Multifrequency discrimination of fish and mysids

The opossum shrimp (Mysis relicta) is common in many lakes in the northern parts of Eurasia and North America. The shrimp is often an important link in the foodweb for fish, either throughout life or in early life stages. Generally, quantitative measurements of mysids in large volumes of water are difficult to obtain with traditional sampling methods. In this pilot study, measurements of volume-backscattering strength (S(v)) at 38, 120, and 200 kHz were used to separate backscattering from fish and mysids. Mysids were sampled with trawls. Where mysids were caught, the correlations between mysid biomass (dry weight) and mean S(v) at 120 and 200 kHz were positive (r(2) = 0.89 and 0.81, respectively). Where mysids were abundant, the S(v) exhibited a characteristic frequency response. This was not found where mysids were scarce or absent. Therefore, areas with great abundances of mysids can be identified, and their biomasses estimated from data collected during ecosystem monitoring.