Atlantic mackerel Scomber scombrus is a small pelagic, migratory fish which supports valuable commercial fisheries. Fishery-independent mackerel abundance estimates in the Northeast Atlantic are currently based on egg-production surveys every third year, and do not yet consider acoustic data. Echo-integration surveys could provide annual estimates of abundance, with additional scope for studying mackerel distributions throughout the year. However, this requires good knowledge of the mackerel target strength (TS), which is not yet adequate. The present study provides in situ TS values for mackerel from measurements made at sea with split-beam echosounders operating concurrently at 18, 38, 120 and 200 kHz. Acoustic data was analysed using Echoview post-processing software. Empirical results suggest mean TS of -51.22 dB at 18 kHz, -59.83 dB at 38 kHz, -55.51 dB at 120 kHz and -53.43 dB at 200 kHz, for a mean fish length of 33.3 cm. This differs significantly from the values currently assumed in acoustic surveying, which advocates a TS of -53.07 dB for a 33cm fish at 38 kHz. The sensitivity of mackerel abundance estimates to variations in TS estimates was also investigated using data from a dedicated mackerel acoustic survey around the Shetland Islands, Scotland. Confidence limits for the biomass estimates (400,000 to 4,500,000 tonnes) at each frequency were obtained using geostatistical techniques. Total stock biomass estimates at 38 and 200 kHz were in very good agreement with each other (to within 2.2 %) and were in range of values from an independent (i.e. non-acoustic) mackerel stock assessment (5,137,104 tonnes).