Using LIDAR and Sonar to Count Menhaden

LIDAR (for Light Detection and Ranging) and SONAR (for Sound Navigation and Ranging) are technologies that use the strength of reflected pulses of light or sound to distinguish among materials with differing compositions or surface properties, such as water and fish tissue. The goal of this study is to determine whether we can use an airplane-mounted LIDAR unit, a boat-mounted sonar unit, or some combination of these two technologies to detect and quantify menhaden schools, thereby providing a rapid, reliable, and relatively inexpensive means for estimating menhaden populations in Chesapeake Bay.

Traditional fishery surveys (in which scientists tow a net behind a research vessel for a standardized time period along numerous randomly chosen transects) are prohibitively expensive for this purpose, and are also poorly suited for counting menhaden and other fish that travel in discrete schools and instinctively flee oncoming sampling nets.

Our study, which is scheduled to begin in summer 2006 and last two years, will include both field and laboratory components. Year-one field studies, to be run in conjunction with vessels involved in the reduction fishery, will help us identify the "target strength" or reflectivity of menhaden schools using both LIDAR and sonar. We will coordinate LIDAR, sonar, and ship-based sampling of menhaden schools encountered and pursed by reduction-fishery vessels. A technician aboard the fishing vessel will record total biomass of the captured school and take fish samples. Catch data will then be compared with LIDAR and sonar measurements. After the initial calibration of LIDAR and sonar techniques, we will conduct a pilot survey over several days.

Laboratory trials will allow us to further calibrate the LIDAR and sonar signals by measuring the reflectivity of a small school of menhaden under controlled conditions in a large tank.

We will use the experience gained during the first-year pilot study to help design a Bay-wide aerial survey of menhaden schools during summer 2007. We will conduct repetitive flights each month for a period of at least three months (June-August) to study changes in the distribution and abundance of menhaden in Chesapeake Bay. We will coordinate LIDAR and sonar measurements to the extent possible, so that menhaden schools are surveyed simultaneously by both technologies. To accomplish this, the research vessel will be directed by the LIDAR-equipped airplane.

Because both LIDAR and sonar techniques can survey a large area quickly, we expect a significant cost savings as compared to a large-scale survey using traditional fishing gear. Calibration of both techniques during the first year and comparisons between both techniques and the fishery during the second year will facilitate full-scale implementation of subsequent menhaden surveys in the future.