No abstract

At the Crescent Hydroelectric Project (Crescent), New York, the Mohawk River is impounded by two dams separated by an island, which creates two channels; the turbine channel conveys water to the hydroelectric turbines, and the main channel conveys water around the turbines. Our objective was to determine whether ultrasound could be used to divert juvenile blueback herring Alosa aestivalis from entering the turbine channel during their fall downriver migration. Fixed‐location hydroacoustics (420 kHz) was used to continuously monitor fish passage in the main channel upriver and downriver of the 122–128‐kHz ultrasonic projectors. A relative index of abundance was derived from echo integration of acoustic backscatter collected from three horizontally aimed single‐beam transducers sampling across each channel, while the proportion and speed of fish moving downriver were estimated from upward‐facing split‐beam transducers. The presence of juvenile blueback herring was verified by castnetting. Fish migrated downriver episodically for periods of hours rather than days. The mean daily number of downstream migrants (Nd) in the main channel was estimated to be 4.2 times higher at the upriver site than at the downriver site. The estimate for Nd at the main‐channel downriver site (NDM) was compared with the expected Nd (NEDM) based on the proportion of total river flow moving through the main‐channel downriver site as measured by acoustic Doppler current profilers. The mean daily difference between NDM and NEDM was significantly different from zero. The estimated proportion of fish that passed the main‐channel downriver site (31.3%) was almost three times greater than the proportion expected (11.5%). If it is valid to assume that water flow directly influences entrainment and impingement, then the significantly higher‐than‐expected number of blueback herring that migrated downriver in the main channel could be an indication that ultrasound at Crescent was partially effective in diverting fish. Received July 17, 2011; accepted February 2, 2012

Gurshin, C. W. D., Jech, J. M., Howell, W. H., Weber, T. C., and Mayer, L. A. 2009. Measurements of acoustic backscatter and density of captive Atlantic cod with synchronized 300-kHz multibeam and 120-kHz split-beam echosounders. – ICES Journal of Marine Science, 66: 1303–1309. Effective management strategies for Atlantic cod (Gadus morhua) in the Gulf of Maine require stock assessments based on accurate estimates of its abundance and distribution. If multibeam echosounders are to provide data for such estimates, the relationship between acoustic backscatter and fish biology must be better understood. Working towards this goal, a series of acoustic measurements was made using a 120 kHz, split-beam echosounder (Simrad EK60) and a 300 kHz, multibeam echosounder (Kongsberg EM3002). The transducers from both systems were fixed to a platform over a submerged 98 m3 cage made of 5 cm stretched-nylon mesh. After standard-sphere calibrations, the cage was stocked with live, mature Atlantic cod, with a mean total length of 80.7 cm (range: 51.5–105.0 cm). The echosounders synchronously collected acoustic data, while the cod were monitored with two underwater video cameras. Cod were incrementally removed from the cage to provide a time-series of acoustic backscatter at four densities (n = 128, 116, 66, and 23). Backscatter measurements of cod are compared between echosounders and over time, and the factors affecting the acoustically derived density estimates are discussed. The benefits and limitations of the EM3002 are highlighted.

From July to October 2004, five sites in the Hampton-Seabrook Estuary in New Hampshire were sampled with beam and otter trawls. The goals were to describe winter flounder (1) abundance in the estuary, (2) size class distributions, (3) spatial distribution by different size classes, and (4) distribution patterns. Of the 19 species caught, winter flounder was the most abundant and was dominated by young-of-the-year (YOY) fish. The five sites were fairly homogenous in depth, bottom type, salinity, and temperature. However, YOY abundance ranged from 2.1 to 32.1 fish 1,000 m −2 depending on the site. Benthic community was the best indicator of juvenile winter flounder abundance. Catch data of other organisms fluctuated, but no one species was a strong predictor of winter flounder abundance and distribution. During late summer and early fall, the estuary is used primarily by YOY winter flounder, indicating that this estuary functions as a nursery ground.

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