ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on June 27, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(7):1161-1174; doi:10.1093/icesjms/fsn103
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The distribution of chondrichthyans along the northern coast of Norway
1 Norwegian College of Fishery Science, University of Tromsø, Breivika, N-9037 Tromsø, Norway
2 Institute of Marine Research, N-5817 Bergen, Norway
Correspondence to M. Aschan: tel: +47 776 46953; fax: +47 776 46020; e-mail: michaela.aschan{at}nfh.uit.no.
Williams, T., Helle, K., and Aschan, M. 2008. The distribution of chondrichthyans along the northern coast of Norway. – ICES Journal of Marine Science, 65: 1161–1174.The relationship between temperature, latitude, and depth and the distribution and relative abundance of chondrichthyans along the northern coast of Norway was examined based on catches made in scientific trawls north of 62°N from 1992 to 2005. It appears that Chimaera monstrosa, Etmopterus spinax, Squalus acanthias, and Galeus melastomus were more abundant in the south, and Amblyraja radiata more common in the north. Between 1992 and 2005, the distribution and relative abundance did not appear to change significantly, although average water temperatures rose during the period. Current fishing levels do not appear to be impacting the populations of the more common species, but the status of species rarely found in the survey catches is unclear.
Keywords: abundance, bottom trawl survey, Chondrichthyes, distribution, elasmobranchs, Norway, shark, skate
Received 4 February 2008; accepted 16 May 2008; advance access publication 27 June 2008.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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There is increased awareness that stocks of chondrichthyans are particularly susceptible to overexploitation as either target or bycatch species. As large-bodied species with few natural predators when fully mature, they have not evolved strategies to withstand rapid changes in mortality (Stevens et al., 2000). To varying degrees, all these species are slow-growing, have a long lifespan and low fecundity, and attain sexual maturity at a late age (Holden, 1977; Walker and Heessen, 1996). Population growth is influenced by juvenile survivorship and age at maturity (Sosebee, 2005). Although theoretical mechanisms of density-dependence have been looked at, empirical evidence of chondrichthyans (elasmobranchs) is limited and often confounded by methodological issues (Ellis et al., 2008). Their limited ability to compensate for being depleted has often been exemplified by the poor record of sustainability by fisheries that have targeted them (Stevens et al., 2000) and by elasmobranch stocks that have declined either unnoticed or unchecked. The reasons for these stock declines have been described by a number of authors (Bonfil, 1994; Dulvy et al., 2000; Stevens et al., 2000; ICES, 2006).
Fisheries research has traditionally focused on the more commercially important teleost and shellfish species, and little research has been undertaken on chondrichthyans. Catch and landings data from commercial fisheries are often poor because of a general lack of species-specific recording (Johnston et al., 2005), and bycatch data only became more available recently. As a result, even the most basic data are unavailable for quantitative studies of the stock status of most chondrichthyans, including those in the Northeast Atlantic (ICES, 2006). The uncertainties in historical total landings and bycatch data attributable to the common practice of recording catches generically, e.g. as "dogfish and hounds", rather than by species, make fishery-independent data from surveys an important source for studying the distribution and relative abundance of chondrichthyans. To varying degrees, research has been undertaken throughout much of the ICES Area (Walker and Hislop, 1998; Daan et al., 2005; Ellis et al., 2005a), and studies have been conducted along the coast of southwestern Norway (Skjæraasen and Bergstad, 2000, 2001), and northwards into the Barents Sea (Dolgov, 1997, 2006; Dolgov et al., 2004, 2005a). However, the chondrichthyan species along the northern coast of Norway have received little attention and are poorly understood.
Norwegian fisheries targeted Greenland shark (Somniosus microcephalus) until 1960 and basking shark (Cetorhinus maximus) until 2006 (ICES, 2006). Other chondrichthyans have not been and are not currently targeted by commercial fisheries operating along the northern coast of Norway, but are taken as bycatch in the coastal fishery. The coastal fleet consists of vessels using a variety of gears, including gillnet, longline, trawl, Danish seine, handline, and pots. Gillnet and longline fisheries targeting demersal fish (e.g. cod, Gadus morhua, and haddock, Melanogrammus aeglefinus) generate the bulk of the chondrichthyan bycatch (Table 1), and management strategies are in place to minimize the bycatch of undersized commercial species, though not relating directly to chondrichthyans (Nakken, 2003). The introduction of sorting grids (Nordmøre grid) in the shrimp fishery in 1992 reduced the bycatch significantly, and only juvenile chondrichthyans (generally <25 cm) have been caught since (Reithe and Aschan, 2004).
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Climate may also play a part in determining the biogeographical distribution of the group. Therefore, studies on the distribution and movement of elasmobranchs should examine environmental parameters associated with the distribution of the various species (Pawson and Ellis, 2005). Dolgov et al. (2005a) suggested that the distribution of various skate species in the Barents Sea appeared to be related to sea temperature. Since the 1990s, there has been a marked increase in sea temperature, particularly in the southern part of the coast of northern Norway (Pawson and Ellis, 2005). Many chondrichthyans found along this coast are close to their geographical limits, so changes in environmental conditions may be expected to affect their local abundance.
The aim of this study is (i) to identify species present along the northern coast of Norway between 1992 and 2005, (ii) to describe their distribution and abundance, (iii) to identify significant spatial or temporal differences between species, and (iv) to evaluate whether changes observed in distribution and abundance over time are affected by sea temperature. Such information will, we believe, help to resolve the issue of identifying appropriate stock units for management.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The study area consists of fjords and offshore banks along the northern coast of Norway from south of Ålesund (62°00'N 4°50'E) to the Russian border near Kirkenes (69°50'N 30°50'E; Figure 1). Fjords north of 68°N are mainly shallower than 300 m, though those to the south are generally deeper. The coastal banks outside the fjords range in depth from 50 to 400 m (Berg and Albert, 2003). Despite the high latitude, sea temperatures are generally warmer than in other northern coastal areas because of the influence of the Norwegian Current, a branch of the Gulf Stream that flows northeast along the coast. As the current passes through higher latitudes, there is an overall reduction in sea temperature. Temperatures are not constant and fluctuate in short- and long-term intervals (Gyory et al., 2005), and average sea temperatures have increased over the past century (Berstad et al., 2003).
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Temperature data
During the period 1935–1947, several permanent hydrographical sampling stations were established along the Norwegian coast by the Institute of Marine Research (IMR, 2007). Temperature data from four stations, Bud (62°56'N 6°47'E), Eggum (68°22'N 13°38'E), Ingøy (71°08'N 24°01'E), and Vardø (70°45'N 31°03'E), were used in this study (Figure 1). Water temperature and salinity were measured using CTD sensors deployed from research vessels. Approximate bottom temperatures were measured as close to the seabed as the equipment would allow (generally within 10 m) and registered to an accuracy of 0.01°C. Annual temperatures at each station were calculated as the mean of quarter-year values.
Survey data
The survey data were from the annual combined trawl and acoustic surveys conducted each autumn by the Norwegian Institute of Fisheries and Aquaculture Research (Fiskeriforskning) from 1992 to 2001 and by IMR during the period 2002–2005 (Table 2). The main aim of these coastal surveys was to investigate commercial species such as coastal cod, haddock, saithe (Pollachius virens), and juvenile herring (Clupea harengus).
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At the start of the survey in 1992, the intended survey area was divided into three sections to determine the feasibility of the survey and to facilitate the development of a practical design. One section was intensely sampled each year, the northern section in 1992, the central section in 1993, and the southern section in 1994 (Table 2). Since 1995, the entire area has been sampled annually. The surveys then began at the northeastern limit of the area, covered the Norwegian coast to 62°N, and lasted
30 d (Figure 1). Sampling was evenly distributed along the coast, and included fjords and offshore areas near the coast (Figure 1). Sampling stations were not selected randomly, because the seabed in fjords and over the shelf is often too rough to permit trawling (Berg and Albert, 2003). The same stations were approximately fixed for each survey from 1995 to 2005, although poor weather conditions or technical difficulties resulted in some stations occasionally being omitted. Catches were considered reasonably representative of substrata suitable for trawling at 30–700 m. The deepest average depth trawled was around 65°N (Figure 2).
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The sampling trawl was a Campelen 1800 shrimp trawl with a 30-m headline, 19-m groundrope, 80–42-mm knot-to-knot stretched mesh in the body, and 20-mm standard mesh size in the inner net (Table 2; Aschan and Sunnanå, 1997). The gear had 40-m bridles and rock-hopper groundgear, with eight steel spacers between 14 rubber discs. Sensors monitored trawl geometry, and strapping constrained the distance between doors to
47 m while trawling (Aschan and Sunnanå, 1997). At this door spread, the silt plume was directed towards the trawl wings and considered to maximize the herding effect between doors and net.
Species identification
All species were identified, counted, and weighed. In 14 of the trawl samples taken between 1997 and 2004, all the skate species (Rajidae) were grouped and recorded generically, so these observations were excluded from this study. Originally, the names were in Norwegian, according to the species list in the IMR quality control system (Mjanger et al., 2004), but the species list of 2004 included Latin names. These names have been monitored and, where appropriate, updated to valid scientific names according to the Integrated Taxonomic Information System (ITIS, 2008).
The reliability of the results from this study depends on the species identifications being accurate and consistent. Unfortunately, no voucher specimens were collected because the main target of the survey was the commercially important teleosts. Therefore, uncertainties in species identification were analysed during the 2006 survey. Participating scientists were observed sorting and identifying fish, then interviewed to understand better the limitations of the identification procedures, especially for skates, which are a problematic taxonomic group (e.g. Raja montagui is sometimes confused with Raja brachyura, according to J. R. Ellis, pers. comm., and Raja clavata and Amblyraja radiata are often confused—Daan, 2001). Because of the uncertainties involved in modifying the raw data, changes in identification (described by Williams, 2007) were limited to clear inconsistencies that were demonstrated between personnel shifts during some surveys. Individual A. radiata may have been misidentified as R. clavata during the 1990s, but R. clavata has rarely been taken, then mainly south of 68°N (W. Richardsen, pers. comm.). However, we cannot confirm these recordings, so have excluded R. clavata from our analyses.
Abundance and distribution mapping
The estimated spatial distribution for a species was based on data from the entire survey period (1992–2005). Abundance was expressed as number of individuals km–2 at each station. Abundance was estimated based on the area trawled at each station using the method of Jakobsen et al. (1997):
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s denotes the abundance in number of fish km–2 at sample station s, fs the number captured (frequency) at sample station s, and as the area swept (km2) at sample station s (nautical miles trawled x 1.852 x 0.047 km). The door spread of 47 m was assumed to be the upper limit of the mean effective catching width of the gear. A more precise estimate of catch efficiency was beyond the scope of this study.
The annual mean catch (number km–2) was used to compare species abundance. Species with an annual mean catch >5 animals km–2 were grouped as common species, and included for further statistical analysis. Species with lower catch rates were either grouped as infrequent, if previously recorded in the survey area, or rare if not recorded previously.
Pethon (2005) and FishBase (Froese and Pauly, 2007) give previously estimated distributions. The two sources were generally consistent, although they differed for Dipturus batis, Leucoraja fullonica, Dipturus linteus, Bathyraja spinicauda, and Galeus melastomus. All five of these species are described by Pethon (2005) as having a more northerly distribution than given in FishBase (Froese and Pauly, 2007). In our opinion, the species distributions of Pethon (2005), which are updated based on recent data, were more accurate than those in FishBase, so were taken as the standard distributions for the present study.
Statistical analysis
For seven common species, the relative abundance (number km–2) in each trawl sample was used to assess the statistical significance of temporal and spatial variation in abundance. Year, depth, and area were the independent variables. The survey area was divided into seven subareas by degree latitude from 62 to 69°N, and a northern region (from 69 to 71°N) was divided into two subareas east and west of 25°E (Figure 1). Samples were identified by 50-m depth interval, encompassing the depth range covered by the survey. Average abundance for each species was calculated, and differences in abundance were tested against the three variables, depth, latitude, and year, using one-way single factor ANOVA. The significance level was set at p = 0.05.
A constrained (canonical) correspondence analysis (CCA) was run in R 2.5.0 (R Development Core Team, 2007) using the vegan package (Oksanen, 2007). As the species data contained many zeros, they were analysed using unimodal methods (ter Braak and Verdonschot, 1995). The ten most common species were included in the analysis, and their abundance estimates were log(x + 1) transformed. Potential explanatory variables were longitude, latitude, depth, and year of sampling. Shifts in abundance of each species were shown as percentages of deviation from the average abundance within the total survey area and presented with respect to the strongest explanatory variable.
Correlations were tested for all species with distributions significantly correlated with latitude. The locations of the four temperature stations determined the areas 63°N, 68°N, West (69–71°N), and East (69–71°N; Figure 1) used for assessing correlations between abundance and temperature. Spearman’s rank-order correlation () was used to test whether latitudinal or annual differences in temperature significantly influenced the distribution of nine common species.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Abundance and distribution
During the survey period 1992–2005, 18 species of Chondrichthyes were recorded at 1932 stations (Table 3). The most diverse orders observed were skates (Rajiformes, 13 species) and dogfish sharks (Squaliformes, 3 species). In contrast, only one species of catshark (Scyliorhinidae) and rabbitfish (Holocephali) was identified in the samples. Six species were observed across the entire latitudinal range (62–71°N), and many species seemed to have a boundary in the north (Figures 3 and 4). Mean annual catch rates and frequency of occurrence for each species reveal that Chimaera monstrosa, Etmopterus spinax, G. melastomus, A. radiata, Squalus acanthias, and Dipturus oxyrinchus were the main species (Table 4). For many species, abundance sometimes varied greatly between years, but there were no obvious negative or positive trends, indicating no recent changes in abundance of the more common species (Figure 5). One exception was Rajella fyllae, which exhibited an increasing trend in the data. Annual changes in abundance were only significant (p < 0.05) for A. radiata (Table 5), which increased significantly between 2002 and 2003, decreased by the same extent between 2004 and 2005, but had no apparent long-term trend (Figure 5). Latitude was a significant factor (ANOVA, p < 0.05) affecting the abundance of all common species (Table 6).
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Chimaera monstrosa and E. spinax (Figures 3a and b) were observed in all areas except in the far northeast (East 69–70°N). Abundance was greatest in the south, where catch rates were occasionally >2000 fish per haul, suggesting an aggregating behaviour by the two species. Distributions of G. melastomus, S. acanthias, and D. oxyrinchus (Figures 3c, e, and f) appeared to be constrained to areas south of 68°N, with most catches south of 65°N. Catches of G. melastomus of >200 animals in each of 10 hauls highlights the aggregating nature of this species. Amblyraja radiata was the dominant skate species and was caught throughout the survey area, but in greater abundance in the north. The distribution of R. fyllae was similar (Figures 3d and g).
Abundance appeared to be greatest for C. monstrosa at 450–550 m, for G. melastomus deeper than 150 m, for A. radiata at 600–650 m, and for R. fyllae and D. batis shallower than 500 m (Table 4; Figure 6). However, differences in depth-dependent abundance were only statistically significant for G. melastomus (Table 7), with greatest abundance in the 500-m interval. The three other common species were more evenly distributed with respect to depth (Figure 6).
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The constrained correspondence biplot shows the species scores that may be taken as the optimal location for nine species in the environment field spanned by the site scores (Figure 7). The constrained axis CCA1 (eigenvalue 0.457, 88%) has a much larger explanatory value than CCA2 (eigenvalue 0.049, 9%). Latitude seemed to structure the chondrichthyan assemblage into three groups (Figure 7), a northern component consisting of A. radiata and R. fyllae, a central component with C. monstrosa, D. batis, E. spinax, and L. fullonica, and a southern component consisting of D. oxyrinchus, G. melastomus, and S. acanthias. Relative abundance along the latitude shows the same pattern for species of each group (Figure 8). Depth and year were of little importance in defining species distribution (Figure 7).
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Of the four species ranked as infrequent, D. batis was observed along the entire coast at depths of 85–425 m, and L. fullonica from 77–512 m in all areas except the eastern sector of 69–70°N (Table 4; Figure 4). Dipturus nidarosiensis was found as far north as 68°N at depths of 140–590 m, whereas its known distribution was primarily south of 65°N (Pethon, 2005). A single S. microcephalus was caught 480 m deep at 69°10'N 16°19'E in 1993.
From 1992 to 2005, six rare species were reported, extending the distributions given by Pethon (2005). Raja brachyura, R. montagui, and Leucoraja circularis were all caught south of 64°N, but at higher latitudes than previously documented (Pethon, 2005). Of these three, L. circularis was caught most frequently and in greatest number, 23 animals in six trawls. The depths of capture for L. circularis were 88–244 m, and for R. montagui 63–120 m, and four R. brachyura were caught at 99 m. Totals of 11 B. spinicauda and nine Amblyraja hyperborea were caught in trawls north of 67°N at depth ranges of 48–410 m and 125–620 m, respectively, and one D. linteus was caught at 588 m off Lofoten at 68°N.
Abundance and temperature
Except the eastern sector at 69–71°N, all areas appeared to show an overall increase in sea temperature between 1992 and 2005 (Figure 9). Except R. fyllae, the abundance of chondrichthyans showed no obvious trend during the study period (Figure 5). Therefore, the Spearman rank analysis showed no significant correlations between annual sea temperature and abundance for each species in the areas 63°N, 68°N, 69–71°N (West), and 69–71°N (East). Bottom temperature decreased in a northward direction along the coast. Latitudinal shifts in abundance for C. monstrosa, E. spinax, G. melastomus, and S. acanthias showed positive correlations ( > 0.9) with bottom temperature, whereas A. radiata abundance showed a negative correlation ( > –0.9) with temperature.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Survey data uncertainty
The surveys were designed primarily to assess the commercially important teleosts, so chondrichthyans were not taken into account in the design. Consequently, it is uncertain how accurately the catch rates reflect the relative abundance of the various species (Kotwicki and Weinberg, 2005). As trawl catchability varies with bottom type and species, there is little information available for estimating absolute stock size (Bonfil, 1994; Abella and Serena, 2005; Daan et al., 2005; Dolgov et al., 2005a). Moreover, species that favour hard rocky substrata (e.g. D. batis) are likely to be underrepresented because most trawling was conducted on seabed that could be trawled, i.e. was fairly smooth.
Species identification issues are important, although this was corrected where possible (Williams, 2007). Misidentification of skates is common (Daan, 2001), and except the more visually distinct species such as D. nidarosiensis, there is still concern regarding the validity of the skate identifications. The main uncertainty in our results is the soundness of the estimated distribution of the infrequent and rare species such as R. clavata (which was excluded from the analyses). For the common species, the data are considered to be valid for describing their relative abundance and distribution.
An update on chondrichthyan distributions
The porbeagle (Lamna nasus), which is recorded as a bycatch in the area (Table 1), was not caught at all during the coastal surveys, because the species is rarely taken by trawl (Daan et al., 2005). Basking sharks (C. maximus) are caught as a bycatch in gillnet fisheries, but were not in our survey data.
From 1992 to 2005, the distribution and abundance of the common species appeared to remain stable (Figure 5). Latitudinal trends in distribution correlated well with previous distributions given by Pethon (2005). Shark species and C. monstrosa were clearly more abundant south of 65°N. Chimaera monstrosa and E. spinax appeared to be the most abundant species, including north of 70°N (Figure 3). Chimaera monstrosa has recently also been observed in the southern Barents Sea (Dolgov, 2006; Byrkjedal and Høines, 2007). Amblyraja radiata is uniformly and widely distributed, and was the dominant skate, followed by R. fyllae. The dominance of these two species agrees with studies undertaken in the neighbouring Barents Sea, northeastern North Sea, and Norwegian Sea (Skjæraasen and Bergstad, 2001; Dolgov et al., 2005a; Dolgov, 2006; Byrkjedal and Høines, 2007). In the south, D. oxyrinchus appeared to be more abundant than R. fyllae. This may be a fairly localized population, because D. linteus replaces D. oxyrinchus in the skate assemblage that dominates the neighbouring northeastern North Sea and Norwegian Sea (Skjæraasen and Bergstad, 2001). Raja clavata has been recorded as far north as the Barents Sea (Hognestad and Vader, 1979; Fossheim et al., 2006; Byrkjedal and Høines, 2007), but was not observed during Russian surveys from 1996 to 2007 (A. V. Dolgov, pers. comm.). We believe that this species may be a sporadic visitor to the whole northern coast of Norway and may also be taken occasionally in the southwestern Barents Sea. However, the real distribution of R. clavata needs further clarification because identification of this species in our data seems to have been biased; voucher specimens are required to confirm its occurrence in northern Norwegian waters.
Because of a lack of knowledge and infrequency of recordings of the rare and infrequent species, it is impossible to be certain of any distribution shifts. Our observations show that R. brachyura, L. circularis, and R. montagui, commonly associated with the North Sea and Atlantic areas south of 62°N (Dulvy et al., 2000; Pethon, 2005; Froese and Pauly, 2007), may all be present as far north as 64°N. This is probably not attributable to a change in distribution, but rather because of poor data historically. Amblyraja hyperborea and B. spinicauda are associated with offshore areas (Mahon et al., 1998; Pethon, 2005; Fossheim et al., 2006; Byrkjedal and Høines, 2007), but were found closer to the coast in our study.
Spatial distribution
Amblyraja radiata has a wide and uniform distribution throughout the study area, with biomass increasing to the north, and is found also in the Barents Sea (Dolgov et al., 2005a; Byrkjedal and Høines, 2007). Annual distribution maps show that the distributions of C. monstrosa, E. spinax, G. melastomus, and S. acanthias are patchy (Williams, 2007). The large catches (>500 animals in a single haul) underscore the aggregating behaviour of these species. Squalus acanthias occurs often in shoals of the same sex and/or size (Ellis et al., 2005b; Stenberg, 2005), similar to G. melastomus, for which there are also bathymetric patterns (Massuti and Moranta, 2003; Calis et al., 2005; Coelho et al., 2005). These uneven distributions can be linked to the availability of suitable bottom substrata or food availability, as has been suggested to explain similar distribution patterns in the neighbouring North Sea and Skagerrak (Skjæraasen and Bergstad, 2000). Tagging studies indicate little mixing of S. acanthias between northern and southern areas of the North Sea (Aasen, 1962; Holden, 1967), and Holden (1968) claimed that the Norwegian–Scottish and Channel populations are separate stocks. Despite assertions of transatlantic migration (Holden, 1967; Templeman, 1984), recent analysis of tag returns indicates that Northeast and Northwest Atlantic stocks should be managed independently. No decline over time was observed for S. acanthias in our study area, but declines have been documented for the North Sea, Celtic Sea, and off Northwest Scotland (Daan et al., 2005; Dobby et al., 2005; Ellis et al., 2005a; ICES, 2007). Those studies, however, were based on surveys that started in the 1970s and 1980s. The few large catches (7–19 fish) indicate that, although D. oxyrinchus is relatively scarce, local aggregations may occur.
The abundance of some species appeared to be depth-dependent. Dipturus batis and R. fyllae were confined to water shallower than 500 m (Table 4). However, R. fyllae probably exists over a wider depth range (Dolgov et al., 2005a; Jørgensen et al., 2005; Pethon, 2005) and probably prefers deeper waters in the south (Skjæraasen and Bergstad, 2001). As observed in other areas, G. melastomus preferred depths deeper than 150 m (Magnussen, 2002; Massuti and Moranta, 2003; Rey et al., 2004; Costa et al., 2005; Serena et al., 2006). Chimaera monstrosa was found mainly in deeper water (>400 m), but may migrate to shallower water in spring and summer to deposit egg capsules (Bristow, 1992; Pethon, 2005). Etmopterus spinax was found in both deep and shallow water in the north and the south of the survey area, and the depth range was not clearly dependent on latitude in the survey area, as suggested by Pethon (2005). Amblyraja radiata and D. oxyrinchus were present at all depths.
Species assemblages
The northern, central, and southern species assemblages shown by the constrained correspondence biplot revealed that factors linked to latitude play an important role in determining species distribution and abundance. The sharp decline in abundance north of 65°N for species in the southern and central group was particularly noteworthy because it suggests a latitudinal change in conditions, resulting in a shift in biodiversity. The 65°N region is, on average, deeper than the other areas surveyed, and the deep trench may function as a biogeographical barrier (Figure 2). However, depth alone does not appear to explain this trend, and the Lofoten Peninsula may function as a physical barrier.
Temporal and spatial shifts with regard to sea temperature
Because of the small number of stations with temperature measurements, sea temperature had limited ability to explain shifts in abundance in our data. Skate distribution in the Barents Sea has been linked to changes in bottom temperature (Dolgov et al., 2005a). In some areas of the Northeast Atlantic, the increase in sea temperature in recent years has coincided with a gradual northward shift in the distribution of some species (Perry et al., 2005; Dolgov, 2006). So far, though, there appear to have been no such shifts along the northern coast of Norway. Demersal fish species such as cod seem to be able to adapt to moderate changes in their ambient temperature and do not necessarily respond to it with a change in distribution (Ottersen et al., 1998), at least to the same extent as do pelagic fish, such as blue whiting (Micromesistius poutassou), capelin (Mallotus villosus), and herring (Bergstad et al., 1999; Toresen and Østvedt, 2000; Fossheim et al., 2006). Depth and temperature are unlikely to be the only factors involved in determining the apparent trends in distribution and general stability in abundance of each chondrichthyan listed here. As the distributions of the southern and central species assemblages have not expanded north in response to ocean warming, other factors may be playing a role in determining abundance and distribution.
Little is known about how fisheries have impacted chondrichthyan populations along the northern coast of Norway. However, we know that they provide a significant and probably underestimated bycatch in gillnet and longline fisheries (Table 1). It is therefore reasonable to assume that the demersal fisheries in particular have a negative impact on chondrichthyan stocks (Bonfil, 1994; Stevens et al., 2000; Dolgov et al., 2005b; Drevetnyak et al., 2005). Any major changes in population structure in response to fishing probably took place in the survey area before 1992, when the most significant development in the fisheries there took place. As mentioned, the introduction of sorting grids in the shrimp fishery reduced bycatch to include only juveniles. This has not resulted in an obvious increase in chondrichthyan abundance, but may be one reason the stocks are not decreasing. The populations of A. radiata and R. fyllae in the neighbouring Barents Sea appear to be stable at current fishing levels (Drevetnyak et al., 2005), so both species are probably similarly tolerant to current fishing pressure within our study area.
The processes influencing the population dynamics of the chondrichthyans described here are undoubtedly complex. Geographical barriers, particularly the deep trenches in the region of 67°N (Vestfjord) and the Lofoten Peninsula, could restrict passage and inhibit an expansion in the distribution of a species. Also, chondrichthyans are generally long-lived, and their distribution may to some degree be the consequence of territorial behaviour and a slow rate of migration. For example, some skates do not migrate great distances (Hunter et al., 2005) and may show clear gaps between areas of high concentration, perhaps indicating that they may form local stock units (Daan et al., 2005). However, A. radiata seems to be an exception, because the continental shelf edge apparently does not present a barrier to its migration, and there are no significant population gaps in the North Atlantic generally (Chevolot et al., 2007). This is one explanation for the uniform distribution of A. radiata in the study area and in the Barents Sea.
Although chondrichthyans are potentially vulnerable to fisheries (Stevens et al., 2000; Priede et al., 2006), the current populations in the study area of the more common species appear not to be adversely affected by bycatch at the current levels of fishing activity (although historical estimates of abundance are not available). Stocks of C. monstrosa, E. spinax, S. microcephalus, G. melastomus, S. acanthias, D. batis, and recently also A. radiata have declined in the North Sea (Daan et al., 2005; Ellis et al., 2005a; Jones et al., 2005; ICES, 2007), but there has been no such decline in our area of interest. This may be due to the lower effort in the demersal fishery in general and in the elasmobranch fishery in particular along the northern coast of Norway over the period 1992–2005. However, because of the limitations of our data, we cannot be certain that this is the case for the less frequently caught species, especially given the comparatively short period covered by the study. For example, species such as S. microcephalus are reported to have had longer term declines in Norwegian waters (Ruud, 1968).
As species identification is currently difficult and potentially inaccurate, effort should be put into quality controlling available taxonomic keys for northern areas, especially for skates, which are known to have high morphological interspecific variability (Serena et al., 2005). To address the distribution of skate species in Norwegian waters, future surveys should place more emphasis on species identification, including the collection of voucher specimens, and the development of appropriate field identification guides should be given high priority. Knowledge of chondrichthyan species along the coast of Norway remains limited compared with that for the neighbouring North Sea (ICES, 2007) and further work is required.
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Acknowledgements |
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An anonymous referee and Jim Ellis provided valuable advice and recommendations for improving the manuscript, and Frøydis Strand kindly helped us finalize the figures. We thank Erik Berg and Ivan Ahlquist (IMR) for making the fish abundance data available, and Øyvin Strand (IMR) and all others who were involved in helping to find suitable temperature data. Andrey Dolgov supported us through interesting discussions. Finally, we thank the crew of RV "Jan Mayen" for their support.
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