Experiments were conducted with a multibeam dual‐frequency identification sonar (DIDSON) to evaluate the accuracy and precision of estimating lengths from images of tethered fish insonified at side aspect in an Alaskan river. Live tethered Chinook salmon Oncorhynchus tshawytscha and sockeye salmon O. nerka were suspended in front of a long‐range DIDSON (1.2 MHz, 48 beams) equipped with an ultra‐high‐resolution lens. Lengths measured manually from DIDSON images were highly correlated with the actual lengths (R2 = 0.90, RMSE = 5.76 cm). No range dependency in the accuracy of the range estimates was documented. We conclude that relatively accurate and precise estimates of fish length are now possible with certain DIDSON system configurations at up to 21 m.
Burwen, D. L., Nealson, P. A., Fleischman, S. J., Mulligan, T. J., and Horne, J. K. 2007. The complexity of narrowband echo envelopes as a function of fish side-aspect angle. – ICES Journal of Marine Science, 64: 1066–1074. High-frequency, narrowband acoustic signals may contain more information on fish size and orientation than previously thought. Our observations of dual frequency identification sonar (DIDSON) images of fish orientation paired with split-beam echo envelopes helped clarify why metrics such as echo duration have performed better than target strength measurements when predicting salmon lengths at side aspect. Fish orientation has a pronounced effect on the duration and shape of split-beam echo envelopes from large (80–130 cm) salmon insonified at side aspect. At near-normal aspect angles, echo envelopes are unimodal, symmetrical, and resemble echo envelopes from calibration spheres. With increasing oblique-aspect angle, echo shapes become less symmetrical as the number of peaks increases, and echo duration and amplitude become more variable. Using angle and range coordinates, peaks in an echo envelope can be traced to their origin on a DIDSON image. At oblique-aspect angles, discrete peaks develop that are reflected from regions close to the head and tail. In addition, the distance between peaks increases with increasing aspect angle and is larger than can be explained by swimbladder length.
Side-looking, fixed-location sonar is used to estimate the abundance of migrating chinook salmon Oncorhynchus tshawytscha in the Kenai River, Alaska. For this application, echo-envelope length has previously been shown to predict fish size better than target strength. Using tethered-fish experiments we generalize these findings to other hydroacoustic descriptors based on time measurements, including range-measurement variability and fish lateral movement. These variables are all descriptors of the echo signal through time. Measurements of these attributes were correlated with daily indices of the species composition of unrestrained fish passing the sonar site. We hypothesize that time-based characteristics are superior predictors of fish size because they capitalize on, or are robust to, the factors which compromise amplitude-based measurements with side-looking sonar.
For this side-looking, 200 kHz, split-beam sonar application, echo-envelope length has been shown to be predictive of fish size. In this study, this relationship is exploited to estimate the abundance of (large) chinook salmon (Oncorhynchus tshawytscha) in the presence of (smaller) sockeye salmon (Oncorhynchus nerka). The echo-length to fish-size relationship is too imprecise to ascertain the species of individual fish in the classic sense. However, the frequency distribution of echo-length measurements contains information on the relative abundance of chinook and sockeye salmon. The use of echo-length measurements in a mixture model is explored in order to estimate the proportion of total fish passage that comprised chinook salmon. Inputs to the model include empirical estimates of the length–frequency distribution for each species, parameter estimates from the regression relationship of echo-length to fish-length, and echo-length measurements from individual, ensonified fish. Outputs are estimates of the proportions of chinook and sockeye salmon in the river. The advantages of the mixture-model approach over threshold-based discrimination are discussed. Conditional maximum likelihood and Bayesian versions of the model are described. The method can be generalized to other hydroacoustic measurements, including target strength and other discrimination problems.
Chinook salmon passage has been estimated in the Kenai River using fixed location, side-looking split-beam sonar since 1996. Estimation of Chinook salmon passage is complicated by the presence of smaller, more abundant species of salmon. Accurate estimates depend on the ability to distinguish large from small fish. Originally, a target strength threshold was used to classify fish species. However, in situ tethered-fish experiments revealed that target strength was a poor predictor of fish size for side-looking sonar. Measurements based on echo envelope length (duration) provided better predictive ability for tethered fish, but this could not be conclusively verified for free-swimming migratory fish. In this paper we describe our current efforts to estimate Chinook salmon passage in the Kenai River using dual frequency identification sonar (DIDSON). Length is manually measured from DIDSON still images, and from fish captured by gillnets drifted onsite. A species/age mixture model is fitted to the data to provide estimates of Chinook salmon passage. Passage estimates for large Chinook salmon can be produced using a simple threshold applied to DIDSON lengths.
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