by Kevin Johnson
Freshwater Fisheries Long Term Monitoring (LTM) Program began in 2006 with the intent to obtain data to be used by managers to determine trends in sportfish abundance, species composition, mortality, growth, size structure, and utilization by anglers for Florida’s important freshwater fisheries. The LTM Program has 30 core lakes that are sampled annually for these metrics. With this program in place, there was a growing need to develop an efficient method of collecting habitat information in these lakes so that habitat quality and quantity could be monitored over time.
Therefore, our objective was to develop sampling protocols that would provide accurate lake-wide estimates of the percent of area covered and percent volume infested with submersed and emergent vegetation in lakes that are part of the LTM Program. After investigating different techniques used for sampling aquatic vegetation, we determined that two methods of remote sensing fit our objective. These include hydroacoustic sensing for mapping submersed vegetation and the interpretation of satellite imagery for mapping emergent vegetation.
Hydroacoustic sensing for mapping submersed vegetation will entail traversing sampling transects with a boat-mounted Lowrance HDS sonar unit. Point-intercept sampling along transects will also take place to determine species occurrence within a lake with the use of a vegetation rake. Transect spacing will be a function of point-intercept intensity, with the program’s largest lakes having a spacing of 285 meters which is equivalent to 1 point per 20 lake acres. Recorded Lowrance sonar logs will be uploaded to CiBioBase, a company that provides rapid automated web-based processing software which uses a series of algorithms to interpolate and extrapolate raw acoustic sonar data into whole-lake submersed vegetation coverage and density raster heat maps. These raster maps will then be converted into ArcGIS raster maps in order to add point-intercept data.
Interpretation of satellite imagery for mapping emergent vegetation uses Landsat images to delineate the emergent vegetation boundary of a lake with ArcGIS software.
With sampling protocols developed, annual mapping efforts will take place during the peak growing season, beginning summer 2015.