Age characteristics of walleye pollock school echoes

Abstract

Abstract The purpose of this study was to investigate the possibility of identifying the age of walleye pollock (Theragra chalcogramma) using acoustic information. Acoustic data targeting walleye pollock were collected at 38 and 120 kHz from 16 June to 12 July 2000 in the Pacific, off Hokkaido, Japan. To complement these data 33 trawl hauls were made and the species and age of the sample fish were accurately examined. The echoes of walleye pollock schools according to age were used to determine the morphological and bathymetric characteristics such as mean height, maximum length, centre depth, seabed depth, and distance from the seabed, as well as the frequency characteristics, this latter being the difference of mean volume backscattering strengths at 38 and 120 kHz, respectively (ΔMVBS). The ΔMVBS method is elaborated using MVBS (mean volume backscattering strength) from an integration cell of optimal size, the cell being examined by means of various integration periods to highlight the characteristics of the walleye pollock schools resulting in 20 pings (120 m), and by applying this method only in a common observation range for two frequencies. The ages of the schools are identified by a combination of morphological and bathymetric characteristics, and ΔMVBS characteristics. Age-0 groups are easy to distinguish from other age groups because they exist in distinct, small schools, are close to the coast, and have a narrow range of ΔMVBS regardless of time of day. Age-1 schools are low in height and very long, are distributed close to the sea floor, and have an ΔMVBS range of −1 to 8 dB, with most between 3 and 5 dB. These characteristics of age-1 schools are distinct from other age groups. As age-2 and age-5 schools have similar maximum length and distribution depth, it is almost impossible to identify these two by just morphological and bathymetric characteristics. However, the ΔMVBS of age-2 and age-5 schools show characteristic patterns that can be used as a means of identification. The pattern of ΔMVBS, which reflects an internal structure (swimming angles) of a school, is different for each age class, and is essential in the identification of the age of a walleye pollock school.