Target strength of mesopelagic lanternfishes (family Myctophidae) based on swimbladder morphology

Abstract

This article reports theoretical values of target strength (TS) for mesopelagic lanternfishes based on morphological measurements of their swimbladders. Three species of lanternfishes, Diaphus theta (26.9–77.4 mm standard length (SL)), Symbolophorus californiensis (85.0–108.4 mm SL), and Notoscopelus japonicus (126.0–133.2 mm SL), were examined. After external morphological measurement of the fish body, a specialized “soft X-ray” imaging system was used to map the swimbladders and obtain their morphological parameters. The swimbladder was inflated in D. theta, uninflated in S. californiensis, and was absent in N. japonicus. For D. theta, the swimbladder length does not increase in proportion to the body length, suggesting that the contribution of the swimbladder to acoustic reflection is reduced with growth in this fish. Based on the morphological measurements, the theoretical TS of the fish at 38 kHz was calculated using the approximate deformed-cylinder model (DCM) and the general prolate-spheroid model (PSM). For all three species, the calculations showed about 3 dB difference between the TS indicated by the DCM and PSM. Given that the description of body shape is poor in PSM, the DCM results were adopted for fish without a swimbladder or an empty one. The intercept b20 in the standard formula TS = 20 log SL + b20 was −85.7 dB (DCM) for S. californiensis and −86.7 dB (DCM) for N. japonicus. On the other hand, the PSM model was adopted for D. theta since its swimbladder has too small an aspect ratio to apply the DCM. For D. theta, the relationship between SL and TS is best expressed by TS = 11.8 log SL − 63.5, which implies that its scattering cross-section is not proportional to the square of the body length.

School characteristics of mesopelagic fish at South Georgia

“…In particular, myctophids are difficult to identify to species level using underwater acoustics and further studies are required to gain new insight into the TS relationships and acoustic backscattering properties of myctophid species. New approaches, such as acoustic morphometric descriptors/classification (Charef et al, 2010;Korneliussen and Ona, 2003), in situ TS probes and theoretical TS modelling (Yasuma et al, 2003;Yasuma et al, 2010), together with continued acoustic surveys and scientific netting, are likely to develop the field of myctophid acoustics. However, there is clearly a pressing need to acquire basic data on these fish and their schooling behaviour in these high latitude regions that are experiencing rapid environmental change.…”

“…Differences in myctophid swimbladder structure, composition (e.g. gas-filled or lipid-filled) and orientation between, and within, species can have a large impact on their acoustic response (Butler and Pearcy, 1972;Godø et al, 2009;Marshall, 1960;Yasuma et al, 2003), which further confounds our ability to identify and quantify myctophids using acoustic techniques. Despite the challenges associated with myctophid fish, acoustic techniques offer the best available window of observation into their schooling behaviour in the natural environment at the appropriate temporal/spatial scales.…”

Differences in acoustic target strength pattern between fish with one- and two-chambered swimbladder during rotation in the horizontal plane

“…The reason for this difference is not known but perhaps the two-chambered swimbladder creates more opportunities for sudden overlapping of individual chamber echoes in near-side aspects during rotation, and such overlapping of echoes may result in higher TS values. Many studies have evaluated the effect of swimbladder shape and rotation on TS (McClatchie et al, 1996;Yasuma et al, 2003Yasuma et al, , 2006Pena and Foote, 2008;Jaffe, 2006). Unfortunately, these studies were done in a vertical plane and for angles that are expected to occur in natural swimming only.…”

Swimbladder Morphology and Target Strength of Myctophid Fishes in the Northwestern Pacific

“…PSM Sawada et al 8) Yasuma et al 3) 10°C 33.5 psu Table 1 Tilt angle, aspect ratio (major axis / minor axis), and ratio of swimbladder volume in fish body. Table 3 TS…”