This study incorporated hydroacoustic sampling for bathymetry, substrate type, underwater vegetation, and fish distribution in Lake Washington, Seattle, USA. Data were collected using three independent echo sounder systems, to maximize data accuracy and vessel use, and geo-referenced using a Differential Global Positioning System enabling the acoustic data to be used in a Geographic Information System. This method proved to be an effective way to study various habitat influences on fish distribution, as well as map and monitor important physical and seasonal habitat parameters such as bathymetry, bottom character, and aquatic vegetation distribution. A survey map was designed with a commercial navigation software package using BSB format maps produced by the United States National Oceanic and Atmospheric Association (NOAA). The navigation software was operated simultaneously with the hydroacoustic acquisition software in order to collect data along preestablished transects. The signal from an onboard Differential Global Positioning System (DGPS) was fed into both the navigation package and acoustic data collection software so that all data collected were concurrently geo-referenced. Upon completion of the data acquisition, researchers processed the data to extract bathymetry, bottom substrate, SAV height and cover, and fish abundance and distribution. Accurate two-dimensional ground measurements (e.g., length, area) required that the DGPS data be mathematically transformed into a map projection system appropriate to the area of interest. All other data was subsequently imported into a GIS database for integration and mapping. Researchers collected data on bathymetry and substrate type using a BioSonics DE Series 70 kHz, 6º single-beam echo sounder set to 0.4 ms pulse-width, 5 pings s-1, and a threshold of 70 dB. BioSonics EcoSAV software was used to analyze the data acquired during testing, in order to determine the bathymetry. After completion of the bathymetric analysis and mapping, regions of potentially different bottom substrate were identified based upon assumptions regarding bathymetrically related substrate distribution. Geospatial surface models were developed from final data on percent plant biovolume, percent bottom cover, sediment type, and bathymetry. The dominant bathymetric feature was a long shipping channel that extended from the very north end of the lake to deep water. A shallow shelf was evident on the north end of the lake, as well as a small alluvial deposition zone at the outflow of the Sammamish River. Approximately 1 km south of the river outflow, the lake quickly deepened and the basin sides became much steeper, greatly limiting the distribution of aquatic vegetation. In general, the sampling area was shallow and the depth did not exceed 15 m. Overall, the study demonstrated that the various echo sounder systems could be combined to yield a wealth of data on aquatic habitat and fish distribution. Integrating the data in GIS was an effective way to evaluate fish distribution in regards to bathymetric features, substrate types, and patches of aquatic vegetation. |
As part of the nutrient criteria development project bathymetric data were collected on nine coastal lakes in Oregon. The data are a result of recent technology that makes data collection more efficient, thus allowing the collection of many more data points that previous methods. Previous mapping exercises for these lakes took place in the seventies when technology limited the amount of data that could be efficiently collected. This project used digital echosounder equipment connected to a gps unit allowing very accurate maps to be created from high data density.
