© 2005 International Council for the Exploration of the Sea
Geographic variation of golden redfish (Sebastes marinus) and deep-sea redfish (S. mentella) in the North Atlantic based on otolith shape analysis
Federal Research Centre for Fisheries, Institute for Sea Fisheries Palmaille 9, D-22767 Hamburg, Germany
*tel: +49 40 38905 228; fax: +49 40 38905 263. e-mail: christoph.stransky{at}ish.bfa-fisch.de.
The unresolved interrelationships of North Atlantic redfish stocks have prevented adaptive fisheries assessment and management in the past. Otolith shapes of golden redfish (S. marinus) and deep-sea redfish (S. mentella) were analysed for geographic variation within the entire distribution range in the North Atlantic, in order to evaluate this technique for stock separation. Multivariate analysis of elliptical Fourier shape descriptors revealed high similarity of S. marinus within the central North Atlantic areas (West and East Greenland, Iceland) that were relatively well separated from the Flemish Cap and Barents Sea samples. A similar separation of the Barents Sea area was also observed for S. mentella, whereas the western (Flemish Cap, Davis Strait) and central areas were overlapping to a greater extent. The overall classification rate given by discriminant analysis was poor for both species (<50%) but increased to 72–74% by combining sampling areas to regions (west, central, east). Geographic variation in otolith shapes of both redfish species suggests a separation of the Northeast Arctic stocks (Barents Sea) of both species from the other redfish stocks assessed within ICES and NAFO, whereas similarities observed for the highly migratory S. mentella give reason for integrated management of demersal and pelagic occurrences of this important fisheries resource that straddles the ICES/NAFO boundaries.
Keywords: Fourier analysis, North Atlantic, otolith shape, redfish, Sebastes marinus, Sebastes mentella, stock identification
Received 30 August 2004; accepted 24 May 2005.
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Introduction |
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Redfish of the genus Sebastes provide important fishery resources in the North Atlantic. Among the four species found in the North Atlantic, golden redfish (S. marinus) and deep-sea redfish (S. mentella) are the most widely distributed and commercially exploited representatives, while Acadian redfish (S. fasciatus) are generally limited to the Northwest Atlantic, and Norway redfish (Norway haddock; S. viviparus) are only found in the Northeast Atlantic (Whitehead et al., 1986). S. marinus grows larger than S. mentella and inhabits continental shelves off eastern Canada, Greenland, Iceland, and the Faroe Islands, Norwegian waters, the Barents Sea, and Svalbard, mainly in depths between 100 m and 300 m. S. mentella is generally distributed deeper than S. marinus and found in the pelagic zone of the Labrador and Irminger Sea down to 1000-m, in addition to the areas where S. marinus occurs (ICES, 1998; Saborido-Rey et al., 2004).
Several attempts have been undertaken to investigate the geographic variation of redfish, especially for the highly migratory and widely distributed S. mentella. Body morphometric and meristic characteristics were used by Nagel et al. (1991a), Reinert and Lastein (1992), Saborido-Rey (1994), and Saborido-Rey and Nedreaas (2000) to identify sub-units in the distribution. The geographic variation of infestation by the copepod parasite Sphyrion lumpi was the subject of several investigations (Bakay, 1988, 2000, 2001; Nagel et al., 1991b; Magnússon, 1992; Marcogliese et al., 2003), as well as abnormal external coloration and ectolesions (e.g. Bogovski and Bakay, 1989). A series of biochemical and genetic studies were carried out to relate the observed geographic differences to population structure. Gel electrophoresis of haemoglobin and tissue enzymes (Dushchenko, 1987; Nedreaas and Nævdal, 1989, 1991; Nedreaas et al., 1994; Johansen et al., 2000a) was recently followed by microsatellite DNA analyses by Roques et al. (2002). The few studies on intraspecific variation of S. marinus were also carried out using haemoglobin and enzyme patterns (Nedreaas and Nævdal, 1991; Nedreaas et al., 1994; Johansen et al., 2000b).
In addition to traditional body morphometry, otolith shape analysis has been successfully applied to stock identification of other North Atlantic fish species such as mackerel (Castonguay et al., 1991), cod (Campana and Casselman, 1993; Cardinale et al., 2004), Atlantic salmon (Friedland and Reddin, 1994), haddock (Begg and Brown, 2000), and herring (Turan, 2000). The majority of these studies used Fourier transformations of the outline coordinates (see Lestrel, 1997) to quantify differences between proposed stocks. Although most otoliths are ellipsoidal (Chauvelon and Bach, 1993), only very few preliminary applications of elliptic Fourier analysis (Kuhl and Giardina, 1982) for the identification of species (Petry, 2001) and stocks (Murta et al., 1996; Stransky, 2001, 2002, 2003) have been reported to date.
For fisheries management, a clear separation of stocks, ideally resembling self-sustaining populations, would be desirable. Past efforts to clarify the complex stock structures of redfish, however, all demonstrated high variation within geographic areas or stock units, and relatively weak differentiation between these (ICES, 1998). The existence of separate stocks of S. mentella in the Irminger Sea and adjacent waters has been controversially discussed and raised unsolved problems in the assessment and management of this important fishery resource (Saborido-Rey et al., 2004). In a multidisciplinary approach, species and population structure of redfish was investigated using extensive material from all important distribution areas in the North Atlantic. As part of this project, the two-dimensional outline shapes of redfish otoliths were analysed for differences between species and proposed stocks, using elliptical Fourier analysis and multivariate techniques. This study investigates geographic variation in otolith shapes of S. marinus and S. mentella from the entire distribution area in the North Atlantic in order to provide information on the inferred stock structure of these species.
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Material and methods |
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Sample collection
Otolith samples were taken from S. marinus and S. mentella, collected on research vessels and commercial trawlers operating in various redfish fishing areas in the North Atlantic (Table 1, Figure 1). Aiming at a complete coverage of the distribution area of both species, S. marinus samples could be obtained from fish caught with bottom trawls on the shelf areas of the Flemish Cap, around Greenland, Iceland, and in the Barents Sea, while otoliths from S. mentella were additionally taken from demersal catches in the Davis Strait and around the Faroe Islands, and from pelagic trawls in the southwestern Irminger Sea. Samples were extracted as sagittal otolith pairs and stored dry in paper envelopes or plastic holders, accompanied by individual fish data (e.g. length, weight, and sex) recorded on the envelopes or on separate sampling sheets with the corresponding station data (e.g. station number, position, date, mean trawl depth). For the Flemish Cap and Davis Strait samples, only fork length data were recorded. These were converted to total length using regressions between fork and total length for S. marinus (TL = 1.028FL + 0.075, r2 = 0.999, n = 601) and S. mentella (TL = 1.033FL – 0.038, r2 = 0.998, n = 432) on the Flemish Cap (unpubl. data, F. Saborido-Rey, Instituto de Investigaciones Marinas, Vigo, Spain). Only otoliths from S. marinus measuring over 20 cm and S. mentella measuring 30–40 cm in total length were selected to reduce the variability of the shape descriptors attributable to the different size of the individuals. A multivariate analysis of covariance (MANCOVA) was used to check whether this procedure effectively reduced the area x length interactions for the shape descriptors. Although otoliths were taken as pairs, only samples from the left side were included in the analysis to ensure that one sample represents one fish. The reduced data set included samples from 399 S. marinus and 586 S. mentella (Table 1). Before further processing by image analysis, adhering tissue or blood remains were removed from the otolith surface.
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Image and shape analysis
Otolith outlines were digitized using an image analysis system consisting of a high-resolution monochrome CCD video camera, mounted on a microscope and connected to a PC framegrabber card via BNC video cable. The microscope magnification was adjusted to the size of the otoliths to ensure as high resolution as possible, varying between 25x and 50x. The image analysis system was calibrated in horizontal and vertical direction separately, to avoid possible distortion effects of the lens system. The otoliths were positioned on a microscope slide with the sulcus down and the rostrum to the left in horizontal line to minimize distortion errors within the normalization process. High-contrast video images were produced using transmitted light, delivering dark two-dimensional objects with bright background. The video signal was analysed using OptimasTM 6.51 (Media Cybernetics, 1999) image analysis software. Shape digitization was performed by sampling 1000 equidistant points on each outline, representing the resolution of the video camera. For the export of outline coordinates, OptimasTM macros were applied.
Digitized outline coordinates were forwarded to Elliptical Fourier Analysis (EFA; Kuhl and Giardina, 1982; Rohlf and Archie, 1984), using C++ modules based on the algorithms of Ferson et al. (1985). The methodology of Fourier analysis has been described in detail by several authors (e.g. Full and Ehrlich, 1982; Bird et al., 1986; Lestrel, 1997) and will therefore not be presented here. In general, the EFA represents a fitting of harmonic functions to the original otolith outlines with an ellipse as the first approximation step. The algorithm for normalizing the rotation and starting angle of the outline was modified to account for deviations from the horizontal axis resulting from the positioning of the otolith on the microscope slide. During the EFA, the size, location, and starting point of the object outlines within the two-dimensional space were normalized. Only the first 15 harmonics were used for multivariate analysis, because these were responsible for more than 99% of the shape variation (Lestrel, 1997). This results in 57 Fourier descriptors (FDs) for each sample, consisting of 3 x 14 FDs (harmonics 2–15 each) of the sine and cosine parts of the x-direction and sine part of the y-direction (where FDs of harmonic 1 become constants after the normalization process, see above) plus 15 FDs (harmonics 1–15) of the cosine part of the y-direction. Before analysing the FDs for differences between areas, the distribution of these data was investigated. All FD amplitudes were normally distributed (Kolmogorov–Smirnov test for normality; p > 0.05), so no transformation of the FD data was necessary.
Size correction and multivariate analysis
The correction of morphometric variables by size, i.e. uncoupling of otolith shape and fish length in this case, is crucial for further data analysis (Bookstein et al., 1985). This was accounted for by using the residuals of the common-within group slopes (Reist, 1985; Claytor and MacCrimmon, 1987) of the linear regressions of each FD on fish total length. After size-adjustment, the average residuals by area were compared in a set of multivariate analyses. After the calculation of a Euclidean distances matrix, linked by unweighted pair-group average (Sneath and Sokal, 1973), multidimensional scaling (MDS) was applied as an ordination technique that offers a detailed comparison of between-group differences (Kruskal and Wish, 1978). The so-called stress factor is a measure for the quality of the ordination, values lower than 0.1 being classified as "good", values lower than 0.05 as "excellent", and values lower than 0.01 as "perfect" (Clarke and Warwick, 1994). As a validation of the grouping patterns, the classification success into these groups was tested within linear discriminant analysis (Klecka, 1980; SPSS Inc., 1999). The permutated (jackknifed) classification results also allow validation of the similarities between groups by listing the misclassification into other areas. Differences between sexes were also tested by discriminant analysis.
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Results |
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The MANCOVA showed no significant area x length interactions (p > 0.05) for the length ranges selected for both species. Furthermore, the size correction of the FDs by residuals reduced the correlation between FDs and fish lengths very effectively. Pearson correlation before size-adjustment was up to r = 0.42, with more than two-thirds of the FDs significantly correlated (p < 0.05) with otolith length, while the residuals showed no correlation with otolith length (r < 0.1, p > 0.05). The FD residuals were significantly different between sexes (S. marinus: Wilks'
= 0.772, p = 0.004; S. mentella: Wilks' = 0.794, p < 0.0001). Since the sex ratio was nearly 1.0 for all areas (Table 1), this fact was not paid further attention. Discriminant analysis of the S. marinus samples revealed high misclassification rates (up to 36%) within the central North Atlantic (Iceland, West, and East Greenland) and a separation of this group from the Flemish Cap and Barents Sea samples (Table 2). The Barents Sea samples were correctly classified by 67%, representing the highest classification success of the S. marinus samples. The overall jackknifed classification success, however, was very low (47%). MDS ordination of the average size-corrected FDs by area (Figure 2) supported close relationships between S. marinus from the central areas that separated fish from the Flemish Cap and Barents Sea. A strong similarity could be observed between West and East Greenland.
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Also for S. mentella, the separation of the Barents Sea from all other areas can be derived from a relatively high classification rate (76%; Table 3), while high misclassification rates point to similarities of varying degree between the other areas, especially between Flemish Cap and Irminger Sea (50%), and between Davis Strait, Flemish Cap, and Iceland (33%). With 47% overall jackknifed classification success into sampling areas, the general separation power was as poor as for S. marinus. This general pattern is also apparent in the MDS plot (Figure 3), where the Barents Sea samples are best separated.
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Combining areas to regions (see Table 1) improved overall correct classification of samples to 74% (S. marinus; Table 4) and 72% (S. mentella; Table 5). Classification into regions was similar for both species, with the central and eastern regions being considerably better defined (67–80%) than the western region (<50%). The western region shows high misclassification into the central region (S. marinus: 31%, Table 4; S. mentella: 45%, Table 5).
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Discussion |
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This study revealed high within-area variation in otolith shapes of redfish and only weak signals for geographic separation. Geographic variation within redfish species was investigated in several studies, predominantly focusing on the widely distributed S. mentella. Body morphometric and meristic measurements have been reported to vary considerably between largely distant areas in the North Atlantic (Nagel et al., 1991a; Reinert and Lastein, 1992; Saborido-Rey, 1994), but also within a relatively limited area in the Northeast Atlantic (Saborido-Rey and Nedreaas, 2000). Natural tags such as the infestation by the copepod parasite Sphyrion lumpi (Bakay, 1988, 2000, 2001; Nagel et al., 1991b; Magnússon, 1992), as well as abnormal external colouration and ectolesions (e.g. Bogovski and Bakay, 1989) pointed to relatively weak sub-structuring of S. mentella occurrences. Recently reported parasitological analyses of Marcogliese et al. (2003), however, suggested separation of S. mentella on the Flemish Cap, in the Gulf of St. Lawrence, and in the Laurentian Channel. A pilot study of Reinert et al. (1992), using Cs-137 as population marker, indicated a closer relationship of redfish from Faroese waters to samples from the Barents Sea than to Icelandic samples. Fatty acid analyses of Joensen and Grahl-Nielsen (2004) separate the occurrences of S. mentella around the Faroe Islands into one group related with fish from the southeastern Icelandic shelf and deeper parts of the northeastern Irminger Sea from another group connected to the Norwegian coast.
Several biochemical and genetic studies were carried out to investigate redfish population structure. Earlier analyses of haemoglobin and enzyme patterns of Northeast Atlantic S. mentella showed low intraspecific variation (Dushchenko, 1987; Nedreaas and Nævdal, 1989, 1991; Nedreaas et al., 1994), while a more extensive study indicated a structured picture (Johansen et al., 2000a). The poor geographic classification by otolith shape of S. mentella, as shown in this study, is in accordance with the earlier genetic studies, as well as recent microsatellite DNA analyses by Roques et al. (2002), pointing to a "pan-oceanic" population spanning from the Grand Banks to the Faroe Islands. Although otolith shapes do not necessarily reflect genetic differences, the high individual variation observed in this study prevents differentiation of S. mentella population units in the central North Atlantic. The relatively high discrimination of the Barents Sea samples from the other areas, however, suggests separation of the Northeast Arctic stock from S. mentella in the central and western areas. Some previous studies pointed to uniformity of S. mentella within the Irminger Sea and close relations with the demersal occurrences on the Greenland and Iceland shelves (Saborido-Rey et al., 2004). The existence of two pelagic stock units of S. mentella in the Irminger Sea, as suggested by differences in haemoglobin and enzyme patterns between "oceanic" and (pelagic) "deep-sea" S. mentella (Johansen et al., 2000a), has also been supported by nuclear DNA methods (microsatellites and AFLP; Daníelsdóttir et al., in press). No differentiation between these occurrences in the Irminger Sea was found with respect to otolith shape, on a vertical or a horizontal scale (Stransky, 2002). S. mentella from the Flemish Cap showed considerable similarity to redfish in the Irminger Sea. The relatively low number of samples from the Flemish Cap, however, does not allow a conclusive statement on the connectivity between these areas. As the formation and growth of otoliths, however, could not only be genetically induced (Söllner et al., 2003), but also be influenced by environmental conditions, the observed similarities in otolith shapes might partly indicate similar habitat conditions (Stransky et al., in press).
Although large overlaps between sampling areas were also observed for S. marinus, the separation of central North Atlantic fish (Greenland, Iceland) from those in the Northwest Atlantic (Flemish Cap), and Northeast Atlantic (Barents Sea) is more pronounced than for S. mentella. Since S. marinus is limited to the shelf areas, a clearer differentiation of geographic units would have been anticipated. The considerable similarity between fish from East and West Greenland points to a close relationship between the occurrences on the Greenland shelf. Otolith shapes of S. marinus from Greenland showed high resemblance to the Icelandic samples, suggesting connectivity between the habitats around Iceland and Greenland. The depth preference of S. marinus (usually between 100 m and 300 m) and the lack of a shallow ridge between these shelf areas, however, likely preclude large-scale horizontal migration of S. marinus between Greenland and Iceland. This assumption is supported by genetic studies of Nedreaas et al. (1994) and Johansen (2003), revealing significant differences between samples from Iceland and Greenland. Some exchange of fry during the larval phase, however, could be possible. Unfortunately, no S. marinus samples from the Faroe Islands were available for this study to investigate the affiliation of this area to adjacent and distant areas.
For fisheries management purposes, several stock units were established for redfish by the International Council for the Exploration of the Sea (ICES) and the Northwest Atlantic Fisheries Organization (NAFO). S. marinus is currently divided into two ICES management units, one comprising the occurrences off Greenland, Iceland, and the Faroe Islands (ICES, 2004a), and the other along the Norwegian coast, in the Barents Sea, and off Svalbard (ICES, 2004b). The same management units apply for demersal S. mentella, whereas pelagic S. mentella are currently assessed within ICES and managed by the Northeast Atlantic Fisheries Commission (NEAFC) as one stock. Considering the transboundary distribution of pelagic S. mentella within ICES/NEAFC and NAFO Areas, common management of this relatively new resource is currently being considered (NEAFC, 2001; NAFO, 2002). In the Northwest Atlantic, demersal redfish, not separated by species, are managed within NAFO in the offshore areas east of Newfoundland, on the Flemish Cap, and off West Greenland, while the inshore areas of Canadian and US coastal waters are assessed nationally (NAFO, 2004). In most of these areas, species separation of redfish would not be problematic and has also been confirmed by otolith shape analysis (Stransky and MacLellan, in press). The geographic variation in otolith shape of both redfish species investigated in this study, however, suggests further separation of the Northeast Arctic stocks of both species from the other redfish stocks assessed within ICES (ICES, 2004a, b) and NAFO, whereas the similarities observed for the highly migratory S. mentella lend weight to integrated management of demersal and pelagic occurrences of this important fisheries resource that straddles across the ICES/NAFO boundaries.
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Acknowledgements |
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Kjell Nedreaas (Institute of Marine Research, Bergen, Norway), Jákup Reinert (Faroese Fisheries Laboratory, Tórshavn, Faroe Islands), Fran Saborido-Rey (Instituto de Investigaciones Marinas, Vigo, Spain), Thorsteinn Sigurdsson (Marine Research Institute, Reykjavík, Iceland), and Margaret Treble (Fisheries and Oceans Canada, Winnipeg, Manitoba) provided redfish otoliths. The Greenland and Irminger Sea samples were collected on board the German FRV "Walther Herwig III" with the help of staff members and volunteers. Jürgen Schlickeisen assisted with image analysis and programming. Cornelius Hammer gave helpful comments on the manuscript. This work was partly funded by the European Commission within the REDFISH project (QLK5-CT1999-01222). The preparation of the image and shape analysis methodology was supported by a doctoral grant from the German National Academic Foundation (Studienstiftung des deutschen Volkes).
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References |
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