Two independent surveys showed that aquatic insects can generate echoes comparable with the echoes of small fish. In the open water of two fishless mountain acidic waters of the Czech Republic, a very distinct population of well-defined single targets resembling fish (target strength –66 to –50 dB at 120 kHz) was discovered. These targets were found in the depth range of 5–7 m during the day (June–July 1997). At night, most targets were recorded by horizontal beaming (depth < 3 m). Towing of ichthyoplankton nets in these horizons revealed the presence of ‘water bugs’, Corixidae (Hemiptera), mostly genus Glaenocorisa. Target strengths of corixids was also verified by direct observations in an experimental tank, where it was possible to track side, dorsal and ventral aspects. In three Dutch reservoirs, Petrusplaat, Honderd-en-Dertig and De Gijster (area 100–300 ha, maximum depth 15–28 m, with fish presence) the targets of ascending dipteran pupae (Chironomidae, Procladius signatus) with target strength of –74 to –65 dB were observed by vertical beaming. The presence of pupae was verified by horizontally towed ichthyoplankton nets and by vertical plankton hauls inside the echosounder beam. The contribution of aquatic insects to the fish biomass was small but they can interfere in juvenile fish assessment.

Observations of acoustic images made along a transect perpendicular to the Coast, off Villefranche-sur-Mer, allow one to recognize the hydrological zone corresponding to a surface density gradient characterizing the Ligurian Sea front (Mediterranean Sea). Combinations of statistical and geometrical parameters from the digitized echoes enable discrimination between different classes of patches in relation to the frontal structure during one transect recorded on December 20, 1990. Patch recognition is based on the extraction of the general trend of each column and each row of the digitized data matrix by Eigenvector filtering. Multivariate analysis (principal component analysis and discriminant analysis) run on a 650 patches × 19 variables matrix shows a high negative relation between statistic and geometric parameters of the patches. The two main discriminant parameters are shape and echo strength of patches. From the convergence of our results in the Ligurian front with those of previous works made in other frontal region, it is suggested that this method could be generalized for quantifying and standardizing studies of the patchiness of plankton.