Feasibility Assessment of Split-Beam Hydroacoustic Techniques for Monitoring Adult Shortnose Sturgeon in the Delaware River
Patrick A. Nealson and Harold M. Brundage, III
Abstract.—As part of an assessment of available remote sensing technologies for monitoring shortnose sturgeon Acipenser brevirostrum, field evaluations of a split-beam hydroacoustic monitoring system were conducted on the Delaware River in December 2002. The survey area selected for evaluation of the system was a section of the river near Bordentown, New Jersey, where adult shortnose sturgeon are known to aggregate during the winter months. Hydroacoustic measurements were collected on eight adult shortnose sturgeon captured in gill nets on December 4–6, 2002 by passing over these netted fish with a 200-kHz split-beam hydroacoustic system sampling a downlooking 15° transducer. The netted sturgeon were recovered following the acoustic sampling, physically measured, and released. The primary objective of this comparison was to determine if shortnose sturgeon could be detected by a hydroacoustic system, given their backscattering characteristics and general close proximity to the bottom. A secondary study objective evaluated shortnose sturgeon acoustic attributes relative to those of other coexisting fish species to assess if sturgeon exhibited any unique characteristics that might be used to distinguish them. The 2002 feasibility assessment determined that shortnose sturgeon could be detected in close proximity to the bottom by a scientific split-beam hydroacoustic system sampling at a relatively narrow (0.2 ms) broadcast pulse width. A netted sturgeon resting directly on the bottom was resolved by the acoustic system. Acoustic measurements of a tungsten carbide sphere determined that the target could be detected to within 12 cm of the substrate. The eight netted sturgeon returned mean target strength (TS) estimates of –26.5 decibels (dB), well above –80 dB ambient background noise levels. Relative to white sucker Catostomus commersonii, the other captured fish species, shortnose sturgeon differed in two measured acoustic parameters, mean fish TS, and distance from the bottom. Shortnose sturgeon were observed to return greater mean TS values (increased acoustic backscatter) and to be generally more associated with the bottom than the other evaluated fish species. Given the limited data set, these observations are only qualitative in nature, but indicate that shortnose sturgeon can be detected using split-beam echo sounding systems and may exhibit some unique acoustic characteristics allowing their differentiation from other fish species. Comparisons of shortnose sturgeon mean TS and total length determined that the attributes were positively correlated. Sturgeon mean TS measured during the experiments was greater than predicted for fish of equivalent length by the empirical TS-length relationship published by Love (1977).