In situ acoustic target strength of juvenile capelin

ICES Journal of Marine Science, Jan 2001

A dispersed, monospecific aggregation of juvenile (0+) capelin was detected acoustically in shallow (20–70 m) water in Bonavista Bay, northeastern Newfoundland in January 2000. This provided a rare opportunity to measure acoustic target strength (TS) of very small (mean length=51 mm) capelin in situ. Mean observed TS at 38 kHz was −61.0 dB. Observed TS was similar to TS predicted by the Norwegian-Icelandic capelin TS-length relationship (TS=19.1 log L (cm)−74), but ∼2 dB lower than predicted by the existing TS-length relationship for capelin in Newfoundland waters at 38 and 49 kHz (TS=20 log L (cm)−73.1). Combining present data with previous 38 kHz data indicates the relationship TS=23.3 log L (cm)−77.1 (r2=0.95, n=6) for capelin of lengths 5–14 cm in Newfoundland waters.

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In situ acoustic target strength of juvenile capelin

Richard L. O'Driscoll 0 George A. Rose 0 0 R. L. O'Driscoll: National Institute of Water and Atmospheric Research , PO Box 14901, Kilbirnie, Wellington , New Zealand. G. A. Rose: Fisheries Conservation , Fisheries and Marine Institute, Memorial University of Newfoundland , PO Box 4920, St. John's A dispersed, monospecific aggregation of juvenile (0+) capelin was detected acoustically in shallow (20-70 m) water in Bonavista Bay, northeastern Newfoundland in January 2000. This provided a rare opportunity to measure acoustic target strength (TS) of very small (mean length=51 mm) capelin in situ. Mean observed TS at 38 kHz was 61.0 dB. Observed TS was similar to TS predicted by the Norwegian-Icelandic capelin TS-length relationship (TS=19.1 log L (cm) 74), but 2 dB lower than predicted by the existing TS-length relationship for capelin in Newfoundland waters at 38 and 49 kHz (TS=20 log L (cm) 73.1). Combining present data with previous 38 kHz data indicates the relationship TS=23.3 log L (cm) 77.1 (r2=0.95, n=6) for capelin of lengths 5-14 cm in Newfoundland waters. - Capelin (Mallotus villosus Mu ller) are small pelagic fish that are an important forage and commercial species in northern waters. Abundance of capelin is routinely assessed using acoustic echo integration methods (e.g. Vilhja lmsson, 1994; Toresen et al., 1998). Despite the importance of acoustic target strength (TS) in scaling echo integrator intensity to fish density, relatively few measurements have been made of capelin TS. Target strength models currently used in Norwegian and Icelandic capelin acoustic surveys (Dommasnes and Rttingen, 1985) and Canadian surveys (Rose, 1998) are based on in situ and/or ex situ experiments conducted with a limited length range of fish. Rose (1998) measured in situ TS of capelin from 9.515.2 cm in Newfoundland waters. The TS model of Dommasnes and Rttingen (1985) incorporates ex situ measurements made by Dalen et al. (1976) on capelin from 1318 cm as well as in situ measurements (by echo trace counting) on capelin of unspecified lengths. It is not known how well existing models describe capelin TS outside these length ranges. In this paper we present in situ measurements of acoustic target strength of very small ( 5 cm) juvenile capelin. We compare observed TS to predicted values from existing TS-length/weight relationships. Materials and methods We encountered a large, monospecific aggregation of juvenile capelin in Sweet Bay in the southern part of Bonavista Bay, northeast Newfoundland (48 26.4 N 53 42.4 W) on 13 January 2000. The aggregation consisted of a thick layer of capelin dispersed throughout the water column (2070 m water depth). Target strength measurements were made using a Simrad EK500 echosounder with a hull-mounted splitbeam 38 kHz transducer. Pulse length was 1.0 ms. The echosounder was calibrated using a standard tungsten carbide sphere prior to (June 1999) and following (April 2000) this experiment according to recommended procedures (Foote et al., 1987). There were no major changes in calibration between these times (<0.2 dB). Measurements of capelin TS were made during darkness (1900 2030 local time) at a vessel speed of 35 knots over 8 km of acoustic transects in Sweet Bay. Data collection was limited to targets with normalised echo lengths from 0.81.8, gain compensation <4.0 dB, standard phase deviation <2.0 steps, and with corrected TS values > 70 dB (SIMRAD, 1997). Following measurements of TS, species composition and size of capelin were obtained by fishing with a Campelen 1800 shrimp trawl fitted with a fine mesh codend liner towed in mid-water ( 25 m depth) through the aggregation. The catch was 100% juvenile capelin. A random sub-sample of 200 fish was measured (total length from tip of the mandible to the end of the ventral lobe of the tail) and weighed. The number of fish per effective reverberation volume (Nv) was calculated from estimates of volumetric fish density (capelin m 3) and ensonified volume (m3) according to Sawada et al. (1993). The beam width of the transducer was 7 between half power points with an equivalent beam angle, =0.0085 sr. Approximate mean volumetric fish densities were calculated for bins 5 m deep by 30 pings (5080 m) long using a backscattering cross-sectional area equivalent to 61 dB. Only TS values from bins with Nv<0.4 were used in this paper (see below). Mean TS was calculated as the logarithm of arithmetically averaged backscattering cross-section values. Results and discussion Both fish length and TS histograms were unimodal (Figure 1). Mean capelin length was 51 mm. Mean weight of individual capelin was 0.30 g. Juvenile capelin were not aged, but observed lengths were consistent with fish spawned the previous summer (0+). Peak larval emergence on Bellevue Beach in adjacent Trinity Bay was on 22 July 1999 (Nakashima and Slaney, 2000), suggesting these capelin were probably 175 days old. We are not aware of any previous TS experiments wi (...truncated)


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Richard L. O'Driscoll, George A. Rose. In situ acoustic target strength of juvenile capelin, ICES Journal of Marine Science, 2001, pp. 342-345, 58/1, DOI: 10.1006/jmsc.2000.1015