ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on July 3, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(6):1235-1245; doi:10.1093/icesjms/fsm092
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Diet of harbour seals and great cormorants in Limfjord, Denmark: interspecific competition and interaction with fishery
1 Department of Arctic Environment, National Environmental Research Institute, Aarhus University, Frederiksborgvej 399, DK-4000 Roskilde, Denmark
2 Institute of Biology, University of Southern Denmark, Odense, Denmark
Correspondence to S. M. Andersen: tel: +45 46 301942; fax: +45 46 301914; e-mail: sia{at}dmu.dk
Andersen, S. M., Teilmann, J., Harders, P. B., Hansen, E. H., and Hjøllund, D. 2007. Diet of harbour seals and great cormorants in Limfjord, Denmark: interspecific competition and interaction with fishery. – ICES Journal of Marine Science, 64: 1235–1245.Comparative studies on seasonal and regional variation in the diet of harbour seals and great cormorants were conducted in Limfjord, a semi-closed water system in northwest Denmark. To compare harbour seal diet from an open water system containing similar prey species, a small diet analysis from the western Baltic is included. Seal diet during spring reflected the abundance of Atlantic herring entering Limfjord to spawn (90% of the weight consumed), whereas during summer and autumn, seal diet was rather more mixed. The diet of seals in the Rødsand area and cormorants in Limfjord showed no marked seasonal trends. During spring, there was little overlap between seal and cormorant diets in Limfjord because seals fed almost exclusively on Atlantic herring, and they consumed significantly larger herring than did the cormorants. During summer and autumn, seal and cormorant diets overlapped markedly, although the fish items consumed by seals were generally larger. Few commercially targeted species were found in the stomachs and scats of seals and casts of great cormorants, but Atlantic herring were taken by the seals at a size greater than that allowed by the fishery.
Keywords: diet, dietary overlap, faecal samples, fishery, great cormorants, harbour seals, interspecific competition, Phalacrocorax carbo sinensis, Phoca vitulina vitulina, seal/fisheries interactions
Received 18 August 2006; accepted 25 May 2007; advance access publication 3 July 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Harbour seals (Phoca vitulina) are generalists that feed on a wide range of fish species (Härkönen, 1987; Härkönen and Heide-Jørgensen, 1991; Andersen et al., 2004), yet their diet is often dominated by a few key species (Härkönen and Heide-Jørgensen, 1991; Tollit and Thompson, 1996). Several studies have noted that the importance of the key species varies both seasonally (Brown and Mate, 1983; Härkönen, 1987; Pierce et al., 1991; Olesiuk, 1993; Tollit and Thompson, 1996; Hall et al., 1998) and regionally (Härkönen, 1987; Olesiuk et al., 1990; Olsen and Bjørge, 1995). Similarly, great cormorants (Phalacrocorax carbo) are described as generalist foragers (Warke et al., 1994), with a diet varying between regions (Hald-Mortensen, 1995) and seasons (Madsen and Spärck, 1950). Hence, harbour seals and great cormorants are mainly piscivorous and, because they often share the same habitats, they potentially compete for the same food resources. In the relatively shallow Limfjord in northwest Denmark, the dietary overlap between these two top predators is expected to be of particular relevance.
Harbour seals and great cormorants are found throughout Danish waters. From an aerial survey in 1997, the Danish population of harbour seals was estimated to be 
11 000 strong, of which 1400 inhabited Limfjord (NERI, unpublished data). Following the total extermination of great cormorants in Denmark in the 1860s, the species took until 1938 to nest again (Madsen and Spärck, 1950). For the past 10 years, there has been a constant 40 000 cormorant nests in Denmark, of which some 6000 were in Limfjord (Eskildsen, 2001, 2005). Up to that point, the populations of harbour seals and great cormorants increased substantially in Denmark, a consequence of their protection from hunting in 1977 and 1980, respectively, and the establishment of several reserves in the late 1970s (Danish Forest and Nature Agency, 2002, 2005). The increases in population size of both combined with declining fish stocks have prompted concern by fishers (Hoffmann et al., 2003). Both have, however, been regulated by epidemics of phocine distemper virus (PDV) in 1988 and 2002, causing the death of up to 50% of the Danish harbour seals then (Härkönen et al., 2006), and a law promulgated in 1994 allowing new cormorant colonies to be removed (Danish Forest and Nature Agency, 2002).
In both Limfjord and the Rødsand area, seal damage to fishing equipment is especially severe. Since 2003 the number of conflicts between fishers and seals has decreased in Limfjord (Danish Forest and Nature Agency, 2005), but conflicts between cormorants and fishers have become more severe. Here, we present data on the diet of harbour seals and great cormorants in Limfjord and analyse trophic interactions between them, and also look at the diet of harbour seals in the western Baltic to compare diets of seals between a semi-closed and an open water system. Finally, we assess potential competition between fishers and these two predators in Limfjord using fish survey results, adapted from Hoffmann (2000).
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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During spring, summer, and autumn of 1997, and spring 1998, 106 harbour seal scats and 198 great cormorant casts were collected from haul-out sites and resting places, respectively, in two different parts of Limfjord, Løgstør Bredning and Nissum Bredning (Figure 1), to allow for examination of the geographic variation in diet of harbour seals and great cormorants in Limfjord. Nissum Bredning, in western Limfjord, is influenced by the North Sea, so the water is supplied with various species from the North Sea and the salinity is higher than that in Løgstør Bredning, in the inner Limfjord.
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Material from the Rødsand reserve, south of Falster Island in southern Denmark, consisted of 13 harbour seal scats collected during the period March–November of 2001–2005 and 17 digestive tracts from harbour seals shot by fishers with permission of the Danish Ministry of Environment. These healthy seals were shot in the vicinity of fishing gear, within a few kilometres of the haul-out sites. Because of the limited number of samples from Rødsand, these are used only in discussing the conclusions from the Limfjord samples.
Processing and identifying prey remains
All scats, casts, and digestive tracts were stored frozen (–20°C) in individual polythene bags until processing. Before analysis in the laboratory, scats, casts, and digestive tracts were left to thaw for 24 h in water, after addition of a few drops of household detergent to emulsify the soft constituents. Scats and digestive tract contents were then washed through three stacked interlocking test sieves with mesh sizes of 300 µm, 750 µm, and 2.0 mm (Endocott). Otoliths were recovered and stored dry until identification. One decilitre of water and 4–5 tablets of sodium hydroxide (NaOH) were added to the casts, and the mixture was stirred until the tablets were dissolved. The mixture was left until the next day for the detergent and NaOH to dissolve the mucous membrane. The mixture was then thoroughly washed through a sieve with mesh size of 180 µm, and otoliths were recovered and stored dry until identification.
Otoliths were identified to the lowest taxon possible, using a reference collection and the otolith identification guide of Härkönen (1986). Otoliths were sorted into right- and left-sided otoliths, and the more numerous side was used to determine the number of fish consumed; if this was not possible because of degrading, the number of fish was estimated by dividing the total number of otoliths by two. Otolith length was measured (accuracy 10–2 mm) parallel to the sulcus, from the anterior tip of the rostrum to the posterior edge, and otolith maximum width was measured perpendicular to the length under a dissecting microscope (Cambridge Instruments) with a binocular micrometer (x6.5 and x40). The weight of the individual fish consumed was estimated from the otolith length and width, using the otolith size : fish weight formulae for each species in Härkönen (1986). From the Rødsand area, some flatfish otoliths were identifiable only to family, i.e. Pleuronectidae, and regressions based on combined data from the two most likely species (plaice, Pleuronectes platessa, and flounder, Platichthys flesus) were used.
Studies from captive feeding experiments found that correction factors can be applied to obtain a more accurate estimate of prey size (Harvey, 1989; Tollit et al., 1997b), but it is still unknown if these correction factors apply to free-ranging pinnipeds; inactivity is likely to have an impact on the digestion and hence on the condition of the otoliths recovered. Grellier and Hammond (2005) showed that only digestion coefficients derived from in situ experiments can be used to estimate fish size accurately, whereas digestion coefficients derived from carrier experiments yield larger digestion coefficients, which tend to overestimate fish size. Correction factors derived from in situ experiments were not available for all prey species relevant to this study, so we did not use them to preclude introducing further bias when comparing interspecies fish weight.
Diet competition between harbour seals and great cormorants
Diet was considered to overlap between harbour seals and great cormorants when a prey species constituted > 5% of both seal and cormorant diet, and when each sample consisted of 
5 specimens of a particular prey species. The estimated length frequency distributions of prey for which seals and cormorants potentially competed were compared using Kolmogorov–Smirnov two-sample tests.
Biomass estimates and length frequency distributions of fish species in Limfjord were produced on research trawl surveys in September 1997 (Hoffmann, 2000). Prey sizes in the seal and cormorant autumn diet were compared with these trawl samples using Kolmogorov–Smirnov two-sample tests to determine whether the range of fish sizes in the diet was consistent with the range of prey sizes in the fjord. Finally, the estimated length frequency distributions of seal and cormorant prey were compared with fishery minimum size limits (Ministry of Food, Agriculture and Fisheries, 2003). The proportion of the various prey species (by mass) in harbour seal and great cormorant diet was compared with their relative abundance in Limfjord.
Seasonal differences in mean number of fish and mean total weight per scat sample were examined using analysis of variance (ANOVA) on log-transformed data, with Tukey–Kramer as the post hoc test (p < 0.05). This analysis was carried out solely on scat samples. All statistical tests were conducted in S-plus 6 (Insightful Corporation).
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Results |
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In Limfjord and the Rødsand area, harbour seals fed on a minimum of 17 and 20 species, respectively, and 22 fish species were found in the cormorant casts from Limfjord (Tables 1–3).
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Seasonal and regional variation in diet
Harbour seals fed almost exclusively on marine fish. However, bones and otoliths from the brackish roach were identified in one harbour seal digestive tract from Rødsand. Roach have been caught several times in pound nets by local fishers on the south coast of Lolland (M. Schjelde, pers. comm., 2005).
Atlantic herring was the principal prey item of the seal diet during spring in Løgstør Bredning, accounting for 90% of the weight consumed (Table 1). The importance of Atlantic herring in seal diet there declined in summer and autumn, then increased again in spring the following year. In summer and autumn, seals fed on a wide variety of prey, although their diet was dominated by a few key species. Eelpouts and plaice were important in summer, accounting for 23% and 18% of the weight consumed, respectively (Table 1), whereas flounders and gobies dominated in autumn, 31% and 36% of the weight consumed, respectively. In contrast to these findings, flounder dominated seal summer diet (65%) in Nissum Bredning (Table 1). In Løgstør Bredning, the mean number of fish recovered in harbour seal scats and the mean total weight per scat varied with season (r2 = 0.14, F2,82 = 6.75, p = 0.002, and r2 = 0.11, F2,82 = 5.33, p = 0.007, for mean fish number and mean total weight, respectively). Hence, there were significantly fewer fish per scat in spring than in summer or autumn, although there was a significantly higher mean total weight per scat in spring than in autumn (Figure 2a).
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There were no seasonal differences in the mean number of fish recovered and the mean total weight per scat in the harbour seal material from the Rødsand area (r2 = 0.00, F2,10 = 0.01, p = 0.986, and r2 = 0.07, F2,10 = 0.40, p = 0.681, for mean fish number and mean total weight, respectively) (Figure 2b). In the Rødsand area, cod (Gadus morhua) dominated both spring and autumn seal diet (42% and 43%, respectively, of the weight consumed) (Table 3). Cod were less common in summer (22%), when flounder and plaice together made up 52% of the weight consumed. In all, seven newly ingested garfish lacking their heads were recovered in two seal digestive tracts from Rødsand.
In contrast to the diet of seals, cormorant diet showed no marked seasonal trends (r2 = 0.02, F2,138 = 1.06, p = 0.348, and r2 = 0.002, F2,138 = 0.14, p = 0.873, for mean fish number and mean total weight, respectively) (Table 2). Bullrout, black goby, and eelpout were the most important prey species in Løgstør Bredning. Bullrout accounted for 30% in all seasons. Summer diet was dominated by eelpout (37%), whereas the diet in autumn was dominated by black goby (34%). In contrast to these findings, the cormorant diet composition in Nissum Bredning was dominated by plaice (36%) in spring. Plaice and cod together dominated the summer diet, constituting 43% and 44% of the weight consumed, respectively.
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Diet competition between harbour seals and great cormorants
In Løgstør Bredning, the spring diet of seals consisted of eight species, in contrast to the 15 prey species in great cormorant spring diet. Seven prey species were preyed on by both, of which only Atlantic herring constituted > 5% in the diet of both. Seals fed almost exclusively on Atlantic herring (90% of the weight consumed) in spring, but that species was of minor importance to cormorants in spring (7% of the weight consumed). Hence, the competition in spring between cormorants and seals in Løgstør Bredning seems to be minimal (Figure 3a). Seal and cormorant summer diet consisted of 15 and 13 prey species, respectively, of which 12 were preyed on by both, but only plaice, eelpout, bullrout, and black goby constituted > 5% in the diet of both. These prey items constituted 55% of seals' summer diet compared with 87% of cormorants' summer diet (Figure 3b). In autumn, seals took 12 prey species and cormorants 9, of which eight were taken by both predators, although only sand goby and black goby constituted > 5% of the diet of both predators. Combined, these prey species in autumn made up 50% of the harbour seal diet and 46% of the cormorant diet (Figure 3c). Competition for black goby is particularly evident, because this species constituted >20% of the diet of both.
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Estimated length frequency distributions of the fish species eaten by both seals and cormorants in Løgstør Bredning are given in Figure 3d–f. Generally, seals preyed on larger fish than cormorants, but the difference was only significant for Atlantic herring in spring. At that time, seals consumed herring of mean length 21.5 (±5.5) cm, whereas the length of herring consumed by cormorants was 10.2 (±6.9) cm (D = 0.3846, p = 0.042) (Figure 3d).
Estimated length distributions of seal and cormorant prey were compared with those derived from trawl catches. The sizes of prey eaten by seals and cormorants reflected the sizes available in Limfjord, so we conclude that seals and cormorants do not seem to select particular prey sizes. Because trawl surveys were carried out in September, this comparison was made only with autumn data (Kolmogorov–Smirnov two-sample tests, p > 0.24 for all tests) (Figure 4). Harbour seals in Limfjord tend to select larger fish (Figure 4). Moreover, the size of herring in the harbour seal diet does not seem to reflect the availability of various sizes of herring in Limfjord (as revealed by the autumn survey trawl) (Figure 5).
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In spring, harbour seals consumed Atlantic herring larger than the allowed minimum size in the fishery, so in spring at least, seals were in direct competition with the fishery (Figure 3d). This was not the case in summer and autumn. All other fish species were consumed at sizes smaller than allowed fishery minimum sizes, so competition was only indirect in that small fish grow to commercial size some years later (Figures 3e and 4).
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Discussion |
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The results of this study confirmed the polyphagous nature of both harbour seals and great cormorants documented in previous studies (Härkönen and Heide-Jørgensen, 1991; Bowen and Harrison, 1996; Tollit and Thompson, 1996; Andersen et al., 2004), and also that both species are generalist feeders (Bigg, 1981; Härkönen, 1987; Tollit et al., 1997a). The changes in diet between seasons documented here should be seen in the light of varying prey availability. Specifically, Atlantic herring enter Limfjord in spring to spawn (Pedersen, 1996), and it was at that time that herring constituted the bulk of harbour seal diet in Løgstør Bredning. Thompson et al. (1991) noted that Scottish harbour seals similarly preyed on clupeids when they were particularly available. This may be due to Atlantic herring being a high-energy food source, generating some degree of prey selection by harbour seals (Thompson et al., 1997). Harbour seals in this study clearly relied on resident fish species, such as eelpouts, black gobies, and flounders in summer and autumn, perhaps related to a lesser availability of Atlantic herring then. Further, in a previous study by Friis et al. (1994), black gobies dominated harbour seal autumn diet in Løgstør Bredning, and the authors suggested that this reflected a marked increase in the population of black gobies after the mid-1980s. Hence, harbour seals are perhaps specialist feeders or maybe specialists and generalists, as suggested by Grellier and Hammond (2006). Our results from Limfjord suggest that seals may prefer to eat herring when the latter are available in spring, whereas they seem to be generalist feeders during the rest of the year when herring have migrated out of the area, so no particular species is preferred.
In contrast to the diet of harbour seals, that of great cormorants did not vary markedly across season, because great cormorants generally relied on bullrouts as prey throughout the year. In an earlier study (Madsen and Spärck, 1950), Danish cormorants responded to increased herring availability, so it was unexpected that Atlantic herring constituted < 7% of the great cormorant spring diet during this study.
Both harbour seal and great cormorant diets varied among localities in Limfjord. Harbour seals generally forage up to 30–50 km from their haul-out sites (Thompson and Miller, 1990; Thompson et al., 1991; Tollit et al., 1998), so it is likely that harbour seals hauling out in Limfjord forage in all parts of Limfjord and also in the North Sea. It is, however, worth noting that all prey species recovered from harbour seal scats are found regularly in Limfjord (Hoffmann, 2000). Most prey species consumed by harbour seals in Nissum Bredning are found in (sandeels) or live on (flatfish) the seabed. No samples were collected during spring in this area, so similar dominance of herring then could not be concluded.
Great cormorants in Nissum Bredning had a broader diet spectrum than cormorants in Løgstør Bredning. Grey gurnard, long rough dab, and haddock are not generally present in Limfjord (Hoffmann, 2000), but they were found in the diet of cormorants at Nissum Bredning, indicating probably that great cormorants also forage in the North Sea. Moreover, there are indications of a marked decline in overall prey availability in Limfjord (Hoffmann, 2000), which may have forced both harbour seals and great cormorants to make longer foraging trips, perhaps out of Limfjord.
Harbour seal diet was apparently limited during autumn. To maintain their energy intake then, they were compelled to consume a large number of small fish that were available. In spring, they could manage with a smaller number of the much larger herring available then. Comparing this result with harbour seal diet in the Rødsand area, it is obvious that harbour seals there consume fish of a much more uniform size through the year, and that they are apparently not as limited in diet. The Rødsand data were pooled across 5 years, and may cover between-year differences (Middlemas et al., 2006).
The fact that harbour seals might have limited diet during some time of the year may increase the importance of possible competition between harbour seals and great cormorants in Limfjord. Both harbour seals and great cormorants feed on the seabed (Härkönen, 1988). Cormorants generally forage in shallow water, whereas harbour seals prefer to feed on plain seabeds down to 30 m (Härkönen, 1988). Hence, in Limfjord, which has an average depth of 5 m (maximum 28 m) (Lissner et al., 2004), dietary overlap between harbour seals and great cormorants would be expected. Competition between harbour seals and great cormorants was minimal in spring, because herring were of only minor importance to cormorants. Moreover, the herring consumed by cormorants were significantly smaller than those taken by seals. In summer and autumn, the extent of dietary overlap increased. In summer, competition seemed to have the greatest impact on cormorants, because the overlapping prey species constituted a larger portion of the diet of great cormorants. Although not significantly, the fish consumed by seals were larger than those consumed by cormorants. Competition eases in autumn, because most of the Danish cormorant population leaves Denmark between August and October only to return in March (Bregnballe et al., 1997).
Some 50 years ago, Madsen and Spärck (1950) argued that herring were so abundant in Danish waters that the 300 Danish breeding pairs of cormorants at that time could feed on them without having any effect on the fishery. In 1997, the total Danish population of cormorants had become 40 000 pairs, the total Danish harbour seal population had increased from 2000 in 1977 to some 11 000 in 1997 (NERI, unpublished data) (1400 in Limfjord), and the fish populations in Limfjord had declined markedly. This scenario caused an increase in interactions between the fishery and cormorants and seals during the 1990s, in terms of both interference around static fishing gear and competition for the same fish resources. Hence, harbour seals and great cormorants are often viewed now as having a negative impact on commercial fish species. However, in this study, only a few commercial species (and sizes) were included in their diet. Flounder, plaice, eelpout, cod, and Atlantic herring are exploited by seals, cormorants, and the fishery. Of these, only Atlantic herring were consumed by harbour seals in sizes larger than the allowed minimum size in the fishery; all other prey species taken by harbour seals and great cormorants were smaller than the minimum allowed sizes. Hence, Atlantic herring was the only prey species on which harbour seal could have had a direct impact. Harbour seals and great cormorants preyed on other prey species at sizes smaller than the allowed minimum sizes in the fishery, which might indicate indirect competition, but this predation might compensate for other mortalities. Moreover, such predation might, through a reduction in intraspecific competition among prey species, give the survivors an increased chance to attain the size required by the commercial fishery size (Friis et al., 1994). However, not applying correction factors in our study has inevitably biased the size distribution towards smaller sizes, and this fact must be kept in mind when comparing fish size distribution in harbour seal and great cormorant diets, and the fishery.
The harbour seal population in Limfjord (1400 animals) consumed some 424 t (based on a daily consumption of 5 kg per seal (Bonner, 1982) of herring during 1997 compared with the 2680 t of herring landed by the commercial fishery. Hence, seals do compete with the fishery, because they consume herring of size larger than the allowed minimum size in the fishery. However, the amount taken is still six times less than that taken by the fishery (without any correction factors being applied). Other prey species were caught at sizes smaller than the minimum fishery sizes, by both harbour seals and great cormorants. If it is assumed that there are sufficient small fish to sustain the stocks of fish at commercial size, the impact on the fishery in Limfjord will be minor. However, if the seals and cormorants are limiting recruitment to commercial size, there will be competition.
By not applying correction factors in our study, we underestimated fish sizes and therefore our values are minimum estimates, but unfortunately the otoliths from the various prey species do not reduce in size by the same amount. Grellier and Hammond (2005, 2006) showed, in a controlled study with captive grey seals, that larger otoliths had greater digestion coefficients, which leads to even greater underestimation of prey size in the case of larger otoliths. Moreover, Grellier and Hammond (2005) demonstrated that otoliths from larger fish have a greater possibility of being recovered (greater recovery rates) in samples, which results in overestimation of their number as well. However, we believe that the extent of competition between harbour seals, great cormorants, and the fishery, as well as the limited values calculated for the food resources for harbour seals in Limfjord, will persist even if correction factors were applied.
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Acknowledgements |
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We thank E. Hoffmann for supplying raw trawl data from the Limfjord fish trawls, P. H. Mortensen for assistance with otolith identification, and A. Linnet for help in the field. M. Schjelde, a Rødsand fisher, supplied valuable information about the fish stocks in the Rødsand area. The contributions by EHH and DH were used for partial fulfilment of Masters of Science degrees at the University of Southern Denmark. Carol Bang-Christensen is thanked for comments on an earlier draft of this manuscript.
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References |
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