Hydroacoustic Target Strength Distributions of Alewives in a Net‐Cage Compared with Field Surveys: Deciphering Target Strength Distributions and Effect on Density Estimates

Abstract

AbstractAcoustic estimation of fish abundance requires knowledge of the target strength (TS) distribution associated with the fish present. We measured the TS distribution at 70 kHz for five size‐groups of alewives Alosa pseudoharengus (mean total length = 68‐138 mm) swimming freely in a large net‐cage. Although the mean and mode of the distributions were significantly related to fish length, the TS range obtained from each length‐class was over 25 decibels (dB). The TS distributions were negatively skewed and had a tendency toward bimodality. The TS distributions at 70 and 123 kHz were similar for one alewife size‐group that was measured with both frequencies. Small but inconsistent differences were found between day and night for the same groups of fish. Shapes of the TS distributions were similar when centered on the mode, and we derived a probability density function (PDF) for TS at 70 kHz as a function of fish length to describe these distributions. We used this PDF to predict TS distributions based on alewife gill‐net catch from six lake surveys and compared predictions with in situ TS distributions from each acoustic survey. Observed and predicted distributions were similar for TS values greater than −50 dB, but the observed number of small TS values exceeded the predicted number for several of the surveys. Some of these smaller TS values were probably from invertebrates or smaller fish or were indicative of noise. Estimates of alewife density from lake surveys were highly affected by inclusion or exclusion of these smaller targets. We used the predicted TS distribution derived either from catch data or from the TS values greater than −50 dB to calculate the proportion of smaller TS values that were probably from alewives. Both methods resulted in similar abundance estimates, which were higher than estimates based on an existing TS‐length regression for alewives.