QUANTIFYING SIDE ASPECT TARGET STRENGTHS OF TETHERED AND FREE-SWIMMING SILVER CARP (HYPOPHTHALMICHTHYS MOLITRIX) USING SPLIT-BEAM HYDROACOUSTICS

Invasive bigheaded carp (Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis) are a major nuisance throughout the Midwest. Horizontally oriented (or side-looking) hydroacoustic split-beam sampling is used throughout the Mississippi River basin to quantify the density and size distribution of bigheaded carp. Accuracy of these estimates relies on target strength (TS) returns from individual fish, and orientation is generally known to affect TS. Specifically, fish swimming perpendicular to a transducer generate a stronger TS return than one swimming towards or away from it. This study conducted two experiments to determine the effect of orientation on TS return. The purpose of this study was to 1) determine the statistical relationship between the max TS return and TL of Silver Carp, 2) quantify the relationship between body orientation and the mean TS return for Silver Carp, 3) compare the relationship between body orientation and the mean TS return for Silver Carp and Smallmouth Buffalo and 4) compare echosounding the tethered and free-swimming Silver Carp. To address objectives 1-3 a study was conducted where tethered Silver Carp and Smallmouth Buffalo were echosounded at multiple orientations. Each fish was tested every 10 degrees from 0°-180° or 180°-360° (where 0° and 360° are the same). The fourth objective was addressed using a free-swimming study where Silver Carp were echosounded in a controlled environment for a week. The tethered study consisted of Silver Carp (range 436mm-715mm total length (TL)), and Smallmouth Buffalo (range 361mm-576mm TL) suspended in a 0.1ha research pond on a carousel at orientations from three categories (lateral, oblique and head/tail). The free-swimming study consisted of Silver Carp (range 425mm-790mm TL) in a 0.1ha research pond where the fish were able to behave normally, and data was collected for 7 days. Target strength returns were measured using a horizontally facing 200kHz split-beam transducer for both studies. Consistent with expectations, we found that tethered orientation affected the TS return (P = \u3c0.001) for both species. Total length resulted in mixed results as Silver Carp had no relationship between TL and TS return (P= 0.37) but Smallmouth Buffalo did (P = \u3c0.001). Estimating TL from TS of tethered fish using an inverse of Love’s side aspect multispecies equation provided mixed results. Silver Carp estimates of TL were generally overestimated compared to Smallmouth Buffalo which were also overestimated but still resulted in no difference between distributions. Target strengths of Silver Carp in the tethered and free-swimming studies were able to be compared due to no difference between the TL distribution of the actual TL distributions. Free-swimming study yielded similar results to the tethered study for Silver Carp. Total length estimates were underestimated, and mean TS values were similar to the tethered study results. Silver Carp appear to be acoustically “noisy”, which may be common in streamlined fish that swim under high velocities. Field estimates of TS likely underestimate the true density of Silver Carp in the field. Further investigation is needed to determine the reason as to why Silver Carp are acoustically “noisy”