Target strength (TS) was estimated from in situ and ex situ measurements of juvenile blueback herring Alosa aestivalis during their fall downriver migration at the Crescent Hydroelectric Project in the Mohawk River, New York. The blueback herring is an ecologically important anadromous species that must transit fish passage facilities at hydroelectric dams. Measurements of TS are necessary to distinguish juvenile blueback herring from other fishes as well as to scale echo integration results to numerical fish densities during hydroacoustic studies. This study presents the first measurements of TS for juvenile blueback herring. The TS measurements were collected from an echosounder operating at 420 kHz with two split‐beam transducers (one down‐looking and one side‐looking). Single echo detections associated with tracked fish echoes and the echoes at the periphery of schools in the headrace of the powerhouse were used in estimating in situ TS. Mean total length (TL) of 192 individuals captured by cast net was 75 mm. A single dead fish (70 mm TL) was tethered and suspended by monofilament line in both transducer beams, each orientated down; the mean TS of this fish was −52.3 decibels (dB) for the 15° beam and −52.8 dB for the 6° beam. The mean in situ TS was −46.0 dB in the down‐looking beam and −48.6 dB in the side‐looking beam. The in situ TS estimates were similar to those reported for similar‐sized fish from published TS–TL relationships. The mean in situ TS for the down‐looking beam was 0.1–0.5 dB higher than the predicted TS for similar‐sized alewives A. pseudoharengus measured at 70 kHz in two studies. Results indicate that blueback herring and alewives have similar TSs and emphasize the importance of swim bladder condition and body orientation in influencing TS estimates for scaling acoustically derived fish density estimates.
Received July 17, 2011; accepted December 30, 2011