much lighter signature. Mean scallop stock density among all samples trawled from the "pale" seabed facies was 115 against a mean of 15 for the "dark" facies. Total catch from the light facies was higher in all cases but one. These initial qualitative analyses and correlations will be further refined during the coming months in the light of statistical classifications leading to the construction of sediment/biotope maps in which the most potentially productive scallop areas are delineated. These maps will thus provide the spatial basis to underpin future stock management plans. It also anticipated that that the methodologies developed during this work can be validated and geographically extended enabling the identification of new areas with high scallop fishery potential.

References