A generalized target strength model for euphausiids, with applications to other zooplankton

A hybrid model has been developed that generates predictions of euphausiid target strengths similar to those observed by direct measurement. The controlling variables of the model account for the size, shape, orientation, and stochastic nature of orientation. Sound speed and density contrast values used are within the range of those directly observed. This suggests that it is possible to produce a generalized target strength prediction for euphausiids like those available for fish. Euphausiids appear to be better generalized as a bent cylindrical or bent spheroid targets rather than spherical targets, at least for frequencies greater than 100 kHz. Although the model is based on a single, limited data set, its predictions compare well with the results, of a number of recently published measurements and models of target strength. The model also resolves some puzzling contradictions of target strength frequency dependency previously observed, but unexplained. A number of items were determined to be needed to complete or further refine such hybrid models: Target strength measurements for more sizes, species, and frequencies (especially below 50 kHz and above 420 kHz) are most prominent among these.