1839 Acoustic characteristics of forage fish species in the Gulf of Alaska and Bering Sea based on Kirchhoff-approximation models Stéphane Gauthier and John K. Horne 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. Acous- tic 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 discrimi- nate 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. Résumé : Les inventaires acoustiques sont utilisés régulièrement pour estimer l’abondance des poissons. Pour obtenir des estimations précises, les caractéristiques des échos provenant des différentes espèces doivent être quantifiées. Des modèles de réflexion acoustique fondées sur l’approximation de Kirchhoff ont été utilisés pour mesurer les propriétés acoustiques de plusieurs espèces dans le Golf de l’Alaska et la Mer de Béring : le capelan (Mallotus villosus), le ha- reng du Pacifique (Clupea pallasii), la goberge de l’Alaska (Theragra chalcogramma), le maquereau Atka (Pleurogram- mus monopterygius) et l’eulakane (Thaleichthys pacificus). Le maquereau Atka et l’eulakane n’ont pas de vessies natatoires. La rétro-diffusion acoustique a été estimée en fonction de la fréquence sonore, de la taille et de l’orientation des poissons. L’intensité des échos et leur variance ont été comparées pour examiner la possibilité de discerner les es- pèces. Nous avons découvert que les espèces peuvent être séparées en deux groupes selon la présence ou l’absence de vessie natatoire. L’incidence de la taille et de l’orientation sur l’intensité des échos dépendaient de la fréquence sonore. La variabilité de l’indice de réflexion (IR) due aux différences morphométriques était élevée pour les espèces sans vessie. Des équations mettant en rapport la taille des poissons et l’IR ont été élaborées pour chaque espèce en fonction des fréquences sonores les plus couramment utilisées. Gauthier and Horne 1850 Introduction 1982), TS depends on fish size (Nakken and Olsen 1977), an- atomical features (e.g., presence of a swimbladder; Foote Acoustic surveys are used to monitor the distribution, abun- 1980a), morphology (e.g., swimbladder shape; McClatchie et dance, and habitat use of fish within ecosystems. These sur- al. 1996a), and physiological state (e.g., gonadal maturation veys are appealing for assessment purposes as large volumes and gut fullness; Ona 1990). Fish behavior can also signifi- of water are rapidly sampled at high spatial and temporal res- cantly alter TS through changes in fish orientation relative to olutions. To convert acoustic signals to estimates of fish abun- the wave front (Nakken and Olsen 1977; Foote 1980b; dance, species must be properly partitioned within survey Blaxter and Blatty 1990) and vertical movement within the areas and the echo energy of constituent species must be water column, which changes swimbladder volume (Ona known. The intensity of an echo is generally expressed on a 1990; Mukai and Iida 1996; Rose and Porter 1996). Ideally, logarithmic scale as the target strength (TS). Acoustic proper- TS should be measured under the same conditions as those ties of fish are species-specific and change over time. In addi- encountered when populations are surveyed. Suitable in situ tion to physical characteristics such as frequency (Foote conditions for the measurement of TS can be difficult to ob- Received 2 October 2003. Accepted 16 April 2004. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 11 December 2004. J17772 S. Gauthier1 and J.K. Horne. University of Washington, School of Aquatic and Fishery Sciences, Box 355020, Seattle, WA 98195-5020, USA. 1Corresponding author (e-mail: [email protected]). Can. J. Fish. Aquat. Sci. 61: 1839–1850 (2004) doi: 10.1139/F04-117 © 2004 NRC Canada 1840 Can. J. Fish. Aquat. Sci. Vol. 61, 2004 tain, especially in mixed species aggregations or within dense Indiana). The fish were placed at a focal distance of 40– schools of fish. 45 cm on cassettes or redipaks containing rare earth film. A modeling approach provides a practical alternative to Fish lateral and dorsal planes were exposed at 15 mA for 2 s measurements and can be used to examine the amplitude and at 95 kVp on the redipaks (generally used for <25-cm fish) variability of acoustic backscatter as a function of single or or for 0.18 s at 70 kVp on cassettes with intensifying screens multiple variables. Modeling exercises can also be used to (generally used for >25-cm fish). Fish without swimbladders compare and contrast acoustic properties among species and were photographed using a 3.3-megapixel digital camera identify potential metrics for species discrimination and (photoPC 3000z; Epson America Inc., Long Beach, Calif.). identification. The Kirchhoff-ray mode (KRM) model uses Lateral and dorsal images of the fish body and swimbladder low-mode solutions and Kirchhoff-ray approximations to es- (when present) were traced on acetate sheets, scanned, and timate resonant and geometric backscatter using planar im- then digitized at 1-mm resolution. Fins and tail were not in- ages of the fish body and swimbladder (Clay and Horne cluded in the trace. Orientation of the swimbladder relative 1994). KRM model predictions have been successfully to the body was maintained, and the body parts were scaled matched to empirical measures (Jech et al. 1995; Horne et to their true size using maximum and minimum body and al. 2000; Horne 2003). Matches of KRM model predictions swimbladder measurements. When necessary, trace lines to empirical backscatter measurements of pollack (Pollachius were smoothed and rotated so that the sagittal axis of the pollachius) and saithe (Pollachius virens) were comparable fish body was horizontal. The resulting dorsal and lateral to those obtained using the boundary element model (BEM) images were elliptically interpolated into 1-mm-thick cylin- over a frequency range of 38.1 to 120.4 kHz (Foote and ders to give a three-dimensional (3D) representation of the Francis 2002). In the present study, KRM models are used to fish body and swimbladder (Fig. 2). A series of morpho- characterize acoustic properties of forage fish in the Gulf of metric descriptors (including swimbladder volume and area) Alaska and the Bering Sea. Five abundant and widely dis- were estimated using these 3D fish representations. Species tributed species are considered: capelin (Mallotus villosus), were partitioned into groups of similar-sized individuals (10- Pacific herring (Clupea pallasii), walleye pollock (Theragra cm length groups). The ratio of the major to minor axes of chalcogramma), Atka mackerel (Pleurogrammus monoptery- the body and swimbladder (maximum length and width) gius), and eulachon (Thaleichthys pacificus). Atka mackerel were measured on the lateral traces as an index of elonga- and eulachon do not possess swimbladders. Capelin and Pa- tion. The eccentricity (e) of the fish body or swimbladder cific herring are physostomous (open swimbladder), whereas was also measured as the ratio of the distance between the walleye pollock is physoclistous (closed swimbladder). foci and the vertices of an ellipse having the same major and These fish species are important to the diet of many apex minor axes as the fish trace. Eccentricity varies between 0 predators such as Steller sea lions (Eumetopias jubatus). Re- and1(0≤ e < 1, where 0 is a circle). cent declines in western Steller sea lion populations in the Swimbladders were modeled as a series of gas-filled cyl- Gulf of Alaska and the Bering Sea have increased interest in inders, and fish bodies were modeled as a series of fluid- the assessment of forage fish species (Alverson 1992). It has filled cylinders. The energy backscattered by the fish body been hypothesized that changes in the composition, distribu- and swimbladder was obtained by estimating the scattering tion, and availability of fish in Steller sea lion habitat are in from each cylinder (for details see Clay and Horne 1994). part responsible for their decline and lack of recovery (Rosen The scattered sound pressure (Pscat at time t and range R)of and Trites 2000; Stickney 2000; Trites and Donnelly 2003). an object insonified by a plane incident wave (of amplitude Some of the challenges associated with the acoustic assess- Pinc at the object) is ment of forage fish around Steller sea lion rookeries, haul- = ikR()−2π ftᏸ outs, and feeding grounds include accurate conversion of (1) PtRPRscat(, ) ( inc / )e ()