The copepod Calanus euxinus is a key prey species for fish in the Black Sea. To estimate the distribution and biomass of the late developmental stages of this species in July 2013, we analysed multi-frequency (38, 120, and 200 kHz) echo-sounder data from a fisheries survey of the Black Sea. The dependence of acoustic backscatter on frequency, i.e. the frequency response, was estimated for daytime scattering layers, which were confirmed by net catches to be dense, post-copepodite-stage (C4) aggregations of C. euxinus with prosome lengths greater than 2 mm. The high-resolution acoustic observations revealed that the nighttime, shallow distribution was bounded by the lower portion of the thermocline and that the daytime, deep distribution was bounded by oxygen. The dense and isolated aggregations were observed in seawater with a specific density, sigma(T), of between 15.2 and 15.9 kg m(-3). These results show that fisheries acoustic surveys, typically targeting only commercially exploited fish species, may also provide information on the lower trophic levels and thereby serve as an ecosystem-monitoring tool.