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Reactions of vendace (Coregonus albula, Linnaeus 1758) towards an approaching pelagic pair-trawl observed by split-beam echosounding
Fisheries Research 96 (2009) 95–101
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Reactions of vendace (Coregonus albula, Linnaeus 1758) towards an approaching pelagic pair-trawl observed by split-beam echosounding Marc Bodo Schmidt ∗ Fisheries Association of Northrhine-Westphalia, Nevinghoff 40, D-48147 Muenster, Germany
a r t i c l e
i n f o
Keywords: Vendace Pair-trawling Reservoir Escapement behaviour Target strength Swimming speed
a b s t r a c t The reactions of vendace (Coregonus albula L.) shoals and single fish to an approaching pelagic pair-trawl deployed for mass removal purposes were studied by stationary hydroacoustic data acquisition and target tracking in the mesotrophic Bigge Reservoir (Germany) during November 2005. A Simrad EY 500 splitbeam echosounder with a frequency of 120 kHz was used to observe fish in the mouth of the pelagic trawl. During six hauls with a duration of 20 min each, 61 single fish were detected and had a mean swimming speed between 0.7 and 2.4 m s−1 (mean = 1.5, S.D. = 0.4) which was comparable to the observed gear speed (1.1 m s−1 ). Calculated aspect-angles (fish tilt to the transducer axis) ranged between 74◦ and 102◦ (mean = 91.5, S.D. = 4.4) indicating that most of these fish showed horizontal swimming behaviour. The mean target strength (TS) of tracked fish varied between −54.2 and −32.9 dB and corresponded well to the length composition of vendace caught by trawling, as well as to a recently obtained species-specific TS–length relationship. Trawl catches comprised only vendace and varied between 10 kg (n = 350) and 60 kg (n = 2098). Whenever the trawl entered dense vendace shoals, single-echo detection and tracking analysis failed due to multiple-echo detections and single fish could not be distinguished. Based on the echograms the avoidance reactions of the vendace shoals were generally weak; the moment when trawlers stopped to haul the net was found to be the most critical point in the trawling process. Large numbers of fish were able to leave the gear at this stage and thus escaped capture. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Scientific echosounders and sonar systems have been used to obtain in situ information about fish and their behaviour during trawling in several studies (e.g. Suuronen, 1988; Ona and Godø, 1990; Misund and Aglen, 1992; Godø and Totland, 1996; Luyeye et al., 1997; Suuronen et al., 1997; Handegaard et al., 2003). Almost all investigations in this field are based on the assumption that fish behaviour significantly influences their capture and so more information on such interactions may lead to enhanced knowledge about fish affected by trawls, making gears more precise or increasing catch rates (Wardle, 1986). As cited above, most recent literature on fish behaviour observed in situ by acoustic methods during trawling is related to marine fish stocks exploited by commercial fisheries (e.g. herring (Clupea harengus L.), sprat (Sprattus sprattus L.), haddock (Melanogrammus aeglefinus L.) and cod (Gadus moruha L.)). Nevertheless, trawling is also commonly used in inland fisheries,
∗ Tel.: +49 251 56618; fax: +49 251 42831. E-mail address: [email protected]. 0165-7836/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2008.09.006
e.g. in terms of removal (Hamrin, 1999), and especially in commercial vendace (Coregonus albula L.) fisheries (Jurvelius, 1991; Muje and Lahti, 1992; Auvinen and Jurvelius, 1994; Marjomäki and Huolila, 2001). Most studies on vendace trawling and combined hydroacoustics have focused on stock density estimates, temporal and spatial distributions of fish, and population dynamics. Only one study has specifically addressed the escapement of vendace from trawl cod-ends and their physiological status (Suuronen et al., 1995). In situ observations of vendace and other freshwater fish by hydroacoustics during trawl surveys have been investigated to a much lesser extent than those of marine fish species. In Germany, observations on trawling in inland waters mainly originate from studies carried out in the 1970s (e.g. Steinberg and Dahm, 1972, 1978). Some of these results were summarised by Steinberg (1982) and included perspectives for the efficient application of bottom and pelagic trawls in German inland waters. Based on these basic experiences, pelagic pair-trawling on two vendacedominated reservoirs in Northrhine-Westphalia (Bigge and Henne Reservoir) has been carried out since the middle of the 1990s (Kühlmann, 1997) for fish removal and water-quality management (Schmidt et al., 2005). An initial observation of the short-term effects of pelagic trawling on the distribution and behaviour of
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M.B. Schmidt / Fisheries Research 96 (2009) 95–101
these vendace populations using scientific echosounders was conducted by Schmidt et al. (2007). It had previously been shown elsewhere that stationary hydroacoustic data acquisition performed from a stationary boat (Suuronen, 1988; Suuronen et al., 1997) or independent acoustic systems (Godø and Totland, 1996; Handegaard et al., 2003; Handegard and Tjøstheim, 2005) can produce detailed information about the behaviour and movement of fish, at least in the front parts of trawl nets passing below an echosounder. In relation to numerous trawl investigations in the marine environment, the aims of this study were to prove (i) that reactions of vendace shoals and individual fish towards approaching trawl gear can be obtained by stationary hydroacoustics even in a large freshwater reservoir and (ii) that related general (i.e. trawl performance based on echograms) and specific (i.e. swimming speed and aspect-angle based on tracking) information can be used by fisheries scientists to enhance knowledge about fish behaviour during trawl sampling.
2. Materials and methods 2.1. Study area The mesotrophic Bigge Reservoir is part of the Bigge and Lister reservoir system and is situated in the south-eastern part of Northrhine-Westphalia, Germany, at 7◦ 53 E and 51◦ 06 N and an altitude of 308 m (Fig. 1A). The reservoir has a maximum surface area of 7 km2 and a catchment area of nearly 287 km2 , dominated by forestry and agriculture. The mean depth is 20 m and the maximum depth is 49.5 m. It has a dimictic circulation and is thermally stratified during summer. From 1995 to 2003, the mean phosphorus and NH4+ concentrations were 18 g L−1 and 2.61 mg L−1 respectively. The mean chlorophyll-a concentration for this period was 4.9 g L−1 . The pelagic fish stock of Bigge Reservoir is dominated by a strong self-reproducing population of vendace successfully introduced at the end of the 1960s (Tack, 1972). Other species are eel (Anguilla
Fig. 1. Map of Bigge Reservoir with trawl area and site in Europe (A), and schematics (plan view) of the experimental setup used to study reactions of vendace in relation to the pelagic trawl gear: boat survey (B) and data acquisition (C).
M.B. Schmidt / Fisheries Research 96 (2009) 95–101
anguilla L.), roach (Rutilus rutilus L.), bream (Abramis brama L.), tench (Tinca tinca L.), carp (Cyprinus carpio L.), brown trout (Salmo trutta f. lacustris L.), pike (Esox lucius L.), perch (Perca fluviatilis L.) and pikeperch (Sander lucioperca L.). The vendace stock is controlled and reduced by a trawl fishery during their spawning period in November and December each year (Kühlmann, 1997; Schmidt et al., 2007). 2.2. Data sampling and analysis Trawling and hydroacoustics were conducted between 5 and 22 November 2005 during daytime in the dam area of Bigge Reservoir. One trawl was carried out each day on 5, 6, 9, 10, 14 and 22 November in calm weather. The pelagic trawl net was towed by two aluminium boats (length 6.6 m, width 1.9 m) with 19 hp diesel inboard engines. The polyethylene net had a theoretical opening of 10 m width and 8 m height and a total length of 38 m; a 12-mm nylon cod-end (nominal bar length as stated by the net manufacturer, 1-mm diameter twine) with a funnel was used. A detailed description of the trawl system is given by Kühlmann (1997) and Schmidt et al. (2007). The net was towed at water depths where vendace showed a layered distribution or were shoaling (around 30 m) over an effective distance of 1.2 km in a south-western direction in the dam area; one haul took about 20 min. The two trawlers were connected by a rope to optimise the opening of the net during the tow and the boat speed was maintained at 4 km h−1 (i.e. 1.1 m s−1 ). All fish caught by trawling were identified to species level, and fish taken from three sub-samples (vendace, n = 313) were weighed (±1 g) and measured (total length, ±0.5 cm). The boat used for hydroacoustic data sampling (length 6.5 m, width 1.8 m, 15 hp four-stroke outboard engine) was equipped with a Simrad EY 500 split-beam echosounder (120 kHz) mounted from the bow at 0.4 m water depth. The nominal beam angle of the elliptical transducer was 4 × 10◦ at the 3-dB-down level (smaller angle orientated alongship). Pulse duration was 0.1 ms and pulse rate was set to 0.2, i.e. 5 pulses s−1 . The single-echo detector was set
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to accept echoes with a minimum target strength (TS) of −60 dB, and minimum and maximum echo lengths of 0.8 or 1.3 times the transmitted pulse length, respectively. The maximum gain compensation was 3 dB and the max. phase deviation was 0.8◦ . The system was calibrated with a standard copper sphere before the surveys and TS gain was checked regularly. Reliability of the TS distribution in relation to the total length of fish (TL) caught by trawling was assessed using a recently described species-specific formula calculated by Mehner (2006) based on trawl catches of vendace (TS = 25.5 log(TL) − 70.9 dB). A general description of the setup (boat survey and data sampling) is given in Fig. 1B and C. After the trawlers started their run, the echosounder boat moved slowly over the net from behind to check the net performance. Then the boat moved ahead of the net laterally, turned around and stopped equidistantly between two surface buoys marking the outsides of the gear tows. Stationary data sampling was carried out until the net passed the boat completely. The latter procedure was repeated at least three times during each trawl. Data were sampled consistently during the whole survey; an example of the echograms produced is shown in Fig. 2. Hydroacoustic data were analysed using Sonar 5 Pro postprocessing software (Balk and Lindem, 2004) with a time-varied gain (TVG) of 40 log R assuming a point-spread model. Single fish observed in the mouth of the trawl were localised first from the amplitude echograms (AMP). Tracking was then carried out based on single-echo detections (SED). Tracking criteria were set to accept fish tracks with at least five single-echo detections and a maximum gap of 2 pings. Mean off-axis compensated target strength (TSc) of single fish was calculated as the mean of all single-echo TS from a fish track:
mean TSc = TSc = 10 log
1 TSci n n
(1)
i=1
To estimate the mean swimming speed (m s−1 ) of a single fish, the velocities (V) between neighbouring echoes of a track were
Fig. 2. Amplitude-echograms (threshold −60 dB) as revealed from the split-beam echosounder based on the survey design for an evenly layered (A) and clustered (B) distribution of vendace: check of the net performance when the boat is crossing the gear from behind (left), turnaround of the boat (dashed line), and the trawl gear passing the stopped boat and echosounder (note that due to the slow boat speed during the performance check (left) the lengths of the trawl net were similar compared to those produced during stationary data acquisition (right)).
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Fig. 3. Amplitude-echogram (threshold −60 dB) of the trawl gear near the bottom (A), xy-mode position (◦ ) from the acoustic axis of a single fish in the beam (B, 1–6), and related single echoes as revealed from the single-echo detector in the range (z) domain (C, 1–6; see text for further explanation).
summed and divided by the number of summed speeds. 1 Vi n
3. Results
n
Mean (V ) = V =
(2)
i=1
Distances of single echoes from the acoustic axis (x = athwartship and y = alongship components) were calculated by the following equation:
ri =
xi2 + yi2
(3)
where i is the ith echo in the track and r is the horizontal distance from the axis. The aspect-angle (i.e. fish tilt towards the transducer axis) was found by linear regression between all ri and absolute values of zi for each single echo (z = range from the transducer; for further explanation see Balk and Lindem, 2004).
Trawl catches comprised only vendace and varied between 10 kg (n = 350) and 60 kg (n = 2098) in weight. Based on sub-samples, the total length of vendace varied between 9.0 and 20.5 cm with a mean of 15.5 cm and an average weight of 28.5 g. In total, 61 single fish were tracked in the mouth of the trawl net; an example is given by Fig. 3. The number of single-echo detections in each track varied between 5 (the minimum criterion) and 14; 26 fish were tracked without ping gaps. Mean TS of single fish varied between −54.2 and −32.9 dB, which corresponds to fish lengths ranging from 4.5 to 30.9 cm (Fig. 4). The measured aspect-angles of tracked fish varied between 74.0◦ and 102.1◦ (mean = 91.5, S.D. = 4.4) and were not significantly different from the dorsal (i.e. horizontal) aspect of 90◦ (one-sample t-test, 90% confidence interval of the difference, 60 degrees of freedom).
Fig. 4. Target strength (TS) of tracked single fish in relation to modelled total length (TL) of fish based on the regression of Mehner (2006), and total-length distribution of vendace caught by trawling in Bigge Reservoir in November and December 2005.
M.B. Schmidt / Fisheries Research 96 (2009) 95–101
The calculated swimming speeds of tracked fish ranged between 0.7 and 2.4 m s−1 (mean = 1.5 m s−1 , S.D. = 0.4) and were generally comparable to the observed gear speed (1.1 m s−1 ); the difference was not significant (one-sample t-test, 90% confidence interval of the difference, 60 degrees of freedom). Whenever a larger shoal of fish was encountered by the net, the tracking analysis failed due to high levels of multiple-echo detections, i.e. single fish could not be distinguished (Fig. 5A). From the echograms it was also obvious that shoaling fish showed a generally weak avoidance reaction (see Fig. 5B) and no escapement of small-sized fish from the cod-end could be observed.
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In comparison to the visually observed number of fish shoals entered by the trawl, the rather low catch results were surprising. Hydroacoustic data sampling at the moment when trawlers stopped to haul the net showed the most critical point in the trawling process: as revealed from the echograms the weighted footrope of the net dropped down to the reservoir bottom while the floats of the headline moved up in the water column; in consequence the gear opened to the full height (Fig. 5C). The echogram showed clearly that fish inside the net, excepting those in the cod-end, were able to leave the gear at this stage.
Fig. 5. Amplitude-echograms (threshold −60 dB) showing crucial aspects during the trawling process: the trawl gear reaching a dense shoal of fish (A), weak reaction of fish in relation to the approaching trawl net near the footrope (B) and fish swimming out of the trawl net at the moment when the trawler stops and begins to haul the gear (C, see text for further explanation).
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4. Discussion Based on the length distribution of vendace in trawl catches and the calculated length from TS measurements, 33 single fish could be characterised as ‘vendace-sized’. It is known that different orientation of fish towards the transducer (e.g. triggered by fish behaviour) affects TS measurements (Torgersen and Kaartvedt, 2001). In relation to other body parameters, Frouzova et al. (2005) found the aspect-angle to be most important. Based on measurements carˇ ried out in the vertical plane only (as done here), Cech and Kubeˇcka (2002) found that the TS of individual fish changed maximally by about 13 dB. Given the mean length of vendace caught (15.5 cm) and their modelled TS based on the regression of Mehner (2006), target strengths between −54 and −28 dB were indicated. Although the possibility cannot be excluded that species other than vendace were also tracked (especially predators like pike, pikeperch, brown trout, and eel), the range of measured TS is in agreement with the length distribution of the catches. However, the present trawl catches in Bigge Reservoir contained only vendace and for this trawling system a by-catch of less than 0.1% has been observed during more extensive sampling (Schmidt et al., 2007). Swimming speeds of coregonids monitored by hydroacoustics under natural conditions have been reported by several recent studies (e.g. Jurvelius et al., 2000; Gjelland et al., 2004; Jurvelius and Marjomäki, 2004) to range between 0.08 and 0.5 m s−1 . These studies considered fish in ‘normal’ swimming with no influence from an approaching fishing gear. Based on a review of swimming performance, Wolter and Arlinghaus (2003) calculated critical speeds (Ucrit = 0.019 TL0.75 , r2 = 0.69, TL in mm) and burst speeds (Uburst = 0.028 TL0.79 , r2 = 0.77) for several freshwater fish families and found burst speeds up to 2 m s−1 for 20 cm long fish to be common. On the basis of these regressions and the length distribution of vendace caught by trawling in Bigge Reservoir, critical speeds between 0.6 and 1.0 m s−1 and burst speeds ranging from 1.0 to 1.9 m s−1 could be calculated. Although data sampling was carried out in absolutely calm weather, possible errors in the tracking process caused by marginal transducer movements (Handegaard et al., 2003) or the location and orientation of fish in the acoustic beam have to be considered (see e.g. Arrhenius et al., 2000; Mulligan and Chen, 2000). Any influence on fish and their swimming behaviour caused by the acoustic sampling boat can be excluded. Draˇstík and Kubeˇcka (2005) found that distances between small acousticsurvey boats and observed fish greater than 10 m produced no appreciable problems. A detailed description of fish and their behaviour during trawls for some marine species was given by Wardle (1986). With increasing tow duration, fish inside the gear get exhausted from swimming at the trawl speed and swim or drop back into the cod-end (‘catching by exhaustion’, see Wardle, 1986; Godø et al., 1990). Due to the relatively short tow duration in Bigge Reservoir (about 20 min), it can be assumed that most of the vendace entering the trawl were able to escape from the gear by fast swimming (e.g. through the larger mesh panels in the front part as shown by Suuronen, 1988), or maintained swimming with the trawl. The latter assumption was confirmed by the echograms recorded at the moment when the trawlers stopped (see Fig. 5C); most of the fish inside the gear were able to swim out of the net at this moment. Therefore, sustained exhaustion seems to be a negligible factor in the catching process, at least for a large part of the vendace size classes caught by trawling in Bigge Reservoir. Escapement of small pelagic fish from trawl nets due to decreasing towing speeds while hauling the gear was also found by Luyeye et al. (1997). A trawling technology that allows hauling the gear without stopping the boats (commonly used in many trawl fisheries) could overcome this problem, but due to technical constraints on the present trawling technique it was not
possible to start hauling before the trawlers stopped in this study. Although trawl catches up to 180 kg have been reported using the same technique in Bigge Reservoir (Schmidt et al., 2007), it can be assumed that this limitation is a determining factor for trawling success. Generally, the towing speed of 1.1 m s−1 is within the range normally used for vendace trawling (Suuronen et al., 1995), and increasing towing speed does not inevitably lead to larger catches (Hamrin, 1999). However, increasing towing speed for a short time (e.g. 5 min) just before the end of a haul may enhance the probability of getting more fish into the cod-end. An additional turnaround by the pair-trawlers at the end of the towing distance to increase tow duration seems to be an appropriate technique but is limited by bottom and shore structures of the canyon-shaped reservoir basin (Kühlmann, pers. commun.). It has to be considered that all hauls were carried out during daytime and that twilight or night trawling, as well as trawling in a different season, might produce different results (see e.g. Suuronen et al., 1997). Learning by the vendace with regard to their escape behaviour could not be observed and Schmidt et al. (2007) showed that short-term effects of trawling on vendace abundance and distribution in Bigge Reservoir could only be observed for maximally 24 h after one haul. In conclusion, the results show that the information generated by the approach adopted in this study is appropriate to enhance scientific knowledge about general and individual fish behaviour and gear performance through trawl sampling of pelagic fish in freshwater ecosystems. Such information can also be used to improve equipment, working practices, and trawling success in fishing operations. Acknowledgements I thank the fishermen of the Ruhrverband Markus Kühlmann, Lars Brackwehr, Florian Vesper and Jan Schneider for their professional help and support during field work. This study was financed by the Fisheries Associations of Northrhine-Westphalia e.V. and Westphalia and Lippe e.V., Münster, and the Ruhrverband, Essen. I also thank two anonymous reviewers for constructive and helpful comments on an earlier draft of the manuscript. References Arrhenius, F., Benneheij, B.J.A.M., Rudstam, L.G., Boisclair, D., 2000. Can stationary bottom split-beam hydroacoustics be used to measure fish swimming speed in situ? Fish. Res. 45, 31–41. Auvinen, H., Jurvelius, J., 1994. Comparison of pelagic vendace (Coregonus albula) stock density estimation methods in a lake. Fish. Res. 19, 31–50. Balk, H., Lindem, T., 2004. Sonar4 and Sonar5-pro. Post processing system. Operator Manual. Lindem Data Acquisition (Norway). V5.9.3, 326 pp. ˇ Cech, M., Kubeˇcka, J., 2002. Sinusoidal cycling swimming pattern of reservoir fishes. J. Fish Biol. 61, 456–471. Draˇstík, D., Kubeˇcka, J., 2005. Fish avoidance of acoustic survey boats in shallow waters. Fish. Res. 72, 219–228. Frouzova, J., Kubecka, J., Balk, H., Frouz, J., 2005. Target strength of some European fish species and its dependence on fish body parameters. Fish. Res. 75, 86–96. Gjelland, K.Ø., Bøhn, T., Knudsen, F.R., Amundsen, P.-A., 2004. Influence of light on the swimming speed of coregonids in subarctic lakes. Ann. Zool. Fenn. 41, 137–146. Godø, O.R., Totland, A., 1996. A stationary acoustic system for monitoring undisturbed and vessel affected fish behaviour. ICES CM 1996/B: 12. Godø, O.R., Pennington, M., Vølstad, J.H., 1990. Effects of tow duration on length composition of trawl catches. Fish. Res. 9, 165–179. Hamrin, S.F., 1999. Planning and execution of the fish reduction in Lake Ringsjön. Hydrobiologia 404, 59–63. Handegaard, N.O., Michalsen, K., Tjøstheim, D., 2003. Avoidance behaviour in cod (Gadus moruha) to a bottom-trawling vessel. Aquat. Living Resour. 16, 265–270. Handegard, N.O., Tjøstheim, D., 2005. When fish meet a trawling vessel: examining the behaviour of gadoids using a free-floating buoy and acoustic split-beam tracking. Can. J. Fish. Aquat. Sci. 62, 2409–2422. Jurvelius, J., 1991. Distribution and density of pelagic fish stocks, especially vendace (Coregonus albula (L.)), monitored by hydroacoustics in shallow and deep southern boreal lakes. Finn. Fish. Res. 12, 45–63.
M.B. Schmidt / Fisheries Research 96 (2009) 95–101 Jurvelius, J., Lilja, J., Hirvonen, E., Riikonen, R., Marjomäki, T.J., 2000. Under ice density and mobility of fish in winter-seining area of two Finnish lakes as revealed by echo-survey. Aquat. Living Resour. 13, 403–408. Jurvelius, J., Marjomäki, T.J., 2004. Vertical distribution and swimming speed of pelagic fishes in winter and summer monitored in situ by acoustic target tracking. Boreal Environ. Res. 9, 277–284. Kühlmann, M., 1997. Einsatz eines Zwei-Schiff-Schwimmschleppnetzes zum Maränenfang auf den Talsperren des Ruhrverbandes. Fischer und Teichwirt 5, 220–222. Luyeye, N., Coetzee, J., Boyer, D., Misund, O.A., 1997. Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day. ICES CM 1997/W: 12. Marjomäki, T.J., Huolila, M., 2001. Long-term dynamics of pelagic fish density and vendace (Coregonus albula (L.)) stock in four zones of a lake differing in trawling intensity. Ecol. Freshw. Fish 10, 65–74. Mehner, T., 2006. Prediction of hydroacoustic target strength of vendace (Coregonus albula) from concurrent trawl catches. Fish. Res. 79, 162–169. Misund, O.A., Aglen, A., 1992. Swimming behaviour of fish schools in the North Sea during acoustic surveying and pelagic trawl sampling. ICES J. Mar. Sci. 49, 325–334. Muje, P., Lahti, E., 1992. The trawl fishery for vendace (Coregonus albula L.) in Lake South Kallavesi, Central Finland: a case study. Pol. Arch. Hydrobiol. 39, 873–878. Mulligan, T.J., Chen, D.G., 2000. Comment on ‘Can stationary bottom split-beam hydroacoustics be used to measure fish swimming speed in situ?’ by Arrhenius et al. Fish. Res. 49, 93–96. Ona, E., Godø, O.R., 1990. Fish reaction to trawling noise: the significance for trawl sampling. Rapp. P.-v. Réun. Cons. Int. Explor. Mer. 189, 159–166. Schmidt, M.B., Gassner, H., Meyer, E.I., 2005. Distribution and total biomass of a vendace, Coregonus albula L., population in a mesotrophic German reservoir. Fish. Manage. Ecol. 12, 169–175.
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Reactions of vendace (Coregonus albula, Linnaeus 1758) towards an approaching pelagic pair-trawl observed by split-beam echosounding Marc Bodo Schmidt ∗ Fisheries Association of Northrhine-Westphalia, Nevinghoff 40, D-48147 Muenster, Germany
a r t i c l e
i n f o
Keywords: Vendace Pair-trawling Reservoir Escapement behaviour Target strength Swimming speed
a b s t r a c t The reactions of vendace (Coregonus albula L.) shoals and single fish to an approaching pelagic pair-trawl deployed for mass removal purposes were studied by stationary hydroacoustic data acquisition and target tracking in the mesotrophic Bigge Reservoir (Germany) during November 2005. A Simrad EY 500 splitbeam echosounder with a frequency of 120 kHz was used to observe fish in the mouth of the pelagic trawl. During six hauls with a duration of 20 min each, 61 single fish were detected and had a mean swimming speed between 0.7 and 2.4 m s−1 (mean = 1.5, S.D. = 0.4) which was comparable to the observed gear speed (1.1 m s−1 ). Calculated aspect-angles (fish tilt to the transducer axis) ranged between 74◦ and 102◦ (mean = 91.5, S.D. = 4.4) indicating that most of these fish showed horizontal swimming behaviour. The mean target strength (TS) of tracked fish varied between −54.2 and −32.9 dB and corresponded well to the length composition of vendace caught by trawling, as well as to a recently obtained species-specific TS–length relationship. Trawl catches comprised only vendace and varied between 10 kg (n = 350) and 60 kg (n = 2098). Whenever the trawl entered dense vendace shoals, single-echo detection and tracking analysis failed due to multiple-echo detections and single fish could not be distinguished. Based on the echograms the avoidance reactions of the vendace shoals were generally weak; the moment when trawlers stopped to haul the net was found to be the most critical point in the trawling process. Large numbers of fish were able to leave the gear at this stage and thus escaped capture. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Scientific echosounders and sonar systems have been used to obtain in situ information about fish and their behaviour during trawling in several studies (e.g. Suuronen, 1988; Ona and Godø, 1990; Misund and Aglen, 1992; Godø and Totland, 1996; Luyeye et al., 1997; Suuronen et al., 1997; Handegaard et al., 2003). Almost all investigations in this field are based on the assumption that fish behaviour significantly influences their capture and so more information on such interactions may lead to enhanced knowledge about fish affected by trawls, making gears more precise or increasing catch rates (Wardle, 1986). As cited above, most recent literature on fish behaviour observed in situ by acoustic methods during trawling is related to marine fish stocks exploited by commercial fisheries (e.g. herring (Clupea harengus L.), sprat (Sprattus sprattus L.), haddock (Melanogrammus aeglefinus L.) and cod (Gadus moruha L.)). Nevertheless, trawling is also commonly used in inland fisheries,
∗ Tel.: +49 251 56618; fax: +49 251 42831. E-mail address: [email protected]. 0165-7836/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2008.09.006
e.g. in terms of removal (Hamrin, 1999), and especially in commercial vendace (Coregonus albula L.) fisheries (Jurvelius, 1991; Muje and Lahti, 1992; Auvinen and Jurvelius, 1994; Marjomäki and Huolila, 2001). Most studies on vendace trawling and combined hydroacoustics have focused on stock density estimates, temporal and spatial distributions of fish, and population dynamics. Only one study has specifically addressed the escapement of vendace from trawl cod-ends and their physiological status (Suuronen et al., 1995). In situ observations of vendace and other freshwater fish by hydroacoustics during trawl surveys have been investigated to a much lesser extent than those of marine fish species. In Germany, observations on trawling in inland waters mainly originate from studies carried out in the 1970s (e.g. Steinberg and Dahm, 1972, 1978). Some of these results were summarised by Steinberg (1982) and included perspectives for the efficient application of bottom and pelagic trawls in German inland waters. Based on these basic experiences, pelagic pair-trawling on two vendacedominated reservoirs in Northrhine-Westphalia (Bigge and Henne Reservoir) has been carried out since the middle of the 1990s (Kühlmann, 1997) for fish removal and water-quality management (Schmidt et al., 2005). An initial observation of the short-term effects of pelagic trawling on the distribution and behaviour of
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these vendace populations using scientific echosounders was conducted by Schmidt et al. (2007). It had previously been shown elsewhere that stationary hydroacoustic data acquisition performed from a stationary boat (Suuronen, 1988; Suuronen et al., 1997) or independent acoustic systems (Godø and Totland, 1996; Handegaard et al., 2003; Handegard and Tjøstheim, 2005) can produce detailed information about the behaviour and movement of fish, at least in the front parts of trawl nets passing below an echosounder. In relation to numerous trawl investigations in the marine environment, the aims of this study were to prove (i) that reactions of vendace shoals and individual fish towards approaching trawl gear can be obtained by stationary hydroacoustics even in a large freshwater reservoir and (ii) that related general (i.e. trawl performance based on echograms) and specific (i.e. swimming speed and aspect-angle based on tracking) information can be used by fisheries scientists to enhance knowledge about fish behaviour during trawl sampling.
2. Materials and methods 2.1. Study area The mesotrophic Bigge Reservoir is part of the Bigge and Lister reservoir system and is situated in the south-eastern part of Northrhine-Westphalia, Germany, at 7◦ 53 E and 51◦ 06 N and an altitude of 308 m (Fig. 1A). The reservoir has a maximum surface area of 7 km2 and a catchment area of nearly 287 km2 , dominated by forestry and agriculture. The mean depth is 20 m and the maximum depth is 49.5 m. It has a dimictic circulation and is thermally stratified during summer. From 1995 to 2003, the mean phosphorus and NH4+ concentrations were 18 g L−1 and 2.61 mg L−1 respectively. The mean chlorophyll-a concentration for this period was 4.9 g L−1 . The pelagic fish stock of Bigge Reservoir is dominated by a strong self-reproducing population of vendace successfully introduced at the end of the 1960s (Tack, 1972). Other species are eel (Anguilla
Fig. 1. Map of Bigge Reservoir with trawl area and site in Europe (A), and schematics (plan view) of the experimental setup used to study reactions of vendace in relation to the pelagic trawl gear: boat survey (B) and data acquisition (C).
M.B. Schmidt / Fisheries Research 96 (2009) 95–101
anguilla L.), roach (Rutilus rutilus L.), bream (Abramis brama L.), tench (Tinca tinca L.), carp (Cyprinus carpio L.), brown trout (Salmo trutta f. lacustris L.), pike (Esox lucius L.), perch (Perca fluviatilis L.) and pikeperch (Sander lucioperca L.). The vendace stock is controlled and reduced by a trawl fishery during their spawning period in November and December each year (Kühlmann, 1997; Schmidt et al., 2007). 2.2. Data sampling and analysis Trawling and hydroacoustics were conducted between 5 and 22 November 2005 during daytime in the dam area of Bigge Reservoir. One trawl was carried out each day on 5, 6, 9, 10, 14 and 22 November in calm weather. The pelagic trawl net was towed by two aluminium boats (length 6.6 m, width 1.9 m) with 19 hp diesel inboard engines. The polyethylene net had a theoretical opening of 10 m width and 8 m height and a total length of 38 m; a 12-mm nylon cod-end (nominal bar length as stated by the net manufacturer, 1-mm diameter twine) with a funnel was used. A detailed description of the trawl system is given by Kühlmann (1997) and Schmidt et al. (2007). The net was towed at water depths where vendace showed a layered distribution or were shoaling (around 30 m) over an effective distance of 1.2 km in a south-western direction in the dam area; one haul took about 20 min. The two trawlers were connected by a rope to optimise the opening of the net during the tow and the boat speed was maintained at 4 km h−1 (i.e. 1.1 m s−1 ). All fish caught by trawling were identified to species level, and fish taken from three sub-samples (vendace, n = 313) were weighed (±1 g) and measured (total length, ±0.5 cm). The boat used for hydroacoustic data sampling (length 6.5 m, width 1.8 m, 15 hp four-stroke outboard engine) was equipped with a Simrad EY 500 split-beam echosounder (120 kHz) mounted from the bow at 0.4 m water depth. The nominal beam angle of the elliptical transducer was 4 × 10◦ at the 3-dB-down level (smaller angle orientated alongship). Pulse duration was 0.1 ms and pulse rate was set to 0.2, i.e. 5 pulses s−1 . The single-echo detector was set
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to accept echoes with a minimum target strength (TS) of −60 dB, and minimum and maximum echo lengths of 0.8 or 1.3 times the transmitted pulse length, respectively. The maximum gain compensation was 3 dB and the max. phase deviation was 0.8◦ . The system was calibrated with a standard copper sphere before the surveys and TS gain was checked regularly. Reliability of the TS distribution in relation to the total length of fish (TL) caught by trawling was assessed using a recently described species-specific formula calculated by Mehner (2006) based on trawl catches of vendace (TS = 25.5 log(TL) − 70.9 dB). A general description of the setup (boat survey and data sampling) is given in Fig. 1B and C. After the trawlers started their run, the echosounder boat moved slowly over the net from behind to check the net performance. Then the boat moved ahead of the net laterally, turned around and stopped equidistantly between two surface buoys marking the outsides of the gear tows. Stationary data sampling was carried out until the net passed the boat completely. The latter procedure was repeated at least three times during each trawl. Data were sampled consistently during the whole survey; an example of the echograms produced is shown in Fig. 2. Hydroacoustic data were analysed using Sonar 5 Pro postprocessing software (Balk and Lindem, 2004) with a time-varied gain (TVG) of 40 log R assuming a point-spread model. Single fish observed in the mouth of the trawl were localised first from the amplitude echograms (AMP). Tracking was then carried out based on single-echo detections (SED). Tracking criteria were set to accept fish tracks with at least five single-echo detections and a maximum gap of 2 pings. Mean off-axis compensated target strength (TSc) of single fish was calculated as the mean of all single-echo TS from a fish track:
mean TSc = TSc = 10 log
1 TSci n n
(1)
i=1
To estimate the mean swimming speed (m s−1 ) of a single fish, the velocities (V) between neighbouring echoes of a track were
Fig. 2. Amplitude-echograms (threshold −60 dB) as revealed from the split-beam echosounder based on the survey design for an evenly layered (A) and clustered (B) distribution of vendace: check of the net performance when the boat is crossing the gear from behind (left), turnaround of the boat (dashed line), and the trawl gear passing the stopped boat and echosounder (note that due to the slow boat speed during the performance check (left) the lengths of the trawl net were similar compared to those produced during stationary data acquisition (right)).
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Fig. 3. Amplitude-echogram (threshold −60 dB) of the trawl gear near the bottom (A), xy-mode position (◦ ) from the acoustic axis of a single fish in the beam (B, 1–6), and related single echoes as revealed from the single-echo detector in the range (z) domain (C, 1–6; see text for further explanation).
summed and divided by the number of summed speeds. 1 Vi n
3. Results
n
Mean (V ) = V =
(2)
i=1
Distances of single echoes from the acoustic axis (x = athwartship and y = alongship components) were calculated by the following equation:
ri =
xi2 + yi2
(3)
where i is the ith echo in the track and r is the horizontal distance from the axis. The aspect-angle (i.e. fish tilt towards the transducer axis) was found by linear regression between all ri and absolute values of zi for each single echo (z = range from the transducer; for further explanation see Balk and Lindem, 2004).
Trawl catches comprised only vendace and varied between 10 kg (n = 350) and 60 kg (n = 2098) in weight. Based on sub-samples, the total length of vendace varied between 9.0 and 20.5 cm with a mean of 15.5 cm and an average weight of 28.5 g. In total, 61 single fish were tracked in the mouth of the trawl net; an example is given by Fig. 3. The number of single-echo detections in each track varied between 5 (the minimum criterion) and 14; 26 fish were tracked without ping gaps. Mean TS of single fish varied between −54.2 and −32.9 dB, which corresponds to fish lengths ranging from 4.5 to 30.9 cm (Fig. 4). The measured aspect-angles of tracked fish varied between 74.0◦ and 102.1◦ (mean = 91.5, S.D. = 4.4) and were not significantly different from the dorsal (i.e. horizontal) aspect of 90◦ (one-sample t-test, 90% confidence interval of the difference, 60 degrees of freedom).
Fig. 4. Target strength (TS) of tracked single fish in relation to modelled total length (TL) of fish based on the regression of Mehner (2006), and total-length distribution of vendace caught by trawling in Bigge Reservoir in November and December 2005.
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The calculated swimming speeds of tracked fish ranged between 0.7 and 2.4 m s−1 (mean = 1.5 m s−1 , S.D. = 0.4) and were generally comparable to the observed gear speed (1.1 m s−1 ); the difference was not significant (one-sample t-test, 90% confidence interval of the difference, 60 degrees of freedom). Whenever a larger shoal of fish was encountered by the net, the tracking analysis failed due to high levels of multiple-echo detections, i.e. single fish could not be distinguished (Fig. 5A). From the echograms it was also obvious that shoaling fish showed a generally weak avoidance reaction (see Fig. 5B) and no escapement of small-sized fish from the cod-end could be observed.
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In comparison to the visually observed number of fish shoals entered by the trawl, the rather low catch results were surprising. Hydroacoustic data sampling at the moment when trawlers stopped to haul the net showed the most critical point in the trawling process: as revealed from the echograms the weighted footrope of the net dropped down to the reservoir bottom while the floats of the headline moved up in the water column; in consequence the gear opened to the full height (Fig. 5C). The echogram showed clearly that fish inside the net, excepting those in the cod-end, were able to leave the gear at this stage.
Fig. 5. Amplitude-echograms (threshold −60 dB) showing crucial aspects during the trawling process: the trawl gear reaching a dense shoal of fish (A), weak reaction of fish in relation to the approaching trawl net near the footrope (B) and fish swimming out of the trawl net at the moment when the trawler stops and begins to haul the gear (C, see text for further explanation).
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4. Discussion Based on the length distribution of vendace in trawl catches and the calculated length from TS measurements, 33 single fish could be characterised as ‘vendace-sized’. It is known that different orientation of fish towards the transducer (e.g. triggered by fish behaviour) affects TS measurements (Torgersen and Kaartvedt, 2001). In relation to other body parameters, Frouzova et al. (2005) found the aspect-angle to be most important. Based on measurements carˇ ried out in the vertical plane only (as done here), Cech and Kubeˇcka (2002) found that the TS of individual fish changed maximally by about 13 dB. Given the mean length of vendace caught (15.5 cm) and their modelled TS based on the regression of Mehner (2006), target strengths between −54 and −28 dB were indicated. Although the possibility cannot be excluded that species other than vendace were also tracked (especially predators like pike, pikeperch, brown trout, and eel), the range of measured TS is in agreement with the length distribution of the catches. However, the present trawl catches in Bigge Reservoir contained only vendace and for this trawling system a by-catch of less than 0.1% has been observed during more extensive sampling (Schmidt et al., 2007). Swimming speeds of coregonids monitored by hydroacoustics under natural conditions have been reported by several recent studies (e.g. Jurvelius et al., 2000; Gjelland et al., 2004; Jurvelius and Marjomäki, 2004) to range between 0.08 and 0.5 m s−1 . These studies considered fish in ‘normal’ swimming with no influence from an approaching fishing gear. Based on a review of swimming performance, Wolter and Arlinghaus (2003) calculated critical speeds (Ucrit = 0.019 TL0.75 , r2 = 0.69, TL in mm) and burst speeds (Uburst = 0.028 TL0.79 , r2 = 0.77) for several freshwater fish families and found burst speeds up to 2 m s−1 for 20 cm long fish to be common. On the basis of these regressions and the length distribution of vendace caught by trawling in Bigge Reservoir, critical speeds between 0.6 and 1.0 m s−1 and burst speeds ranging from 1.0 to 1.9 m s−1 could be calculated. Although data sampling was carried out in absolutely calm weather, possible errors in the tracking process caused by marginal transducer movements (Handegaard et al., 2003) or the location and orientation of fish in the acoustic beam have to be considered (see e.g. Arrhenius et al., 2000; Mulligan and Chen, 2000). Any influence on fish and their swimming behaviour caused by the acoustic sampling boat can be excluded. Draˇstík and Kubeˇcka (2005) found that distances between small acousticsurvey boats and observed fish greater than 10 m produced no appreciable problems. A detailed description of fish and their behaviour during trawls for some marine species was given by Wardle (1986). With increasing tow duration, fish inside the gear get exhausted from swimming at the trawl speed and swim or drop back into the cod-end (‘catching by exhaustion’, see Wardle, 1986; Godø et al., 1990). Due to the relatively short tow duration in Bigge Reservoir (about 20 min), it can be assumed that most of the vendace entering the trawl were able to escape from the gear by fast swimming (e.g. through the larger mesh panels in the front part as shown by Suuronen, 1988), or maintained swimming with the trawl. The latter assumption was confirmed by the echograms recorded at the moment when the trawlers stopped (see Fig. 5C); most of the fish inside the gear were able to swim out of the net at this moment. Therefore, sustained exhaustion seems to be a negligible factor in the catching process, at least for a large part of the vendace size classes caught by trawling in Bigge Reservoir. Escapement of small pelagic fish from trawl nets due to decreasing towing speeds while hauling the gear was also found by Luyeye et al. (1997). A trawling technology that allows hauling the gear without stopping the boats (commonly used in many trawl fisheries) could overcome this problem, but due to technical constraints on the present trawling technique it was not
possible to start hauling before the trawlers stopped in this study. Although trawl catches up to 180 kg have been reported using the same technique in Bigge Reservoir (Schmidt et al., 2007), it can be assumed that this limitation is a determining factor for trawling success. Generally, the towing speed of 1.1 m s−1 is within the range normally used for vendace trawling (Suuronen et al., 1995), and increasing towing speed does not inevitably lead to larger catches (Hamrin, 1999). However, increasing towing speed for a short time (e.g. 5 min) just before the end of a haul may enhance the probability of getting more fish into the cod-end. An additional turnaround by the pair-trawlers at the end of the towing distance to increase tow duration seems to be an appropriate technique but is limited by bottom and shore structures of the canyon-shaped reservoir basin (Kühlmann, pers. commun.). It has to be considered that all hauls were carried out during daytime and that twilight or night trawling, as well as trawling in a different season, might produce different results (see e.g. Suuronen et al., 1997). Learning by the vendace with regard to their escape behaviour could not be observed and Schmidt et al. (2007) showed that short-term effects of trawling on vendace abundance and distribution in Bigge Reservoir could only be observed for maximally 24 h after one haul. In conclusion, the results show that the information generated by the approach adopted in this study is appropriate to enhance scientific knowledge about general and individual fish behaviour and gear performance through trawl sampling of pelagic fish in freshwater ecosystems. Such information can also be used to improve equipment, working practices, and trawling success in fishing operations. Acknowledgements I thank the fishermen of the Ruhrverband Markus Kühlmann, Lars Brackwehr, Florian Vesper and Jan Schneider for their professional help and support during field work. This study was financed by the Fisheries Associations of Northrhine-Westphalia e.V. and Westphalia and Lippe e.V., Münster, and the Ruhrverband, Essen. I also thank two anonymous reviewers for constructive and helpful comments on an earlier draft of the manuscript. References Arrhenius, F., Benneheij, B.J.A.M., Rudstam, L.G., Boisclair, D., 2000. Can stationary bottom split-beam hydroacoustics be used to measure fish swimming speed in situ? Fish. Res. 45, 31–41. Auvinen, H., Jurvelius, J., 1994. Comparison of pelagic vendace (Coregonus albula) stock density estimation methods in a lake. Fish. Res. 19, 31–50. Balk, H., Lindem, T., 2004. Sonar4 and Sonar5-pro. Post processing system. Operator Manual. Lindem Data Acquisition (Norway). V5.9.3, 326 pp. ˇ Cech, M., Kubeˇcka, J., 2002. Sinusoidal cycling swimming pattern of reservoir fishes. J. Fish Biol. 61, 456–471. Draˇstík, D., Kubeˇcka, J., 2005. Fish avoidance of acoustic survey boats in shallow waters. Fish. Res. 72, 219–228. Frouzova, J., Kubecka, J., Balk, H., Frouz, J., 2005. Target strength of some European fish species and its dependence on fish body parameters. Fish. Res. 75, 86–96. Gjelland, K.Ø., Bøhn, T., Knudsen, F.R., Amundsen, P.-A., 2004. Influence of light on the swimming speed of coregonids in subarctic lakes. Ann. Zool. Fenn. 41, 137–146. Godø, O.R., Totland, A., 1996. A stationary acoustic system for monitoring undisturbed and vessel affected fish behaviour. ICES CM 1996/B: 12. Godø, O.R., Pennington, M., Vølstad, J.H., 1990. Effects of tow duration on length composition of trawl catches. Fish. Res. 9, 165–179. Hamrin, S.F., 1999. Planning and execution of the fish reduction in Lake Ringsjön. Hydrobiologia 404, 59–63. Handegaard, N.O., Michalsen, K., Tjøstheim, D., 2003. Avoidance behaviour in cod (Gadus moruha) to a bottom-trawling vessel. Aquat. Living Resour. 16, 265–270. Handegard, N.O., Tjøstheim, D., 2005. When fish meet a trawling vessel: examining the behaviour of gadoids using a free-floating buoy and acoustic split-beam tracking. Can. J. Fish. Aquat. Sci. 62, 2409–2422. Jurvelius, J., 1991. Distribution and density of pelagic fish stocks, especially vendace (Coregonus albula (L.)), monitored by hydroacoustics in shallow and deep southern boreal lakes. Finn. Fish. Res. 12, 45–63.
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