© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Vertical distribution and feeding patterns in fish foraging on the krill Meganyctiphanes norvegica
Department of Biology, University of Oslo PO Box 1066, 0316 Oslo, Norway
*Correspondence to S. Kaartvedt: tel: +47 22854739; fax: +47 22854438. e-mail: stein.kaartvedt{at}bio.uio.no.
Fish and krill were studied at a 120 m deep site in the Oslofjord, Norway. Herring (Clupea harengus), whiting (Merlangius merlangus), and Norway pout (Trisopterus esmarkii) were foraging on krill (Euphausiacea, Meganyctiphanes norvegica) during both day and night. During daytime, herring and whiting were foraging in the upper and middle part of the krill assemblage, while the deep-living, and often benthopelagic Norway pout was approaching the krill from below. Krill and fish ascended and fish schools dispersed at dusk. At night, herring and whiting were feeding near the surface, with the shallowest distribution suggested for herring. Norway pout foraged in midwater. Krill antipredator behaviour comprised diel vertical migration and instantaneous escape reactions, and the krill also appeared to actively seek out strata with low acoustic recordings of fish. Fish accumulated beneath the research vessel when the ship was anchored at a fixed location during acoustic studies, apparently resulting in artificially high local fish abundances. Since we suggest that krill respond to the presence of fish, such high fish abundance may bias studies of interactions between the fish predators and their krill prey.
Keywords: antipredator behaviour, fish accumulation, fish and krill distribution, fish–krill interaction
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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The vertical distributions of plankton and fish often reflect a trade-off between optimizing food intake and minimizing risk of predation (Johnsen and Jakobsen, 1987; Lazzaro, 1987; Aksnes and Giske, 1990; Bollens and Frost, 1991; Milinski, 1993; Loose and Dawidowicz, 1994), being further mediated by temperature owing to its regulating effect on metabolic rates (Wurtsbaugh and Neverman, 1988; Giske et al., 1990; Dawidowicz and Loose, 1992). Fish are normally visual predators, and feeding is most efficient in daylight and in shallow water. Small planktivorous fish are themselves subject to predation by piscivores. They may reduce risk of daytime predation by schooling (e.g. Gallego and Heath, 1994; Connell, 2000), or by diel vertical migrations (e.g. Clark and Levy, 1988). Some small planktivores live in close association with the bottom during the day (Albert, 1994; Kaartvedt et al., 1996), migrating upwards into the water column at night (Beamish, 1966; Bailey, 1975; Albert, 1994, 1995).
Vertical gradients in visual encounter rates between predators and prey will be species-specific, partly as a function of eye size (Aksnes and Giske, 1993), but will be absent in darkness at great depths. Models of planktivore foraging typically assume negligible foraging by particulate feeders below their visual foraging thresholds (Giske and Salvanes, 1995). However, many particulate feeders can forage non-visually (Montgomery et al., 1988; Janssen, 1996; Ryer et al., 2002). Laboratory studies suggest that some fish locate their prey by means of the mechanosensory lateral-line system when low light renders vision ineffective (Dijkgraaf, 1963; Janssen et al., 1995; Janssen, 1997; Ryer et al., 2002). Some planktivorous fish species respond to mechanical stimuli imitating prey (Janssen, 1996), and both swimming behaviour (Barham, 1966; Janssen et al., 1992), and lateral-line morphology (Janssen et al., 1995) suggest that they may be partially tactile predators.
Euphausiids, and in the North Atlantic specifically the krill Meganyctiphanes norvegica, are key organisms in food webs, utilizing diverse food sources and being prominent prey organisms for planktivores. Previous studies in the Oslofjord have revealed that M. norvegica is the main constituent of an acoustic scattering layer (SL) that stays below ca. 70 m by day and migrates upwards at night (Onsrud and Kaartvedt, 1998; Kaartvedt et al., 2002). The krill layer typically remains below schooling fish by day (Onsrud and Kaartvedt, 1998; Bagøien et al., 2000), although records of fish schools in the upper fringe of the krill layer have been made (Klevjer, 2001; Kaartvedt et al., 2002). Their vertical distributions overlap at night. Another group of fish seems to be associated with the krill during both day and night, and a third group leaves the benthic boundary zone near sunset, migrating into midwater (Onsrud and Kaartvedt, 1998; Kaartvedt et al., 2002). The identities and the feeding patterns of these fish have not been established.
Knowledge of distribution and behaviour of planktivorous fish is essential for understanding plankton ecology. Likewise, plankton distribution and behaviour are essential driving forces for the distribution and behaviour of fish. Therefore, in a series of studies, we aimed to address both the fish and plankton components of the ecosystem in the Oslofjord, Norway (e.g. Onsrud and Kaartvedt, 1998; Bagøien et al., 2000; Kaartvedt et al., 2002; Klevjer and Kaartvedt, 2003). In the present study, we investigate the distribution and feeding of the fish co-occurring with the krill during the diel cycle. Our aims were to identify the main component of the fish assemblage, establish their vertical distributions, and assess their diel feeding patterns.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The study was conducted at a 120 m deep site in the Oslofjord, Norway, March 1998, March 2000, November 2000, March and June 2001. Additionally, trawl catches from March 1997 were included. Descriptions of the study site are given by Onsrud and Kaartvedt (1998) and Bagøien et al. (2000). The RV "Trygve Braarud" was anchored at a fixed position (59°48'N 10°34'E) during diel acoustic studies. Trawling was conducted in transects adjacent to the anchor station.
Sampling procedures for temperature, salinity, oxygen and light measurements were the same as detailed in Onsrud and Kaartvedt (1998). Chlorophyll a concentrations were analysed in water samples from discrete depths according to the procedures of Strickland and Parsons (1972). Subsurface light extinction was measured in the upper 25 m using a 2
LiCor 190SA quantum sensor.
Mesozooplankton was collected by a WP2 plankton net (Tranter, 1968). The net was hauled vertically at 50 cm s–1, and filtered volumes were estimated by multiplying towing distance with net aperture. Duplicate daytime-series (five depth intervals covering the whole water column) were taken in November 2000, March and June 2001. Previous studies have shown little variation between replicated net tows (Vedal, 1997), and identification and enumeration was therefore made for only one of the series for each survey.
Acoustic studies
SIMRAD EK500 echosounders (software version 5.3) with 38- and 120-kHz split-beam transducers were used for studies of fish and krill. Fish are recorded on both frequencies, while krill are mainly detected at 120 kHz, where they appear as recognizable blue scattering layers (Onsrud and Kaartvedt, 1998; Kaartvedt et al., 2002; Klevjer and Kaartvedt, 2003).
We applied hull-mounted transducers in various combinations with submerged transducers (Table 1). The submerged transducers were used to obtain better resolution of deep-living targets (partly for use in other studies), but also to observe if the abundance of acoustic targets deviated immediately beneath and at some distance away from the ship. For each frequency, only one transducer could be used at a given time. The submerged 120-kHz transducer was used in a downward-looking mode immediately beneath the ship, while the 38 kHz was placed on the sea bottom, facing upwards (at various distances from the ship; Table 1), and was coupled to the EK500 on board "Trygve Braarud" by a 300-m cable. This transducer was mounted to a steel frame with gimbal couplings to ensure horizontal orientation of the transducer surface. Total weight of the transducer with frame was ca. 100 kg.
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Data were stored by the software program Echo Receiver (Mork, 2000) and later processed by the EP500 postprocessing software, version 5.4 (Lindem and Al Houari, 1993) and by the Sonar 5 postprocessing software, version 5.8.7 (Balk and Lindem, 2002). The EK500 was operating at maximum pulse repetition frequency, i.e. ca. 2 s–1 for the 120-m depth range. The vertical resolutions were 10 cm and 3 cm for the 38 and 120 kHz, respectively. Pulse durations were 1 ms (38 kHz) and 0.3 ms (120 kHz). The main engine of the vessel was turned off, and only navigation lights were lit during acoustic studies.
Trawling
Pelagic and bottom trawling were used to identify acoustic targets, establish their size distribution, and provide stomachs for diet investigation. A total of 66 pelagic and 24 bottom hauls were taken.
We used a pelagic trawl with an aperture of about 100 m2, towed at 2 knots. The mesh size was 20 cm near the opening, declining to 1 cm at the rear end. The pelagic trawling depths were selected so as to target the acoustic recordings. Fishing depth was measured by an autonomous mini-STD (SAIV model 202), attached to the trawl. Since the exact depth could first be established after the tow, fishing depth during trawling was estimated as 1/3 of the wire length, a relationship derived empirically from earlier trawling surveys. Effective trawling time (from the time the trawl was at the desired trawling depth until haulback) was generally between 20 and 30 min.
The bottom-trawl aperture was 57 m2, with a mesh size of 2 cm at the rear end. Bottom trawling was performed with a vessel speed of 1.5 knots. Bottom trawling was only performed during daytime.
All fish were identified to species, counted, measured for total length (TL) according to Fotland et al. (2000), weighed, and thereafter frozen at –18°C. For fish with a total length larger than 25 cm, stomachs were removed, weighed, and frozen separately. Stomach fullness was categorized in groups on a scale from 1 to 5 (1 denotes empty, 5 full/distended) according to Fotland et al. (2000). The volume of krill captured in each tow was noted.
Frozen fish/stomachs from the November 2000, March and June 2001 cruises were later thawed and analysed in the laboratory. Prey organisms in the stomachs were identified to the lowest possible taxon. Prey organisms were counted and the wet weight of the stomach content and krill fraction was determined. State of digestion was categorized to groups on a scale from 1 to 5 (1 denotes undigested, 5 fully digested/unrecognizable; Fotland et al., 2000).
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Environmental data
Below
30 m for all surveys, the vertical temperature and salinity profiles were practically homogenous with depth (Figure 1A). Temperature at the surface varied between surveys, with a minimum temperature of 2.2°C in March 1997 and a maximum of 20°C in June 2001. Salinity declined markedly above the sill depth of 19 m, except for March 2001 when there was virtually no salinity gradient. The water was generally well oxygenated, apart for low oxygen content at midwater depth in November (min. 1.5 ml l–1, oxygen saturation 22.8% at 62 m; Figure 1A). The maximum chlorophyll a concentration in the upper 10 m varied between 1.2 and 3 µg l–1 (Figure 1B). The 1% light depth was particularly shallow in November (4 m), and occurred between 12–15 m for the other study periods.
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Copepod distribution
Copepods were usually most abundant in the uppermost 25 m (Figure 1C). Pseudocalanus sp. dominated the upper layer in November, while over-wintering Calanus spp. were the most abundant species at depth at that time. In March and June, juvenile Calanus spp. and Temora spp. dominated the upper stratum. The most abundant cyclopoid species at all depths were Oithona spp. and Oncaea spp.
Distribution of krill and fish
The krill layer was restricted to waters deeper than ca. 70 m during the day, mainly concentrated in a 20–40-m thick layer (Figure 2A, B). The krill ascended at dusk, followed by a subsequent descent shortly after reaching the surface. However, in March 1998 strong recordings of krill were made close to the surface also later at night. These recordings followed a break for trawling, and represent a period when recordings of fish were low in the upper 30 m (cf. upper left, right-hand part of Figure 2A).
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By day, fish schools occurred above the krill layer, particularly evident the second day of the studies (Figure 2A–C). Some schooling fish were also occasionally seen entering the krill layer (e.g. Figure 3). Non-schooling fish (appearing as a scattering layer (SL) in the compressed plots) were distributed together with the krill. Additionally, fish occurred below the krill layer, apparently associated with the bottom in March and living pelagically below about 90 m in November (Figure 2A–C).
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Fish ascended and schools dispersed at dusk (Figure 2). Fish from the pelagic schools and those associated with the krill layer concentrated in the upper
30–40 m. The benthopelagic fish ascending from near the bottom established a scattering layer in midwater, below 40 m. The upper fringe of this midwater layer maintained its vertical distribution throughout the night, before performing a coherent downward migration in the morning (starting approximately 1 h before sunrise). In contrast, on two of three occasions the lower edge of the layer started sinking long before the approach of daylight. This is most conspicuous in the March 1998 echogram (Figure 2A), when the SL descended at a velocity of about 1.5 cm s–1. An apparent dawn rise of the lower edge was recorded in November.
In March 2001, a loosely aggregated and dielly migrating 15-m thick SL was the prevailing acoustic structure, as seen from the bottom-mounted transducer. However, in the morning of the 28 March, when switching from bottom- to hull-mounted transducer, a distinct difference in backscattering from the two ensonified volumes was observed, with much stronger fish records obtained from the hull-mounted transducer (Figure 4). The 38-kHz volume backscattering (Sv) from the layer below 75 m differed by ca. 9 dB, indicating an about eight times higher biomass directly beneath the vessel compared to 80 m away from the vessel.
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Volume backscattering increased with time beneath the ship throughout the diel studies, as evidenced from the records by the hull-mounted transducers. This phenomenon could not be observed in March (or June), when the upward-looking, bottom-mounted transducer was located away from the vessel.
Repeatedly, virtually all fish targets disappeared from the acoustic beam, resulting in "voids" in the recordings (Figure 2). This coincided with ships passing at a distance of 200–400 m.
Trawling
Krill (Meganyctiphanes norvegica), whiting (Merlangius merlangus), Norway pout (Trisopterus esmarkii), herring (Clupea harengus), and sprat (Sprattus sprattus) were the prevailing species in the pelagic trawl catches. Moreover, catches in June were totally dominated by jellyfish (Cyanea capillata and Aurelia aurita), mainly in the size range 5–35 cm.
Daytime catches increased with depth, apart for sprat, which gave the largest daytime catches in midwater (Figure 5; all hauls pooled). At night, herring, whiting, and sprat were most abundant in the upper 50 m, with the shallowest distribution suggested for herring. Norway pout had the deepest nocturnal distribution, with maximum abundance in midwater and no catches in the upper 25 m.
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Sprat, whiting, and Norway pout were also present in the bottom trawls, but the prevailing species in these catches were rockling (Rhinonemus cimbrius) and American plaice (Hippoglossoides platessoides) (Table 2). Shrimps were numerous in March and June 2001. Catches of krill were insignificant in the bottom trawl.
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Feeding
For whiting, krill constituted up to 99% of the stomach content by weight, on average
82% during both day and night (maximum number of krill found in one stomach was 40 krill; fish length 21 cm). Small sprat, a few decapods, amphipods, and polychaetes were also found in the whiting stomachs. No systematic trends were apparent comparing proportion of stomachs containing krill or state of digestion during the day and night (Table 3).
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The diet of Norway pout was dominated by M. norvegica and calanoid copepods, particularly Calanus spp. Also a few decapods and amphipods were found in the Norway pout stomachs. No significant differences in stomach fullness, or normalized stomach wet weight could be found between day/night (chi-square and Kruskal–Wallis tests, p > 0.05).
Herring stomachs contained krill prey during both day and night. The highest number of krill per stomach was found by day in March, but neither stomach fullness nor state of digestion revealed clear diel patterns in food uptake (Table 3). Copepod remains in November were dominated by Centropages spp., and cladocerans were also identified. These groups were only represented in the WP2-hauls in the upper 25 m. Herring additionally preyed upon polychaetes, isopods, and small decapods, indicating feeding at depth during the day.
Sprat stomachs did not contain krill prey. The stomach contents comprised large numbers of newly eaten (state of digestion 1–3) copepods and zooplankton larvae by night in March, while no stomachs had newly eaten food items during day. In November, stomach fullness was generally low and state of digestion high at all times. However, a few sprat caught in upper layers during the day had newly eaten Temora spp., Calanus spp., and Centropages spp.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Fish distribution and feeding
Schooling fish occurred above and in the upper part of the krill layer by day, while schools dispersed at night, in accordance with previous studies at the station (Onsrud and Kaartvedt, 1998; Bagøien et al., 2000; Røstad, 2000; Kaartvedt et al., 2002). The relative contribution of sprat and herring in these acoustic signatures is hard to assess, as it is inherently difficult to capture schooling fish by small pelagic trawls. However, the catches suggested that a majority of herring belonged to the deepest schools (cf. Figure 5), the acoustic recordings demonstrated spatial interaction between krill and the deep schooling fish by day (cf. Figure 3), and the stomach contents of herring did suggest feeding by day in deep water.
Herring stomachs contained krill during both day and night, but stomach contents were always well digested. Any nocturnal predation on krill would be restricted to the shallowest fringe of the krill population, as indicated from the shallow distribution of herring. During our investigations, nocturnal light levels at the surface would range from about 10–4 to 10–6 µmol m–2 s–1 (full moon to clear nights with no moon). The lower visual threshold for herring feeding by biting (as apposed to filter-feeding) is ranging from 3.6 x 10–2 to 7 x 10–3 lux (Blaxter, 1964), equalling 1.4 x 10–4 to 7 x 10–4 µmol m–2 s–1. This would suggest the possibility of visual feeding close to the surface (cf. Batty et al., 1986; Blaxter, 1988).
The trawl catches showed that whiting were associated with the krill during both day and night, and the single fish targets in the krill SL would largely represent this species. Although the visual threshold for whiting feeding on M. norvegica is not established, whiting is expected to have a lower visual threshold than herring, due to their larger eye size (cf. Aksnes and Giske, 1993; Torgersen, 2001a). Whiting foraging in deep SLs of krill during daytime have been observed in previous studies from Lurefjorden, western Norway, which is characterized by low light penetration in the basin water (Bagøien, 1999; Torgersen, 2001a).
We suggest that whiting was foraging on krill during both day and night in the Oslofjord. The proportion of stomachs with krill structures were similar, or higher by day than by night, while the state of digestion for whiting captured at night suggested that food had been eaten recently.
While both herring and whiting apparently would be most efficient predators in the upper part of the krill layer, Norway pout were distributed deeper than the krill and thus attacking from below. Norway pout has been established as a benthopelagic species from many previous studies. It is a common, and often dominant constituent in bottom trawls from the Skagerrak and the North Sea (Hjort and Ruud, 1938; Raitt and Adams, 1965; Poulsen, 1968; Bergstad, 1990; Daan et al., 1990; Albert, 1994), and Larsen (1998) obtained large catches of Norway pout in the bottom trawl at the study site. However, the degree of bottom association varies, and Kaartvedt et al. (1996) showed how Norway pout left the benthic boundary zone when turbidity caused the light attenuation to increase. Kaartvedt et al. (1996) interpreted this situation as equivalent to "antipredator windows" which may occur at dawn and dusk, when light levels are sufficiently high to detect abundant plankton prey, while being sufficiently low to give relative safety with respect to visually foraging piscivores (Clark and Levy, 1988). Also in their study, the pout was foraging in the lower part of a krill layer.
Norway pout were part of the bottom-associated, dielly migrating acoustic layer. Fish ascending from the bottom at night, establishing a SL in midwater has been a recurrent phenomenon through years of study at this site (Onsrud and Kaartvedt, 1998; Kaartvedt et al., 2002). In November 2000, the source for the nocturnal layer lived pelagically during day. We ascribe this to the particularly high light extinction on this occasion (i.e. a similar phenomenon as outlined by Kaartvedt et al., 1996). The 1% light depth was then found as shallow as ca. 4 m, compared with 12–15 m during the other sampling occasions. The high light extinction was associated with record high run-off from land, caused by extensive rainfall prior to the sampling period.
We conclude that Norway pout were foraging on krill during both day and night. No clear diel feeding pattern was established from the stomach contents in our study. Albert (1994) found predation of Norway pout on M. norvegica at night, while Kaartvedt et al. (1996) documented foraging on krill during daytime. However, the prey-detection mechanism may have varied between day and night. Whether visual or non-visual perception, or a combination of both, is most important in locating the prey will depend on both visual acuity and the sensitivity of the lateral-line system of the species in question (Ryer et al., 2002). Norway pout have particularly large eyes, presumably enabling them to feed visually in relatively deep water during day. Nevertheless, their deep night-time distribution renders visual predation unlikely. The lateral-line morphology of Norway pout suggests that it may be a partially tactile predator (John Janssen pers. comm.). The slowly sinking SL would correspond to the description of non-visual, ambush-feeding fish detecting their prey by using the lateral-line system (Janssen et al., 1995; Janssen, 1997), although cessation of feeding represents an alternative explanation for this pattern. Several reports on midnight sinking in other species have documented a subsequent dawn rise during the morning "antipredation window" (Clark and Levy, 1988; Bagøien et al., 2001). Such a switch to visual predation would represent one possible explanation of the rise of the lower edge of the midwater acoustic layer in November 2000.
Aggregation behaviour
Fish aggregated beneath the research vessel in the course of each acoustic study period, evidenced by a conspicuous build-up in acoustic backscattering as recorded by the hull-mounted transducers (Figure 2). A corresponding increase was also observed from the bottom-mounted transducer in November, when the upward-looking transducer was located on the seabed in close proximity to the ship (Onsrud et al., in preperation), but was not recorded in March and June (the latter not shown), when the bottom-mounted transducer was located 80 m away from the ship. In March we were able to compare the level of acoustic backscattering just beneath and away from the ship, revealing a much higher fish abundance under the ship (Figure 4).
On several occasions, all fish targets disappeared from the ensonified volume, and then reappeared 10–20 min later. This occurred simultaneously with the passing of commercial vessels 200–400 m away. Corresponding patterns have been recorded during previous studies (Onsrud and Kaartvedt, 1998; Kaartvedt et al., 2002). Ship-avoidance behaviour by diving or horizontal swimming in response to the pressure wave ahead of ships and the low frequent noise generated from propellers have been repeatedly reported (Olsen et al., 1983; Diner and Masse, 1987; Fréon and Misund, 1998). There is concern how this may lead to underestimation of fish abundance from a moving vessel. This study, on the contrary, demonstrates how fish abundance may be overestimated in ecological studies at a fixed site. The particular mechanisms that account for this aggregation is beyond the scope of this paper. The aggregation behaviour is, however, of potential importance in evaluating the interactions between the fish predators and their krill prey. In our study, backscattering ascribed to krill became more prominent at low fish abundance.
Krill distribution and antipredator behaviour
Krill was a main food item for whiting, Norway pout and herring. The distribution of fish and krill seemed to be governed by predator–prey relationships, and not by physical properties of the habitat, apart from the light conditions which have a direct impact on predator–prey interactions involving visual predators. Temperature and salinity below sill depth were fairly homogeneous, and could not explain the variation in distribution by day, or nocturnal distributions in the basin water. In the period with lowest oxygen content of the basin water (November 2000; oxygen saturation of 22.8%), both fish and krill were abundant.
Krill were preyed on by planktivorous fish with different vertical distribution and foraging tactics, and were exposed to predators throughout the water column during both day and night. Their diel vertical migration would provide relative safety in the darkness of deep water, but their pelagic predators were still active. Further descent might have been unrewarding, bringing them into the realm of large-eyed, deep-living predators (Norway pout). Furthermore, M. norvegica are themselves visual predators (Torgersen, 2001b), using visual senses in locating deep-living prey (Kaartvedt et al., 2002) so their daytime distribution may be a trade-off between predator avoidance and searching for food.
Most focus on plankton antipredator behaviour has been on diel vertical migration. The current study suggests additional mechanisms. The "voids" in the krill recordings in the proximity of deep fish assemblages (cf. Figure 3) suggest that the krill performed instantaneous escape reactions during day. It appeared that M. norvegica would seek out water with reduced fish abundance at night (cf. Figure 2). Earlier investigations have displayed avoidance of the surface layer by the krill SL at the anchor station, where fish recordings normally are abundant (Onsrud and Kaartvedt, 1998; Kaartvedt et al., 2002). Artificial accumulation of fish under the ship at the anchor station could enhance the surface avoidance behaviour.
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Acknowledgements |
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This study was funded by the Research Council of Norway (Project no. 133355/122). We greatly appreciate the assistance given by Rita Amundsen and the RV "Trygve Braarud" crew, and thank Espen Bagøien and two anonymous referees for valuable comments on the draft manuscript.
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