© 2005 International Council for the Exploration of the Sea
The behaviour of spawning herring in relation to a survey vessel
a Department of Biology, University of Bergen Bergen High-Technology Centre, N-5020 Bergen, Norway
b Institute of Marine Research PO Box 1870 Nordnes, N-5817 Bergen, Norway
*Correspondence to G. Skaret: tel: +47 55 236993; fax: +47 55 238555. e-mail: georg.skaret{at}imr.no.
Vessel avoidance of spawning herring (Clupea harengus L.) was studied off the coast of southwestern Norway in April 2000. In eight repeated night-time passages a demersal layer of herring was recorded acoustically by a small stationary reference vessel (96 GRT), while a survey vessel (710 GRT) passed at short ranges (8–40 m). No avoidance attributable to the survey vessel was observed. We interpret vessel avoidance as a response to a perceived threat and herring are known to exhibit strong avoidance reactions to survey vessels during wintering and the spawning migration. At the spawning site, the high priority given to reproductive activities seems to overrule the avoidance responses to a passing survey vessel.
Keywords: vessel avoidance, herring, spawning behaviour, state-dependent trade-off
Received 10 January 2004; accepted 1 May 2005.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Many pelagic fish species initiate avoidance reactions in response to vessel-generated sound (Olsen et al., 1983), and vessel avoidance therefore represents a potentially important bias in acoustic surveys (Misund, 1997). Vessel avoidance is commonly interpreted as a reaction to perceived threats, and can thus be regarded as anti-predator behaviour (Fréon et al., 1993; Vabø et al., 2002), which is state-dependent (Magurran et al., 1985). However, the biological platform for understanding and quantifying variations in vessel avoidance is weak and the existing literature is scattered and inconsistent. In herring (Clupea harengus L.), avoidance reactions are known to vary with the season and physiological state of the fish (Misund, 1994). Strong vessel avoidance is observed in the wintering areas (Vabø et al., 2002) where herring do not feed (Slotte, 1999), prioritizing survival. During the feeding season, however, the focus on feeding results in relaxed predator vigilance, making them easier to catch (Misund, 1994).
Around spawning, the behaviour and risk-reluctance of herring change rapidly with altering physiological and motivational states (Nøttestad et al., 1996; Axelsen et al., 2000). The strong avoidance reactions to vessels and fishing gear typically observed prior to spawning (Mohr, 1971; Olsen et al., 1983; Vabø et al., 2002) are consistent with herring adopting a low-risk behaviour in order to maximize the probability of successful reproduction. The behaviour changes when herring engage in spawning activities for a period of about 3–7 days (Nøttestad et al., 1996; Axelsen et al., 2000), but in situ avoidance experiments in this period have been lacking.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The study was carried out in a shallow (30–40 m) part of the Boknafjord off the southwestern Norwegian coast on 4 April, 2000. Two research vessels were utilized: the small, coastal research vessel "Hans Brattstrøm" (HB, 96 Gross Registered Tonnage, GRT) and the survey vessel "Håkon Mosby" (HM, 710 GRT). The experiment was conducted after dark between 01:45 and 04:30 local time, in calm weather. An area with layers of spawning herring close to the bottom was located by means of sonar and echosounder and selected for the experiments. For each passage, HM started from a position about one nautical mile from HB, passed HB as close as possible (8–40 m distance) (Table 1) at standard survey speed (10–11 knots), and continued for another nautical mile past HB. The ship described a wide, circular course in order to keep an equal distance to the herring layer between each passage. The passages were separated by 20-min intervals to "normalize" the herring.
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Altogether eight successful passage cycles were completed. From the start to the end of each passage HB was positioned above the herring layer with all deck lights switched off, all work on deck suspended, and the propellers disengaged. In this stand-by position, HB radiated about 1/250 of the sound pressure emitted by RV "Michael Sars" (MS), the sister ship of HM, at cruising speed (Figure 1) within the perception range of herring, i.e. 3–8000 Hz (Blaxter and Hunter, 1982). Between passages, HB manoeuvred carefully in order to maintain stand-by position at a steering speed of three knots maximum. All acoustic equipment on HM was switched off in order to avoid acoustic interference. One person on each of the vessels recorded the passage time and the vessel-to-vessel distance as the transducers of the two vessels were aligned. Herring densities were recorded by HB before, during, and after each passage using a 38 kHz, SIMRAD EK 500/ES38B echosounder transmitting at a ping rate of 4 s–1. Herring samples were obtained using two gillnets with mesh sizes of 37 mm (stretched mesh at 5 kg). The height of the nets was about 4 m. The nets were set prior to the experiments at about 22:00 and hauled the following morning at 11:30; one was set at the bottom and one near the surface.
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The acoustic data were stored using the Bergen Echo Integrator (BEI) system (Knudsen, 1990), and analysed using Sonardata Echoview v.2.20 © post-processing software. The mean acoustic-backscattering cross-section, sA PASS (m2 per square nautical mile) and the centre of the depth distribution, dPASS (m) were calculated for each vessel passage from the recordings obtained during a 5-s time interval surrounding the estimated time of passage (Figure 2). The sA-value and centre depth during reference situation, sA REF and dREF, respectively, were averaged over a 5-s interval that ended 25 s before passage. A 25-s separation interval represents a distance of 128 m at 10 knots, and was chosen in order to minimize the influence of HM without introducing errors caused by natural variation in aggregation density over time. Values from three other reference intervals were tested against the passage values; 60–55, 45, and 30 s prior to passage, respectively. The vessel-avoidance coefficient vab and school-depth coefficient vad (Vabø et al., 2002) were calculated as:
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Sign-tests were used to test the null-hypotheses that the acoustic-backscattering cross-section and the centre of the depth distribution did not change from the reference situation to the passage, i.e. vab = 1 and vad = 1.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The herring layer close to the bottom was present throughout the experiment. In total 17 herring were caught, of which 13 were in the demersal gillnet. Altogether 14 herring were running (Gonad Maturity Index, GMI 6, Anon., 1962), two were maturing (GMI 5), and one was spent (GMI 7). Total fish length ranged from 32.5 to 36.0 cm (mean 33.9 cm ± 1.3 s.d.). One stomach contained few copepods, while all the others were empty. The plankton sample consisted almost entirely of Calanus finmarchicus, and contained a total quantity corresponding to a density of 1.9 individuals l–1 in the sea. The temperature ranged from 4.5 to 5.2°C throughout the water column, i.e. within the typical temperature range at herring spawning grounds in this area (Runnstrøm, 1941).
The vab ranged from 0.63 to 1.63 (1.13 ± 0.36) (Table 1). The sA-value increased during passage in four cases and decreased in the other four (Figure 2), with no significant difference between the reference situation and passage (sign-test, p = 0.72). The school-depth coefficient, vad, ranged from 0.99 to 1.02 (1.00 ± 0.01), with no difference between measurements before and during passage (sign-test, p = 0.72). The sA-value and centre-depth distribution did not differ between the passage and the reference situation with any of the other three reference intervals. There was no linear relation between passing distance and vab (linear regression, p = 0.77, r2 = 0.01).
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Herring spawning in layers close to the bottom did not show any sign of reaction towards a survey vessel passing at standard survey speed (10–11 knots) at a distance of 8–40 m. The continuous exposure to noise from HB in our study could have triggered a response in the herring prior to passage by HM and concealed a response to the survey vessel. However, noise-induced avoidance reactions generally weaken with decreasing sound intensity (Schwartz and Greer, 1984; Gerlotto and Fréon, 1992), and the noise emitted by HB in stand-by mode was minimal compared with MS and well below the reference level recommended by ICES (Mitson, 1995). Assuming similar noise levels between HM and its sister ship, the average sound pressure radiated by HB before and during passage was 1/250 of the pressure radiated by HM within the herring perception range, and hence unlikely to trigger any reaction.
The validity of the observations could be questioned on account of the distance between the survey vessel and the reference vessel. Olsen (1979) found that, at a distance of 40–50 m, vessel reaction in herring was only weakly detectable, whereas significant avoidance reactions occurred at all passing distances <30 m. With the passing distances used in our study and the shallow depths involved, a reaction would most likely have been detected. Vertical avoidance near the bottom would have resulted in reduced backscattering attributable to fish entering the acoustic deadzone, and should also have resulted in some increased depth distribution. Lateral avoidance could have resulted in the reduced echo energy during passage caused by the fish swimming away from both survey and reference vessels, but increased echo energy could also the result caused by fish swimming away from the survey vessel and entering the acoustic beam of the HB. No such trends were observed, however, and there was no relation between passing distance and vab to suggest that such effects were present.
Herring generally adopt low-risk behavioural strategies (Fernö et al., 1998; Axelsen et al., 2000), but at times predator avoidance must be balanced with other activities that affect vigilance. In the feeding season, the reaction towards vessels is low compared with the wintering period (Misund, 1994). Similarly, at the spawning site, current reproduction has high priority (Nøttestad et al., 1996; Skaret et al., 2003), and Mohr (1971) observed that ripe herring swimming close to the bottom showed no avoidance reactions to a moving trawl, consistent with high reaction thresholds towards potential predators. The production of gonads and the spawning migration represent considerable energetic investments where the ultimate outcome in terms of successful spawning on a favourable bottom substrate is determined within a period of 3–7 crucial days. With a year's reproductive success at stake, a plausible biological explanation to the lack of vessel response is that herring engaging strongly in spawning activity have higher reaction thresholds to threatening stimuli than are normally found.
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References |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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