© 2006 International Council for the Exploration of the Sea
Feeding of Atlantic salmon (Salmo salar L.) post-smolts in the Northeast Atlantic
a Institute of Marine Research PO Box 1870, Nordnes, N-5817 Bergen, Norway
b Norwegian Institute for Nature Research PO Box 736 Sentrum, N-0105 Oslo, Norway
*Correspondence to M. Haugland: tel: +47 55 23 68 92; fax: +47 55 23 86 85. e-mail: monika{at}ramson.org.
Stomach samples from 1384 Atlantic salmon, collected from 1991 to 2003 in the Northeast Atlantic, were analysed to fill the gap between studies on post-smolt diet in fjords and coastal areas of the Northeast Atlantic and studies on the diet of pre-adults and adults in the Norwegian Sea. The post-smolts fed largely on 0-group fish. Blue whiting was an important prey only in the slope current transporting the larvae from the spawning areas west of the United Kingdom into the North and Norwegian Seas. Sandeel and herring were important or present in the stomachs throughout most of the area studied. Unusually large quantities of 0-group herring in the Norwegian Sea in summer 2002 coincided with a high condition factor of post-smolts that year. The forage ratio of the post-smolts was positively related to the proportion of herring in the stomachs and the abundance of herring recruits. Despite these findings, the most productive period for Atlantic salmon on record, the 1970s, coincided with the collapse of the Norwegian spring-spawning herring, which raises the question as to whether herring is more important as a competitor than as a food source. Hyperiid amphipods were more important prey than krill, in contrast to the situation for other pelagic fish species.
Keywords: Atlantic salmon, diet, ecology, feeding preference, Northeast Atlantic, wild salmon
Received 4 March 2005; accepted 4 June 2006.
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Introduction |
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Atlantic salmon (Salmo salar L.) leave their home rivers in spring and early summer as smolts, and migrate towards feeding areas in the Nordic Seas and West Greenland. Until recently, the distribution of post-smolts in the open sea during their first months after leaving freshwater was largely unknown owing to a lack of effective sampling and observation technology (Holm et al., 2000, 2003). Recent investigations have shown that Atlantic salmon post-smolts in the Northeast Atlantic are caught mainly in early summer in association with the slope current passing northwards to the west of the United Kingdom and Ireland (Holst et al., 1996, 2000; Shelton et al., 1997; Holm et al., 2000). Entering the Norwegian Sea, the fish are found in the current running north parallel with the western edge of the Vøring Plateau, before captures spread in a fan-like manner over a wider area north of 67°N (Holm et al., 2003). The origins of these post-smolts are unclear, but the age distribution suggests that a major proportion of those caught in the Norwegian Sea between June and September originate from areas south of Norway (Holm et al., 2003). However, post-smolts tagged in southwestern Norway have also been captured in the same areas and times, indicating that the Norwegian Sea is important for many salmon stocks bordering the Northeast Atlantic (Holm et al., 2004).
Stomach analyses of post-smolts taken in fjords and coastal areas in Norway show that the fish start to prey on marine organisms immediately after their transition to saltwater, although insects available in the estuary still form part of the diet. Fish larvae are important prey in this early marine phase, although the amount taken may vary between years depending on local conditions (Levings et al., 1994; Hvidsten et al., 1995; Andreassen et al., 2001; Rikardsen et al., 2004). Similar findings have been reported from coastal areas throughout the range of the salmon (Morgan et al., 1986; Dutil and Coutu, 1988; Levings, 1994; Sturlaugsson, 1994, 1995).
Most salmon growth is during the marine stage (Thorpe, 1988), and it has been suggested that most marine mortality of salmon at sea is during the post-smolt stage (Doubleday et al., 1979; Ritter, 1989). The feeding regime in the ocean during the first summer may therefore be one of the important factors for post-smolt survival. Only two studies describe the diet of post-smolt salmon in the oceanic areas of the Northeast Atlantic. Holst et al. (1993) analysed a few stomachs from the Norwegian Sea. Shelton et al. (1997) found remains of myctophids, juvenile gadoids, and five-bearded rockling (Ciliata mustela), and crustaceans such as amphipods, krill, copepods, and decapod larvae in post-smolt stomachs from the Faroe–Shetland Channel in June. Pre-adult and adult salmon in the Northeast Atlantic prey on several fish species. Mesopelagic fish constitute an important proportion of the prey, but other pelagic species frequent in the area are also common. In the eastern Atlantic, adult salmon frequently prey on amphipods, krill, and mesopelagic shrimp, as well as on squid (Thurow, 1973; Hislop and Youngson, 1984; Hansen and Pethon, 1985; Jacobsen and Hansen, 2000, 2001). In oceanic areas of the Northwest Atlantic, however, fish seem to dominate the diet (Reddin, 1988; Hislop and Shelton, 1993).
The objective of the present study is to fill the gap between the investigations of post-smolt diet in fjords and coastal areas of the Northeast Atlantic (Morgan et al., 1986; Levings et al., 1994; Sturlaugsson, 1994; Andreassen et al., 2001; Rikardsen et al., 2004) and the investigations on the diet of pre-adult and adult salmon in the Norwegian Sea (Hislop and Youngson, 1984; Hansen and Pethon, 1985; Jacobsen and Hansen, 2001). Focusing on the area west of the United Kingdom and the Norwegian Sea, we discuss the implications of our findings in ecological terms both on a discrete latitudinal scale and as a continuous process running from the smolts' entrance into the sea to the arrival of post-smolts in adult feeding areas.
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Material and methods |
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Owing to differences in oceanography and ecology in different parts of the area sampled, as well as differences in the location and time of sampling, the study area was divided into three parts: (i) the southwest area, consisting of samples taken from northwest of Ireland, west and north of Scotland, and in the Faroe–Shetland Channel up to 62°N; (ii) the North Sea, which includes the areas east of England and Scotland up to 62°N; and (iii) the Norwegian Sea (Figure 1). The division is closely related to that of the ICES Working Group WGRED (ICES, 2006b). Most post-smolts examined from the southwest area probably originate from Irish, western English, and western Scottish rivers, fish from the North Sea probably originate mainly from eastern Scottish and southern Norwegian rivers, and fish captured in the Norwegian Sea are a mix from all these areas (Holm et al., 2004). The fish therefore have unique migration and feeding histories, depending on the areas they passed before being caught.
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The post-smolts were collected during scientific summer surveys carried out by the Institute of Marine Research (IMR) between 1990 and 2004. Some of the data included here were analysed by Holst et al. (1993). The distribution in time and space and the catch per unit of effort (cpue) of these post-smolts are shown in Figure 1. Each dot represents a trawl position and, up to 2004, more than 2000 post-smolts have been captured in the open ocean. Because of the objectives and timing of the surveys, most sampling has been concentrated in the central part of the Norwegian Sea, where there is more consistent temporal coverage than in the southwestern part of the study area or in northern parts of the sea.
In all, 1384 stomachs collected over ten years were examined (Table 1), and their distribution by latitude is shown in Figure 2. Most of the fish were sampled in the Norwegian Sea, numbers varying from 20 to 346 stomachs per year. In 1991, 1995, 1996, 1998, and 1999, fish were sampled primarily in July and August in the northern parts, whereas in the years 2000–2003, sampling was carried out primarily in June in the central and southern parts of the Norwegian Sea (Table 1, Figure 3). In 1997, only four fish were caught in this area, so these were not included in the analysis. Stomach samples were obtained from the southwest area in June and early July of 1995–1997, annual sample sizes varying from 46 to 55 (Table 1, Figure 3). In the North Sea the total sample size was 32, and sampling was carried out in mid-June 1997 (Table 1, Figure 3). During most of the early surveys, herring or mackerel were the target species, and salmon were taken as bycatch. During the surveys carried out between 2000 and 2003, however, salmon was the target species.
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In 1991, a pair trawl was used for sampling, but in the other years a modified surface trawl (Valdemarsen and Misund, 1995) fitted with extra flotation on the headline to sample the upper 14 m and hauled at 3–5 knots was used. From 1999 on, the trawl was equipped with a live fish capture device attached to the codend (Holst and McDonald, 2000). Tow duration ranged from 30 min to 1 h.
Most post-smolts in the catch were measured (fork length, LF) onboard to the nearest 1 mm (1995–2003), 0.5 mm (1991, 1997, 1998, 1999), or 1 cm (1999), and weighed (MF) to the nearest 0.1 g. The LF of fish that had to be measured after freezing and thawing were adjusted up by 3% (Rikardsen et al., 2004). Fulton's condition factor (K = 100MFLF–3) was calculated on the basis of length and weight measurements without stomach contents. The fish were frozen for further analyses ashore, and those showing signs of cultivation or farming (adipose fin cut and/or abnormal scale growth pattern) were excluded from the study.
In the laboratory, the post-smolts were thawed, opened, and the stomachs removed. Analysis of stomach contents was performed according to earlier studies on post-smolts of salmon (Andreassen et al., 2001; Rikardsen et al., 2004). Stomach contents were classified into groups of prey (Table 2), which were all weighed (wet mass, MS) to the nearest mg. Forage ratios were calculated as FR = 100MS total (MF–MS total)–1.
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To express the importance of each prey category in the diet, one gravimetric and one numerical/occurrence index were calculated (Berg, 1979; Hyslop, 1980): percentage frequency of occurrence, O% = 100NiNj–1, where Ni is the number of stomachs containing item i, and Nj is the number of non-empty stomachs examined; percentage by mass, M% = 100MijMj–1, where Mij is the mass of item i eaten by fish j, and Mj is the total mass of items eaten by fish j.
Total lengths of Themisto abyssorum were measured according to Dunbar (1957). Year-class strength of Norwegian spring-spawning herring was taken from ICES (2005).
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Results |
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The distribution of the samples in the present study varied between years depending on the coverage by the annual survey (Figure 2). The average lengths of the post-smolts caught in the Norwegian Sea ranged from 208 mm (2001) to 225 mm (2000) for those caught in early summer, and from 234 mm (1995) to 283 mm (1991) for those caught in late summer (Table 1). The post-smolts captured in 1991 were significantly larger than in all other years, and the fish captured between 2000 and 2003 were the smallest (Tukey, p < 0.05). The average condition factor (K) of the post-smolts caught in the Norwegian Sea ranged from 0.97 (1996) to 1.19 (2002) (Table 1). In 2002, it was significantly higher than in all other years. In 2000 and 2003, K was significantly higher than in 1991, 1995, and 1996, and in 1996, it was lower than in all other years (Tukey, p < 0.05).
The total number of stomachs sampled varied along the latitudinal range, with a peak from 65°N to 69°N, at the Vøring Plateau (Figure 3). Different prey groups and species were observed in the stomachs at varying latitudes (Figure 4), and some groups and species were present over only a small range whereas others were present over most of the latitudes sampled. In particular, some of the fish species, such as sandeel (Ammodytes spp.), herring (Clupea harengus), and redfish (Scorpaenidae), were present over a large latitudinal range. Among the zooplankton, krill (Euphausiacea) and amphipods (Themisto spp.) were present over a wide latitudinal range. The proportion of empty stomachs by area and year varied from 0% to 15%, with a mean of 9.5%.
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In the southwest area, fish dominated the stomach contents, with sandeel and blue whiting (Micromesistius poutassou) the most important, accounting for 33% and 26% by mass and 29% and 16% by frequency of occurrence, respectively. Only 8% of the total mass was crustaceans (Table 2). In 1995, blue whiting were the dominant prey (64% by mass and 54% by frequency of occurrence), followed by sandeel and herring (Table 3). In 1996, sandeel dominated (47% and 19%), followed by greater forkbeard (Phycis blennoides), cod (Gadus morhua), and various crustaceans (Table 3). In 1997, sandeel were again the dominant prey (45% and 28%) followed by krill, but with a substantial group of unidentified fish (Table 4, Figure 5). The FRs observed in this area were 0.77 for 1995, 0.42 for 1996, and 0.45 for 1997 (Table 3, Figure 6a). The average length of the post-smolts caught in the southwest area was significantly larger in 1997 than in the two preceding years (Tukey, p < 0.05). The condition factor was significantly lower in 1996 than in the other two years (Tukey, p < 0.05; Table 1).
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In the North Sea, the 32 stomachs analysed from 1997 all contained sandeel, that species consequently contributing 100% by frequency of occurrence and close to 100% by mass. Other groups of prey found in small numbers were copepods, amphipods, and krill. A high mean FR was found in this sample (1.29; Table 2, Figure 6b).
In the Norwegian Sea, the dominant prey group by mass was fish (64%), with crustaceans as the next most important prey (36% of the total; Table 2). Frequencies of occurrence were 64% for both groups. Herring was the most important fish species (41% by mass, 38% by frequency of occurrence). Sandeel accounted for 10% by mass and 11% by frequency of occurrence. Pearlside (Maurolicus muelleri), lanternfish (Myctophidae), gadoids (cod, saithe, haddock), and redfish were also found in the stomachs from this area, and 3% by mass were unidentified fish. Most of the fish found in the stomachs were 0-group. Amphipods were the most important crustacean prey (32% by mass, 47% by frequency of occurrence), with Themisto abyssorum (2–8 mm total length) as the dominant species. Themisto libellula and Themisto compressa were infrequent. Krill and copepods accounted for a small amount by mass, 3% and 1%, respectively, but as much as 22% and 9% by frequency of occurrence. The copepods were mainly the hyponeustonic species Anomalocera patersoni. Caligoids, isopods, and decapods were other crustaceans found in the stomachs. Other than fish and crustaceans, polychaetes, molluscs, and insects were found in small numbers, and 1% by mass was unidentified (Table 2).
Herring and amphipods were the only groups of prey that accounted for more than 50% of the total in one or more years in the Norwegian Sea (Table 4, Figures 5 and 6c). In 1991, amphipods were the most important group of prey (58% by mass, 74% by frequency), followed by herring and sandeel. Amphipods were also the most important group of prey in 1995 (37% and 69%) followed by redfish, lanternfish, and krill. Herring was only found in small numbers in the stomachs that year. In 1996, haddock was the most important group (42% by mass, 6% by frequency of occurrence), followed by amphipods and lanternfish. Herring was by far the most important prey in the 1998 sample (70% and 78%), and dominated together with saithe. Stomachs from 1998, 2000, and 2001 contained a negligible amount of hyperiids. In 1999, however, hyperiids were the dominant group of prey (64% by mass, 86% by frequency of occurrence) followed by herring. In 2000, sandeel and krill were important, and 36% of the stomach contents were unidentified fish. Herring dominated in 2001 (63% by mass, 77% by frequency of occurrence), sandeel accounting for a relatively large contribution. The stomachs analysed from 2002 were also dominated by herring (51% by mass, 56% by frequency), followed by hyperiids and sandeel. In 2003 hyperiids were the dominant group of prey (63% by mass, 81% by frequency of occurrence), with herring contributing about one-fifth of the total.
In the Norwegian Sea, there were considerable variations in forage ratios among years. Highest FRs were in 1998 and 2002 (1.36 and 1.14), with herring as the most important prey. The lowest FRs were in 1995, 1996, and 2000 (0.55, 0.57, and 0.46, respectively) and in those years the number of herring observed in the stomachs was small (1995 and 2000) or absent (1996). The remaining years all had FRs around 0.8, with herring varying from 19% to 63% by mass (Table 4, Figure 6c). There was a significant positive correlation between FR and the abundance of 0-group herring in post-smolt stomachs (Figure 7a; Spearman rank correlation coefficient, rS = 0.85, p < 0.01), and also between FR and year-class strength in the Norwegian spring-spawning herring stock (Figure 7b; rS = 0.80, p < 0.01).
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Discussion |
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The occurrence of post-smolts is closely associated with the North Atlantic Current (NAC), so their diet will depend on the potential prey species carried along by that current system. The slope current transports large numbers of blue whiting larvae from the spawning areas west of the United Kingdom into the North Sea and the Norwegian Sea (Bailey, 1982), and an increasing trend in recruitment of the blue whiting stock has been observed from around 1995 (ICES, 2005). Blue whiting was the dominating post-smolt prey in the southwest area, particularly in 1995. The time-series of the blue whiting spawning-stock biomass (SSB) indicates an increase in stock size during the study period (ICES, 2005), making it reasonable to assume that blue whiting has provided a significant and increasing food source for post-smolts following this western European migration route.
Sandeel has traditionally been very abundant in the North Sea and is known as an important food source for a large number of both aerial (Furness and Tasker, 2000) and aquatic predators in the area (Pierce et al., 1991; Hammond et al., 1994; Greenstreet, 1996; Haug et al., 1996). The limited data set available from the North Sea suggests that 0-group sandeel are, indeed, an even more important food source for post-smolts in the North Sea than in the southwest area. Sandeel stocks have supported the largest industrial fishery in the area during the past decade, with annual catches exceeding 1.1 million tonnes during the late 1990s (ICES, 2006a). Recently, however, there has been an abrupt drop in stock size (ICES, 2006a), so it remains to be seen if such a significant reduction of an important food source will affect the growth and survival of salmon migrating through the North Sea, or whether alternative food sources will be utilized.
In some years, significant quantities of 0-group herring have been observed in the Norwegian Sea between Norway and Jan Mayen Island in late summer (Figure 1). Large year classes of larvae in that area have been observed (e.g. 1950, 1991, 1992, and 2002 year classes; JCH, pers. comm.). Consequently, in 2002, the 0-group of herring were more abundant than normal in the areas where the post-smolts were captured, and the availability of an ample food source is probably the reason for the strikingly high condition factor of post-smolts that year. The high forage ratio in 2002 and the significant positive correlation found between the forage ratio and the proportion of herring in the stomachs, and between the forage ratio and herring recruits, further support a positive relationship between available fish prey and condition factor of post-smolts.
There are several reports from the open ocean of post-smolts and larger salmon feeding on pelagic amphipods of the genus Themisto (Jacobsen and Hansen, 2000). The hyperiid amphipod Themisto abyssorum contributes significantly to zooplankton biomass in the central and western parts of the Norwegian Sea in May and June (Dalpadado et al., 2000), and the same species is distributed in Atlantic and mixed Subarctic waters (Melle et al., 2004). T. abyssorum (3–7 mm long) constitute a substantial part of the herring diet in Atlantic waters in the Norwegian Sea in July and August (Dalpadado et al., 2000), corresponding with the size distribution of this prey item eaten by post-smolts in the same months.
Krill are widely distributed in the Norwegian Sea, the greatest biomass occurring at depths of 200–600 m in Atlantic and mixed Arctic waters (Dalpadado et al., 1998). They make diurnal vertical migrations towards the surface at night, mainly the young stages (Zelickman et al., 1979; Melle et al., 2004), and as indicated by the relatively high occurrence and simultaneous low weight of krill in the stomachs in our study, post-smolts seem to prey mainly on younger stages. While krill appear to be more common than amphipods as food for blue whiting, herring, and mackerel in Atlantic waters in summer (Timokhina, 1974; Mehl and Westgård, 1983; Murta et al., 1993; Bjelland and Monstad, 1997; Dalpadado et al., 2000), the opposite seems to be the case for post-smolts (this study) and for pre-adult and adult salmon (Jacobsen and Hansen, 2001). This might be a consequence of prey selection (Jacobsen and Hansen, 2001), allowing post-smolts to utilize a food source not fully exploited by the large pelagic stocks (i.e. amphipods), or perhaps the spatial and temporal distribution of krill may render them less accessible to post-smolts than to other predators.
Copepods (mainly Calanus finmarchicus) are the most abundant zooplankton by weight in Atlantic and mixed Arctic waters in the Norwegian Sea in summer (May–July). They concentrate in the upper 50 m of the water column (Dalpadado et al., 2000; Melle et al., 2004), and serve as an important part of the diet of several pelagic fish species, though according to our results they are not important prey of post-smolt Atlantic salmon. Salmon do not have well-developed gillrakers as do herring and mackerel, so are unable to switch to filter-feeding when preying on small prey.
Given the importance of the 0-group of some large fish stocks as food for post-smolts, it could be of interest to assess the effects of declines in one or more of these stocks on the feeding and growth of Atlantic salmon. For example, a historical low spawning-stock biomass of sandeel in the North Sea was observed in 2004, and the stock is presently classified as having reduced reproductive capacity (ICES, 2006a). Whether the post-smolts are able to compensate for such a strong reduction in a key food source is uncertain. According to this study, herring constitute a key food source in the Norwegian Sea. However, the best production period for Atlantic salmon on record, the 1970s (ICES, 2004), coincided with the collapse of Norwegian spring-spawning herring (ICES, 2005). After spawning between late February and early March, adult herring leave the spawning grounds and, in May, are found over large areas of the Norwegian Sea feeding close to the surface in the warm eastern areas and deeper in the western parts (Misund et al., 1998; Dalpadado et al., 2000; Holst et al., 2004). In June, herring move farther west and north, feeding generally closer to the surface than in May. Consequently, when post-smolts enter the Norwegian Sea in June and July most of the area has already been utilized by herring for 1–2 months. The disappearance of 5–10 million tonnes of adult herring stock may therefore have resulted in improved availability of plankton for young salmon. The fact that such a small herring stock co-occurred with the most productive period of Atlantic salmon raises the question as to whether herring as a competitor is a much more important factor in salmon growth and survival than herring as a food source.
Recent findings suggest a relationship between post-smolt growth and sea surface temperature, as well as between growth and survival (Friedland et al., 2000, 2005). There is little doubt that post-smolts are influenced by both the thermal regimes and production in the oceanic areas of the Northeast Atlantic (Beaugrand and Reid, 2003; Holm et al., 2004), but it is also clear that we lack essential information on how variations in the biological and physical environment influence growth and mortality of salmon. Such information could be important not only for enhancing knowledge of the ecology of the species, but also for creating models for use in salmon management.
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Acknowledgements |
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We thank the crew of the chartered fishing vessels and RVs "J. Hjort", "G. O. Sars", and "M. Sars" of the Institute of Marine Research, Bergen, Norway. Eilert Hermansen is thanked for help with the stomach analyses and Valantine Anthonypillai for assistance with mapping cpue. The work was supported by the Norwegian Research Council. Finally, we gratefully acknowledge the helpful inputs of editor Pierre Pepin and two anonymous referees.
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References |
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