Target Strength of Atlantic Cod from Synchronized Split-Beam and Multibeam Echosounders

Acoustic surveys have been widely used to assess fish stock abundance, yet the uncertainty of these estimates partly depends on how well the acoustic scattering properties of the individuals being surveyed are understood and represented.   Abundance of Atlantic cod (Gadus morhua), a commercially-important and exploited species throughout most of its range, is traditionally estimated from fishery-independent bottom trawl surveys, such as the Gulf of Maine (GoM) and Georges Bank stocks of the US northwest Atlantic.  Acoustic surveys have been used to provide abundance estimates of cod stocks from other regions such as Newfoundland and the Barents Sea.  The spring spawning aggregation of GoM cod found in Ipswich Bay, because of it spatial and temporal predictabilities, lends itself to acoustic surveying. In working toward the goal of developing an acoustic survey of this aggregation, target strength (TS) of individual mature GoM cod collected from this spring spawning aggregation was estimated from a series of acoustic measurements made using a 38 kHz and 120 kHz split-beam echosounder (Simrad EK60) and a 300 kHz multibeam echosounder (Kongsberg EM3002).   This multibeam echosounder can also be used to collect high-quality bottom backscatter and bathymetry of the benthic habitat where cod are surveyed.  Individual cod, ranging from 59 to 98 cm in total length (TL), were placed inside a 2-m³ monofilament mesh cage.  The cage was then suspended from an anchored vessel at a depth of 8-10 m.  The two echosounders synchronously collected acoustic data of each free-swimming captive cod, while the movements of most individuals were observed with underwater video.  The split-beam transducers provided direct measurements of TS after standard sphere calibration, but the TS from the central single beams of the multibeam transducer was statistically estimated from the echo amplitudes after compensations based on prior beam pattern and standard sphere measurements.  The TS-TL relations at 38 and 120 kHz were compared to those reported in the literature, and to the TS-TL relation at 300 kHz.   The shape of TS distributions differed between calm and erratic swimming behavior as well as between near horizontal angles and tilted angles.