Acoustic characteristics of forage fish species in the Gulf of Alaska and Bering Sea based on Kirchhoff-approximation models

Abstract

Acoustic surveys are routinely used to assess fish abundance. To ensure accurate population estimates, the characteristics of echoes from constituent species must be quantified. Kirchhoff-ray mode (KRM) backscatter models were used to quantify acoustic characteristics of Bering Sea and Gulf of Alaska pelagic fish species: capelin (Mallotus villosus), Pacific herring (Clupea pallasii), walleye pollock (Theragra chalcogramma), Atka mackerel (Pleurogrammus monopterygius), and eulachon (Thaleichthys pacificus). Atka mackerel and eulachon do not have swimbladders. Acoustic backscatter was estimated as a function of insonifying frequency, fish length, and body orientation relative to the incident wave front. Backscatter intensity and variance estimates were compared to examine the potential to discriminate among species. Based on relative intensity differences, species could be separated in two major groups: fish with gas-filled swimbladders and fish without swimbladders. The effects of length and tilt angle on echo intensity depended on frequency. Variability in target strength (TS) resulting from morphometric differences was high for species without swimbladders. Based on our model predictions, a series of TS to length equations were developed for each species at the common frequencies used by fisheries acousticians.

Seasonal changes in the diel vertical migration of walleye pollock (Theragra chalcogramma) in the northern Gulf of Alaska

“…where L ij is the midpoint of the jth size class for species i, and a i and b i are constants for the ith species given in Gauthier and Horne (2004). Note that only pollock were split into different size classes.…”

Acoustic classification of coexisting taxa in a coastal ecosystem

“…als with similar lengths, swimbladder area and volume of herring are more than double that of hake (Gauthier and Horne, 2004;Henderson and Horne, 2007). Since acoustic backscatter intensity is a function of swimbladder cross-sectional area and shape at high frequencies (Ona, 1990), differences in morphological characteristics can result in a difference in MVBS.…”

Rare detections of North Pacific right whales in the Gulf of Alaska, with observations of their potential prey

“…The basis for the technique is that active acoustic backscatter at 18 and 120 kHz is strongly frequency dependent for planktonic organisms such as copepods and euphausiids, but generally exhibits much less frequency dependence in fish (e.g. Gauthier & Horne 2004, Lavery et al 2007. Although it is difficult to distinguish individual species or taxa with active acoustics, fish and macrozooplankton can be distinguished in many cases due to the strong frequency dependence of plankton (e.g.…”