Target strength and vertical distribution of smelt (Osmerus eperlanus) in the IJsselmeer based on stationary 200 kHz echosounder recordings

Ventral target strength (TS) of smelt (Osmerus eperlanus) was measured using a bottom deployed Simrad EK60 200 kHz echosounder in the shallow lake Ijsselmeer (The Netherlands). Ventral TS values recorded (TS = 20 log10(L)–74.39) were lower but comparable to previous studies looking at the dorsal TS in other areas. Biological samples were collected simultaneously close to the bottom and close to the surface. The combination of acoustic measurements and catch information revealed a diurnal migration of smelt. Smelt was observed to be more dispersed during the night than during daytime, but showing no clear avoidance pattern for any environmental variable recorded by CTD measurements. Smelt was found in the upper water column during times of highest light intensity. Such behaviour is likely to occur due to a combination of turbid water conditions and predator avoidance, mainly of piscivorous fish species such as pike–perch (Stizostedion luciperca) and perch (Perca fluviatilis