Recently developed broadband acoustic methods were used to study mixed assemblages of fish spanning a wide range of lengths and species. Through a combination of resonance classification and pulse-compression signal processing, which provides for high-range resolution, a modified commercial broadband echosounder was demonstrated to provide quantitative information on the spatial distribution of the individual size classes within an assemblage. In essence, this system spectrally resolves the different size classes of fish that are otherwise not resolved spatially. This method reveals new insights into biological processes, such as predator–prey interactions, that are not obtainable through the use of a conventional narrowband high-frequency echosounder or previous broadband systems. A recent study at sea with this system revealed aggregations containing bladdered fish 15–30 cm in length (Atlantic herring (Clupea harengus) and silver hake (Merluccius bilinearis)) and a variety of species of smaller fish 2–5 cm in length. These observations infer that the smaller 2–5 cm fish can be colocated in the same aggregations as their predator, the larger silver hake, as well as pre-spawning herring. While this technological advancement provides more information, there remain challenges in interpreting the echo spectra in terms of meaningful biological quantities such as size distribution and species composition.