- Email: [email protected]
Hydroacoustic monitoring of fish in sea cages: target strength (TS) measurements on Atlantic salmon (Salmo salar)
Fisheries Research 69 (2004) 205–209
Hydroacoustic monitoring of fish in sea cages: target strength (TS) measurements on Atlantic salmon (Salmo salar) F.R. Knudsena,∗ , J.E. Fosseidengenb , F. Oppedalb , Ø. Karlsenb , E. Onab a
Simrad AS, Department of Fisheries Research, P.O. Box 111, Horten N-3191, Norway b Institute of marine research, P.O. Box 1870, Nordnes, Bergen, Norway
Received 11 November 2002; received in revised form 6 April 2004; accepted 10 May 2004
Abstract The aim of the present study was to establish the relationship between target strength (TS) and body length of Atlantic salmon (Salmo salar) in sea cages. Five size-groups (mean total length 20, 25, 55, 67 and 78 cm, n = 6–17 fish per group) of Atlantic salmon were used in the experiment. The fish were monitored in a sea cage (12 m × 12 m × 20 m) using an echosounder with two split-beam transducers (120 and 200 kHz). The two transducers were mounted side by side in the centre of the cage. Measurements were made with the transducers in the bottom of the cage transmitting upwards (ventral recordings), and with the transducers at the surface transmitting downwards (dorsal recordings). Underwater video was used to observe the tilt angle of the fish. A bi- and tri-modal TS distribution was found in dorsal recordings of the smallest size-groups (20, 25 and 50 cm), but this was not evident for the two largest groups (67 and 78 cm). As a result, the TS-to-length relationship for dorsal recordings was rather poor. However, when the recording was made ventrally, the groups showed a unimodal TS distribution. A good correlation was then found between TS and fish length, both for 120 and 200 kHz. © 2004 Elsevier B.V. All rights reserved. Keywords: Atlantic salmon; Target strength; Sea cages
1. Introduction Crucial to both size and biomass estimation of salmon in sea cages is information about the target strength (TS)-to-length relationship for salmon under ∗ Corresponding author. Tel.: +47 3303 4253; fax: +47 3304 2987. E-mail address: [email protected] (F.R. Knudsen).
0165-7836/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2004.05.008
realistic conditions in a sea cage. Earlier TS measurements on salmon have mainly been concerned with the side-aspect echo strength due to the relevance to fish monitoring in rivers (Dahl and Mathiesen, 1983; Kubecka and Duncan, 1998; Lilja et al., 2000). A TSto-length relationship for salmon in the side aspect has, therefore, been established (Kubecka and Duncan, 1998; Lilja et al., 2000), but is lacking for dorsal and ventral fish aspects.
206
F.R. Knudsen et al. / Fisheries Research 69 (2004) 205–209
Establishment of the correlation between TS and body length of fish in a sea cage, where the fish are close to the transducer, is not without problems. A typical acoustic transducer has a nearfield of several meters. The fish also have an acoustic nearfield (Dawson et al., 2000), and the combined nearfields of the transducer and the fish can be a substantial proportion of the depth of a sea cage. Recordings within a nearfield are not reliable (MacLennan and Simmons, 1992), so there is reason to question the validity of TS recordings made in sea cages. Furthermore, the echosounder is designed to operate under conditions where the target is much smaller than the range from the transducer to the target (MacLennan and Simmons, 1992). This point-source requirement is violated at close range (Mulligan, 2000), as in a sea cage. One problem is determining the correct time varied gain (TVG) function when the fish is large in comparison with the cross-section of the beam. For the same reason, bias is expected in split-beam phase measurements, causing an unknown error in the compensated TS (Mulligan, 2000). The aim of the present study is to determine whether a meaningful correlation between TS and length can be established for Atlantic salmon (Salmo salar) in sea cages where the fish are close to the transducer.
2. Material and method The study took place from January to March 2001 at the Institute of Marine Research, Austevoll Aquaculture Research Station, near Bergen. Five different size-groups of Atlantic salmon (mean total length 20, 25, 55, 67, 78 cm, n = 6–17 fish per group) were used in the experiment. A small number of fish was used to prevent overlapping echoes from multiple targets being falsely accepted as valid single-target echoes (Ona and Røttingen, 1986). The fish were netted from a storage sea cage, anaesthetised, measured for length and transferred into the test sea cage (12 m × 12 m × 20 m). The fish were left for 12 h before the recordings began. The sea temperature during the experiments was 4–5 ◦ C. Feeding was stopped one week prior to the experiment and no food was given during the experiment. A SIMRAD EK60 echosounder with two 7◦ splitbeam transducers (120 and 200 kHz), each with a maximum nearfield of about 3 m, was used in the experi-
ment. Transmitting power was 100 W, pulse length was 0.256 ms and the pulse repetition frequency was 5 s−1 . The threshold for data analysis was set to −60 dB to eliminate noise. Calibration was performed prior to releasing the fish into the test cage and after finishing the experiment, each time using standard copper calibration spheres for the two frequencies (MacLennan and Simmons, 1992). Negligible differences were found in the two calibration results. Raw data were stored by the echosounder for later analysis. The two transducers were mounted side by side on an aluminium plate suspended in a gimbal in the centre of the cage. Measurements were conducted both with the transducers at the bottom of the cage transmitting upwards (ventral fish aspect), and with the transducers at the surface transmitting downwards (dorsal fish aspect). Twenty-four-hour recordings were performed for each position of the transducers. Underwater video recordings were taken throughout the experiment, in particular to observe behaviour, vertical movement and tilt angles of the fish. To eliminate detections from unwanted sources and to separate individual fish, only detections from singlefish tracks were used. A Windows® -based automatic tracking software, Wintracker, developed by the Institute of Marine Research, Bergen, Norway (Ona and Hansen, 1991) was used to define single-fish tracks. The software used the single-fish echo data from the EK60. The parameters used to define a fish track were a minimum of five detections and a maximum of one missing ping/track within the 6–18-m range from the transducer. Recordings made closer than 6 m from the transducer were discarded to avoid the worst near-transducer phenomena except when the effect of range on TS was analysed. The maximum vertical movement between two consecutive detections was 10 cm. Single-fish tracks with information on beamcompensated and uncompensated TS, range, time, ping number and phase angles for each detection, were converted from Wintracker into a standard statistical software package for analysis. Data from the tracks of each size-group were pooled and converted into linear values (acoustic cross-section, σ) to calculate the mean echo strength of a size-group before recalculating the data into mean TS. Linear regression through mean TS of the size-groups, was used to establish the TS-to-length relationship.
F.R. Knudsen et al. / Fisheries Research 69 (2004) 205–209
207
3.2. Fish position, range and fish track TS variability
3. Results 3.1. TS and range For all size-groups recorded both ventrally and dorsally, TS did not change with depth, as indicated by the slope (0 ± 0.2) of the linear regression line through the TS and depth data. 4
Y angle
2
0 -4
-2
0
2
4
-2
-4 X angle
Fig. 1. Plot of X and Y split-beam angles from a 78 cm salmon at 6 m recorded ventrally.
Regardless of fish size and range, the fish moved from one side of the beam towards the other, following a straight track with little variability. The majority of the fish swam quietly and horizontally with little vertical movement. Video recordings confirmed this. Fig. 1 shows the track of a salmon recorded ventrally from the largest size-group (78 cm) at the closest range used (i.e. 6 m) expressed as a time series of X and Y angles from the 7◦ transducer. At 6 m, the diameter of the acoustic-beam cross-section is 74 cm, slightly shorter than the length of the fish. The number of pings in the track is 38, the maximum and minimum TS are −14 and −32 dB, respectively, with a mean TS of −23 dB. A similar within-track variability of TS was found among all size-groups recorded both dorsally and ventrally. 3.3. TS and fish length Fig. 2 shows the TS distribution for the smallest and largest size-groups recorded both dorsally and ventrally on 200 kHz. Similar results were obtained on 120 kHz. Note that the smallest size-group recorded dorsally has a bi- and tri-modal TS distribution, while larger fish recorded have a unimodal TS distribution. We found
Fig. 2. Target strength distribution for the smallest (20 cm) and largest (78 cm) size-group, recorded both dorsally and ventrally. Multi-modal TS distribution is found on the smallest group recorded dorsally, while the TS distribution recorded dorsally on the largest size-group is unimodal. The TS distribution on ventral recordings is unimodal on both small and large fish.
208
F.R. Knudsen et al. / Fisheries Research 69 (2004) 205–209
Fig. 3. Target strength-to-length relationship for 120 and 200 kHz recorded ventrally. The correlation is positive for both frequencies.
that the difference in mean TS between size-groups was insignificant when recorded dorsally. A poor correlation was, therefore, established between fish length and mean TS for the dorsal recordings (not shown). When recording, ventrally all size-groups showed a predominantly unimodal TS distribution. Note that a much higher mean TS was recorded ventrally than was recorded dorsally on the largest size-groups (Fig. 2). The correlation between the mean ventral TS and length was positive, both for 120 and 200 kHz (Fig. 3): 120 kHz: 25.0 log L (cm) − 69. 0 (ventral fish aspect); 200 kHz: 24.7 log L (cm) − 69.3 (ventral fish aspect).
4. Discussion Target strength was apparently not dependent on depth in our experiment. For practical purposes, TS measured from Atlantic salmon in sea cages can, therefore, be converted to size without any depth correction. This is in agreement with other studies showing that the TS can be quite insensitive to moderate pressure changes similar to those experienced in sea cages (Furusawa, 1988; Blaxter, 1979; Mukai and Iida, 1996; Zhao, 1996; Ona, 1990; Ona, 2003). When a fish is close to the transducer, point-source violation is expected (MacLennan and Simmons, 1992) causing corrupted phase-angle measurements with little possibility of detecting fish tracks (Mulligan, 2000). However, fish tracks were isolated even when the fish body lengths were longer than the diameter of the acoustic-beam. This questions the physical length of the fish versus the acoustical length. Since the swimbladder is responsible for more than 90% of the re-
flected acoustic energy from a fish, it might be argued that only the swimbladder should be considered (Foote, 1985; Furusawa, 1988). However, the length of the swimbladder can occupy a substantial proportion of the acoustic-beam cross-section and will unequivocally cause ambiguities in positional estimates. The magnitude of the error in TS compensation is, therefore, not known. Dorsal recordings gave a bi- or a tri-modal TS distribution on the smallest fish groups, while this was not found on larger fish. Williamson and Traynor (1984) demonstrated that bimodal TS distribution is expected for directive scatters of the same size. Zhao (1996) found bi- and multi-modal TS distribution in herring (Clupea harengus) recorded dorsally using the frequencies 18, 38 and 120 kHz in sea cages at similar ranges to those used in our study. Ona (2003) has reported the same from herring in the wild. The multi-modal TS distribution can be explained by the high directivity of a cylindrical swimbladder. When the fish swims through the acoustic-beam, echoes from both the side lobes and the main lobe of the swimbladder will be detected, producing the multimodal TS distribution. Larger fish will have longer cylindrical swimbladders with a narrower main lobe than smaller fish. The probability of detecting echoes from the main lobe will thereby decrease, and the TS distribution from the larger fish is more likely to become unimodal. Ventral recordings resulted in mostly unimodal TS distribution in all size-groups. The most likely explanation for this finding is described by Foote (1985). Based on both modelling and experiments, he found that the backscattering in the ventral aspect of swimbladdered fish is less directional than in the dorsal aspect due to the morphometry of the swimbladder where the ven-
F.R. Knudsen et al. / Fisheries Research 69 (2004) 205–209
tral side is more rounded than the dorsal side; thereby, reducing directivity. If the salmon swimbladder shows little directivity ventrally, the TS distribution detected from free-swimming salmon is likely to be unimodal. This could also explain the higher mean ventral TS on the largest fish which have a higher representation of detections from the main lobe on ventral recordings than they do on dorsal recordings. Higher mean TS on salmon recorded ventrally rather than dorsally is reported by Acker (1977), and higher echo integrator values recorded ventrally rather than dorsally on salmon, is reported by Burczynski et al. (1990). Directivity increases with frequency (Foote, 1985; Clay and Horne, 1994) and a more pronounced multimodality should be expected on 200 kHz than on 120 kHz. However, we found no clear difference between the two frequencies and it might be that a wider range of frequencies is needed to detect the frequency dependence. One implication of the unimodal TS distribution on all size-groups recorded ventrally was the establishment of a positive correlation between TS and salmon length. It should be noted that this correlation is based on recordings where the fish are near the transducers and it might not be valid at longer ranges.
Acknowledgements This work was funded by the Norwegian Research Council. Referees are thanked for valuable comments on the manuscript.
References Acker, W.C., 1977. Acoustic assessment of North Pacific salmon stock. Rapp. P.-v. R´eun. Cons. int. Explor. Mer. 170, 189–195. Blaxter, J.H.S., 1979. The herring swimbladder as a gas reservoir for the acoustico-lateralis system. J. Mar. Biol. Assess. U.K. 59, 1–10.
209
Burczynski, J.J., Johnson, R.L., Kreiberg, H., Kirchner, W.B., 1990. Acoustic estimation of dense aggregations of fish in sea pens. Rapp. P-v. R´eun. Cons. int. Explor. Mer. 189, 54–64. Clay, C.S., Horne, J.K., 1994. Acoustic models of fish: the Atlantic cod (Gadus morhua). J. Acoust. Soc. Am. 96, 1661–1668. Dahl, P., Mathiesen, O.A., 1983. Measurment of fish target strength and associated directivity at high frequencies. J. Acoust. Soc. Am. 73, 1205–1211. Dawson, J.J., Wiggins, D., Degan, D.D., Geiger, H., Hart, D., Adams, B., 2000. Point-source violations: split-beam tracking of fish at close range. Aquat. Living Resour. 13, 291–295. Foote, K.G., 1985. Rather-high-frequency sound scattering by swimbladdered fish. J. Acoust. Soc. Am. 78, 688–699. Furusawa, M., 1988. Prolate spheroidal models for predicting general trends of fish target strength. J. Acoust. Soc. Jpn. 9, 13–24. Kubecka, J., Duncan, A., 1998. Acoustic size vs. real size relationships for common species of riverine fish. Fish Res. 35, 115– 125. Lilja, J., Marjom¨aki, T.J., Riikonen, R., Jurvelius, J., 2000. Sideaspect target strength of Atlantic salmon (Salmo salar), brown trout (Salmo trutta), whitefish (Coregonus lavaretus) and pike (Esox lucius). Aquat. Living Resour. 13, 355–360. MacLennan, D.N., Simmons, E.J., 1992. Fisheries Acoustics. Chapman and Hall, London (p. 325). Mukai, T., Iida, K., 1996. Depth dependence of target strength of live kokanee salmon in accordance with Boyle’s law. ICES J. Mar. Sci. 53, 245–248. Mulligan, T.J., 2000. Shallow water fisheries sonar: a personal view. Aquat. Living Resour. 13, 269–273. Ona, E., 1990. Physiological factors causing natural variations in acoustic target strength of fish. J. Mar. Biol. Assess. U.K. 70, 107–127. Ona, E., 2003. An expanded target strength relationship for herring. ICES J. Mar. Sci. 60, 493–499. Ona, E., Røttingen, I., 1986. Experience using the ES-400, split-beam echosounder, with special reference to the single-fish recognition criterion. ICES C. M. 1986/B, 38. Ona, E., Hansen, D., 1991. Software for target tracking with splitbeam echosounders: users’ manual. Inst. Mar. Res., Bergen, Norway. Williamson, N.J., Traynor, J.J., 1984. In situ target-strength estimation of Pacific whiting (Merluccius productus) using dual-beam transducer. J. Cons. int. Explor. Mer. 41, 285– 292. Zhao, X., 1996. Target strength of herring (Clupea harengus L.) measured by the split-beam method. Thesis, MS, Department of Fisheries and Marine Biology, University of Bergen, Norway.
Hydroacoustic monitoring of fish in sea cages: target strength (TS) measurements on Atlantic salmon (Salmo salar) F.R. Knudsena,∗ , J.E. Fosseidengenb , F. Oppedalb , Ø. Karlsenb , E. Onab a
Simrad AS, Department of Fisheries Research, P.O. Box 111, Horten N-3191, Norway b Institute of marine research, P.O. Box 1870, Nordnes, Bergen, Norway
Received 11 November 2002; received in revised form 6 April 2004; accepted 10 May 2004
Abstract The aim of the present study was to establish the relationship between target strength (TS) and body length of Atlantic salmon (Salmo salar) in sea cages. Five size-groups (mean total length 20, 25, 55, 67 and 78 cm, n = 6–17 fish per group) of Atlantic salmon were used in the experiment. The fish were monitored in a sea cage (12 m × 12 m × 20 m) using an echosounder with two split-beam transducers (120 and 200 kHz). The two transducers were mounted side by side in the centre of the cage. Measurements were made with the transducers in the bottom of the cage transmitting upwards (ventral recordings), and with the transducers at the surface transmitting downwards (dorsal recordings). Underwater video was used to observe the tilt angle of the fish. A bi- and tri-modal TS distribution was found in dorsal recordings of the smallest size-groups (20, 25 and 50 cm), but this was not evident for the two largest groups (67 and 78 cm). As a result, the TS-to-length relationship for dorsal recordings was rather poor. However, when the recording was made ventrally, the groups showed a unimodal TS distribution. A good correlation was then found between TS and fish length, both for 120 and 200 kHz. © 2004 Elsevier B.V. All rights reserved. Keywords: Atlantic salmon; Target strength; Sea cages
1. Introduction Crucial to both size and biomass estimation of salmon in sea cages is information about the target strength (TS)-to-length relationship for salmon under ∗ Corresponding author. Tel.: +47 3303 4253; fax: +47 3304 2987. E-mail address: [email protected] (F.R. Knudsen).
0165-7836/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2004.05.008
realistic conditions in a sea cage. Earlier TS measurements on salmon have mainly been concerned with the side-aspect echo strength due to the relevance to fish monitoring in rivers (Dahl and Mathiesen, 1983; Kubecka and Duncan, 1998; Lilja et al., 2000). A TSto-length relationship for salmon in the side aspect has, therefore, been established (Kubecka and Duncan, 1998; Lilja et al., 2000), but is lacking for dorsal and ventral fish aspects.
206
F.R. Knudsen et al. / Fisheries Research 69 (2004) 205–209
Establishment of the correlation between TS and body length of fish in a sea cage, where the fish are close to the transducer, is not without problems. A typical acoustic transducer has a nearfield of several meters. The fish also have an acoustic nearfield (Dawson et al., 2000), and the combined nearfields of the transducer and the fish can be a substantial proportion of the depth of a sea cage. Recordings within a nearfield are not reliable (MacLennan and Simmons, 1992), so there is reason to question the validity of TS recordings made in sea cages. Furthermore, the echosounder is designed to operate under conditions where the target is much smaller than the range from the transducer to the target (MacLennan and Simmons, 1992). This point-source requirement is violated at close range (Mulligan, 2000), as in a sea cage. One problem is determining the correct time varied gain (TVG) function when the fish is large in comparison with the cross-section of the beam. For the same reason, bias is expected in split-beam phase measurements, causing an unknown error in the compensated TS (Mulligan, 2000). The aim of the present study is to determine whether a meaningful correlation between TS and length can be established for Atlantic salmon (Salmo salar) in sea cages where the fish are close to the transducer.
2. Material and method The study took place from January to March 2001 at the Institute of Marine Research, Austevoll Aquaculture Research Station, near Bergen. Five different size-groups of Atlantic salmon (mean total length 20, 25, 55, 67, 78 cm, n = 6–17 fish per group) were used in the experiment. A small number of fish was used to prevent overlapping echoes from multiple targets being falsely accepted as valid single-target echoes (Ona and Røttingen, 1986). The fish were netted from a storage sea cage, anaesthetised, measured for length and transferred into the test sea cage (12 m × 12 m × 20 m). The fish were left for 12 h before the recordings began. The sea temperature during the experiments was 4–5 ◦ C. Feeding was stopped one week prior to the experiment and no food was given during the experiment. A SIMRAD EK60 echosounder with two 7◦ splitbeam transducers (120 and 200 kHz), each with a maximum nearfield of about 3 m, was used in the experi-
ment. Transmitting power was 100 W, pulse length was 0.256 ms and the pulse repetition frequency was 5 s−1 . The threshold for data analysis was set to −60 dB to eliminate noise. Calibration was performed prior to releasing the fish into the test cage and after finishing the experiment, each time using standard copper calibration spheres for the two frequencies (MacLennan and Simmons, 1992). Negligible differences were found in the two calibration results. Raw data were stored by the echosounder for later analysis. The two transducers were mounted side by side on an aluminium plate suspended in a gimbal in the centre of the cage. Measurements were conducted both with the transducers at the bottom of the cage transmitting upwards (ventral fish aspect), and with the transducers at the surface transmitting downwards (dorsal fish aspect). Twenty-four-hour recordings were performed for each position of the transducers. Underwater video recordings were taken throughout the experiment, in particular to observe behaviour, vertical movement and tilt angles of the fish. To eliminate detections from unwanted sources and to separate individual fish, only detections from singlefish tracks were used. A Windows® -based automatic tracking software, Wintracker, developed by the Institute of Marine Research, Bergen, Norway (Ona and Hansen, 1991) was used to define single-fish tracks. The software used the single-fish echo data from the EK60. The parameters used to define a fish track were a minimum of five detections and a maximum of one missing ping/track within the 6–18-m range from the transducer. Recordings made closer than 6 m from the transducer were discarded to avoid the worst near-transducer phenomena except when the effect of range on TS was analysed. The maximum vertical movement between two consecutive detections was 10 cm. Single-fish tracks with information on beamcompensated and uncompensated TS, range, time, ping number and phase angles for each detection, were converted from Wintracker into a standard statistical software package for analysis. Data from the tracks of each size-group were pooled and converted into linear values (acoustic cross-section, σ) to calculate the mean echo strength of a size-group before recalculating the data into mean TS. Linear regression through mean TS of the size-groups, was used to establish the TS-to-length relationship.
F.R. Knudsen et al. / Fisheries Research 69 (2004) 205–209
207
3.2. Fish position, range and fish track TS variability
3. Results 3.1. TS and range For all size-groups recorded both ventrally and dorsally, TS did not change with depth, as indicated by the slope (0 ± 0.2) of the linear regression line through the TS and depth data. 4
Y angle
2
0 -4
-2
0
2
4
-2
-4 X angle
Fig. 1. Plot of X and Y split-beam angles from a 78 cm salmon at 6 m recorded ventrally.
Regardless of fish size and range, the fish moved from one side of the beam towards the other, following a straight track with little variability. The majority of the fish swam quietly and horizontally with little vertical movement. Video recordings confirmed this. Fig. 1 shows the track of a salmon recorded ventrally from the largest size-group (78 cm) at the closest range used (i.e. 6 m) expressed as a time series of X and Y angles from the 7◦ transducer. At 6 m, the diameter of the acoustic-beam cross-section is 74 cm, slightly shorter than the length of the fish. The number of pings in the track is 38, the maximum and minimum TS are −14 and −32 dB, respectively, with a mean TS of −23 dB. A similar within-track variability of TS was found among all size-groups recorded both dorsally and ventrally. 3.3. TS and fish length Fig. 2 shows the TS distribution for the smallest and largest size-groups recorded both dorsally and ventrally on 200 kHz. Similar results were obtained on 120 kHz. Note that the smallest size-group recorded dorsally has a bi- and tri-modal TS distribution, while larger fish recorded have a unimodal TS distribution. We found
Fig. 2. Target strength distribution for the smallest (20 cm) and largest (78 cm) size-group, recorded both dorsally and ventrally. Multi-modal TS distribution is found on the smallest group recorded dorsally, while the TS distribution recorded dorsally on the largest size-group is unimodal. The TS distribution on ventral recordings is unimodal on both small and large fish.
208
F.R. Knudsen et al. / Fisheries Research 69 (2004) 205–209
Fig. 3. Target strength-to-length relationship for 120 and 200 kHz recorded ventrally. The correlation is positive for both frequencies.
that the difference in mean TS between size-groups was insignificant when recorded dorsally. A poor correlation was, therefore, established between fish length and mean TS for the dorsal recordings (not shown). When recording, ventrally all size-groups showed a predominantly unimodal TS distribution. Note that a much higher mean TS was recorded ventrally than was recorded dorsally on the largest size-groups (Fig. 2). The correlation between the mean ventral TS and length was positive, both for 120 and 200 kHz (Fig. 3): 120 kHz: 25.0 log L (cm) − 69. 0 (ventral fish aspect); 200 kHz: 24.7 log L (cm) − 69.3 (ventral fish aspect).
4. Discussion Target strength was apparently not dependent on depth in our experiment. For practical purposes, TS measured from Atlantic salmon in sea cages can, therefore, be converted to size without any depth correction. This is in agreement with other studies showing that the TS can be quite insensitive to moderate pressure changes similar to those experienced in sea cages (Furusawa, 1988; Blaxter, 1979; Mukai and Iida, 1996; Zhao, 1996; Ona, 1990; Ona, 2003). When a fish is close to the transducer, point-source violation is expected (MacLennan and Simmons, 1992) causing corrupted phase-angle measurements with little possibility of detecting fish tracks (Mulligan, 2000). However, fish tracks were isolated even when the fish body lengths were longer than the diameter of the acoustic-beam. This questions the physical length of the fish versus the acoustical length. Since the swimbladder is responsible for more than 90% of the re-
flected acoustic energy from a fish, it might be argued that only the swimbladder should be considered (Foote, 1985; Furusawa, 1988). However, the length of the swimbladder can occupy a substantial proportion of the acoustic-beam cross-section and will unequivocally cause ambiguities in positional estimates. The magnitude of the error in TS compensation is, therefore, not known. Dorsal recordings gave a bi- or a tri-modal TS distribution on the smallest fish groups, while this was not found on larger fish. Williamson and Traynor (1984) demonstrated that bimodal TS distribution is expected for directive scatters of the same size. Zhao (1996) found bi- and multi-modal TS distribution in herring (Clupea harengus) recorded dorsally using the frequencies 18, 38 and 120 kHz in sea cages at similar ranges to those used in our study. Ona (2003) has reported the same from herring in the wild. The multi-modal TS distribution can be explained by the high directivity of a cylindrical swimbladder. When the fish swims through the acoustic-beam, echoes from both the side lobes and the main lobe of the swimbladder will be detected, producing the multimodal TS distribution. Larger fish will have longer cylindrical swimbladders with a narrower main lobe than smaller fish. The probability of detecting echoes from the main lobe will thereby decrease, and the TS distribution from the larger fish is more likely to become unimodal. Ventral recordings resulted in mostly unimodal TS distribution in all size-groups. The most likely explanation for this finding is described by Foote (1985). Based on both modelling and experiments, he found that the backscattering in the ventral aspect of swimbladdered fish is less directional than in the dorsal aspect due to the morphometry of the swimbladder where the ven-
F.R. Knudsen et al. / Fisheries Research 69 (2004) 205–209
tral side is more rounded than the dorsal side; thereby, reducing directivity. If the salmon swimbladder shows little directivity ventrally, the TS distribution detected from free-swimming salmon is likely to be unimodal. This could also explain the higher mean ventral TS on the largest fish which have a higher representation of detections from the main lobe on ventral recordings than they do on dorsal recordings. Higher mean TS on salmon recorded ventrally rather than dorsally is reported by Acker (1977), and higher echo integrator values recorded ventrally rather than dorsally on salmon, is reported by Burczynski et al. (1990). Directivity increases with frequency (Foote, 1985; Clay and Horne, 1994) and a more pronounced multimodality should be expected on 200 kHz than on 120 kHz. However, we found no clear difference between the two frequencies and it might be that a wider range of frequencies is needed to detect the frequency dependence. One implication of the unimodal TS distribution on all size-groups recorded ventrally was the establishment of a positive correlation between TS and salmon length. It should be noted that this correlation is based on recordings where the fish are near the transducers and it might not be valid at longer ranges.
Acknowledgements This work was funded by the Norwegian Research Council. Referees are thanked for valuable comments on the manuscript.
References Acker, W.C., 1977. Acoustic assessment of North Pacific salmon stock. Rapp. P.-v. R´eun. Cons. int. Explor. Mer. 170, 189–195. Blaxter, J.H.S., 1979. The herring swimbladder as a gas reservoir for the acoustico-lateralis system. J. Mar. Biol. Assess. U.K. 59, 1–10.
209
Burczynski, J.J., Johnson, R.L., Kreiberg, H., Kirchner, W.B., 1990. Acoustic estimation of dense aggregations of fish in sea pens. Rapp. P-v. R´eun. Cons. int. Explor. Mer. 189, 54–64. Clay, C.S., Horne, J.K., 1994. Acoustic models of fish: the Atlantic cod (Gadus morhua). J. Acoust. Soc. Am. 96, 1661–1668. Dahl, P., Mathiesen, O.A., 1983. Measurment of fish target strength and associated directivity at high frequencies. J. Acoust. Soc. Am. 73, 1205–1211. Dawson, J.J., Wiggins, D., Degan, D.D., Geiger, H., Hart, D., Adams, B., 2000. Point-source violations: split-beam tracking of fish at close range. Aquat. Living Resour. 13, 291–295. Foote, K.G., 1985. Rather-high-frequency sound scattering by swimbladdered fish. J. Acoust. Soc. Am. 78, 688–699. Furusawa, M., 1988. Prolate spheroidal models for predicting general trends of fish target strength. J. Acoust. Soc. Jpn. 9, 13–24. Kubecka, J., Duncan, A., 1998. Acoustic size vs. real size relationships for common species of riverine fish. Fish Res. 35, 115– 125. Lilja, J., Marjom¨aki, T.J., Riikonen, R., Jurvelius, J., 2000. Sideaspect target strength of Atlantic salmon (Salmo salar), brown trout (Salmo trutta), whitefish (Coregonus lavaretus) and pike (Esox lucius). Aquat. Living Resour. 13, 355–360. MacLennan, D.N., Simmons, E.J., 1992. Fisheries Acoustics. Chapman and Hall, London (p. 325). Mukai, T., Iida, K., 1996. Depth dependence of target strength of live kokanee salmon in accordance with Boyle’s law. ICES J. Mar. Sci. 53, 245–248. Mulligan, T.J., 2000. Shallow water fisheries sonar: a personal view. Aquat. Living Resour. 13, 269–273. Ona, E., 1990. Physiological factors causing natural variations in acoustic target strength of fish. J. Mar. Biol. Assess. U.K. 70, 107–127. Ona, E., 2003. An expanded target strength relationship for herring. ICES J. Mar. Sci. 60, 493–499. Ona, E., Røttingen, I., 1986. Experience using the ES-400, split-beam echosounder, with special reference to the single-fish recognition criterion. ICES C. M. 1986/B, 38. Ona, E., Hansen, D., 1991. Software for target tracking with splitbeam echosounders: users’ manual. Inst. Mar. Res., Bergen, Norway. Williamson, N.J., Traynor, J.J., 1984. In situ target-strength estimation of Pacific whiting (Merluccius productus) using dual-beam transducer. J. Cons. int. Explor. Mer. 41, 285– 292. Zhao, X., 1996. Target strength of herring (Clupea harengus L.) measured by the split-beam method. Thesis, MS, Department of Fisheries and Marine Biology, University of Bergen, Norway.