A multivariate analysis of food and feeding trends among Greenland halibut (Reinhardtius hippoglossoides) sampled in Davis Strait, during 1986

ICES Journal of Marine Science, 54: 819 829. 1997 A multivariate analysis of food and feeding trends among Greenland halibut (Reinhardtius hippoglossoides) sampled in Davis Strait, during 1986 D. C. Orr and W. R. Bowering Orr, D. C. and Bowering, W. R. 1997. A multivariate analysis of food and feeding trends among Greenland halibut (Reinhardtius hippoglossoides) sampled in Davis Strait, during 1986. ICES Journal of Marine Science, 54: 819 829. Pelagic invertebrates [northern shrimp (Pandalus borealis), Cephalopoda, Mysidacea, Amphipoda and Euphausiacea] and fish [redfish (Sebastes mentella), R. hippoglossoides and Arctic cod (Boreogadus saida)] were the most important food items found in 4295 Greenland halibut (R. hippoglossoides) stomachs collected from Davis Strait [Northwest Atlantic Fisheries Organization (NAFO) Subareas and 1], during 1986. Ordination methods indicated that predator size and capture depth accounted for 85.2% of the variation in diet, while classification methods were used to illustrate these relationships. Additionally, the study extended into locations not previously sampled, therefore, knowledge was expanded upon pertaining to the distribution ranges of important prey. 1997 International Council for the Exploration of the Sea Key words: Reinhardtius hippoglossoides, Greenland halibut, ordination, classification, food. Received 17 October 1995; accepted 14 September 1996. D. C. Orr and W. R. Bowering: Flatfish and Deepwater Species Section, Groundfish Division, Science Branch, Northwest Atlantic Fisheries Centre, Department of Fisheries and Oceans, P.O. Box 5667, St John s, Newfoundland, Canada, A1C 5X1. Correspondence to W. R. Bowering: tel: +79772254; fax: +797724188; email: bowering@athena.nwafc.nf.ca Introduction Greenland halibut (Reinhardtius hippoglossoides) occurs in both the North Atlantic Ocean and the North Pacific Ocean (Yang and Livingston, 1988; Bowering and Chumakov, 1989). Within the Northwest Atlantic, the known range of the species extends from Smith Sound (78 N in Western Greenland), south along the Labrador coast, throughout the Grand Banks, into the Gulf of St Lawrence and southward to Georges Bank (42 N) (Templeman, 1973; Bowering and Chumakov, 1989; Bowering and Brodie, 1995). Due to the collapse of many major groundfish fisheries in the Canadian Northwest Atlantic (FRCC Report, 1994; Anon, 1994), Greenland halibut has become the most significant groundfish fishery in the Northwest Atlantic. Prior to 1995, it was managed as three separate stocks (Bowering and Chumakov, 1989): Baffin Island West Greenland [Northwest Atlantic Fisheries Organization (NAFO) Subareas and 1], Labrador Eastern Newfoundland (NAFO Subarea 2 and Division 3KL), and Gulf of St Lawrence (NAFO Division 4RST). However, scientific advice in 1994 resulted in the extension of the Labrador Eastern Newfoundland stock boundaries into Divisions 3MNO, both inside and outside Canada s 2 mile zone (see Anon, 1994). The combined annual harvest of these three stocks fluctuated between 3 and 7 t during 197 1992 (Chumakov and Podrazhanskaya, 1986; Bowering and Brodie, 1995); 1993 and 1994 annual harvests were maintained near 7 t (unpublished data). Such wide distributions and high abundances, implied by these catches, suggest that Greenland halibut is likely to be an important member of the Northwest Atlantic ecosystem. Thus it is important to determine the impact that Greenland halibut has upon economically important forage species such as capelin (Mallotus villosus) (Bowering and Lilly, 1992) and northern shrimp (Pandalus borealis) (Pedersen and Riget, 1993; Pedersen, 1994), and non-economically important fauna such as Amphipoda, Mysidacea and Euphausiacea. Studies of food and feeding of Greenland halibut within Davis Strait indicate that northern shrimp, Amphipoda, Euphausiacea, Mysidacea, redfish and Cephalopoda are 154 3139/97/5819+11 $25.//jm96198 1997 International Council for the Exploration of the Sea

82 D. C. Orr and W. R. Bowering important food items (Pedersen and Riget, 1993; Pedersen, 1994). Chumakov and Podrazhanskaya (1986) note that capelin, Myctophidae, sand lance (Ammodytes spp.), roundnose grenadier (Coryphaenoides rupestris), and northern shrimp were most frequently eaten off Labrador and northeastern Newfoundland (NAFO Subareas 2 and 3). Bowering and Lilly (1992) indicate that capelin, Arctic cod, redfish, Greenland halibut, northern shrimp and Amphipoda were the most important prey items of specimens taken off Labrador and northeast Newfoundland (NAFO Division 2J3K). Capelin predominated the diet of Greenland halibut collected from Trinity Bay (Lear, 197). All of these studies indicate that pelagic invertebrates and fish are the predominant prey, and that diets may differ over broad geographic areas. The objective of this paper is to statistically define and graphically illustrate relationships between Davis Strait (NAFO Subareas and 1) Greenland halibut and their prey. Relative importance of various prey are determined by percent frequency of occurrence, percent weight and mean partial fullness indices. Additionally, a review of commonalities and differences in diet among Northwest Atlantic Greenland halibut is completed. Since the study area extended into locations that had not previously been sampled, knowledge is expanded upon of prey distribution ranges. absorbent paper to remove excess moisture, and then weighed to the nearest.1 g. The contents of each stomach constituted one sample (Graham and Vrijenhoek, 1988; Fahrig et al., 1993; Pedersen and Riget, 1993). Relative importance The relative importances of food items (frequency of occurrence 2%) were determined using percent frequency of occurrence, percent weight and mean partial fullness (PFI) indices. The mean PFI of taxon P i in Greenland halibut f j is: where n refers to number of stomachs (Lilly and Fleming, 1981). The data were classified according to predator length (seven, 15 cm size classes beginning at less than or equal to 15 cm). Dietary trends were determined graphically via area plots of indices vs. size class. Materials and methods Greenland halibut were captured between 18 August 2 September 1986, during a stratified random survey of Northwest Atlantic Fisheries Organization (NAFO) Subareas and 1. The survey extended from Cape Chidley in the south (61 N) to Disko Island in the north (71 N) and included depths between 2 and 125 m. The survey was conducted by the 8 m stern trawler R.V. Gadus Atlantica using an Engel 145 high rise bottom trawl with a 29 mm mesh codend liner. The area was stratified by depth; the number of stations per stratum was proportional to its geographic area with a target of one set per 35 square nautical miles (nmi 2 ) and a minimum of two sets per stratum. A total of 194 standard 3 min tows were completed (Fig. 1). Bottom temperatures were determined at each station using expendable bathythermographs (Atkinson and Bowering, 1987). At each station, a maximum of five stomachs were collected within each 5 cm total length group of Greenland halibut. A total of 4295 Greenland halibut stomachs were excised at sea and preserved in 1% formalin. Fish and decapods were identified to species whenever possible, while other groups were identified to higher taxonomic levels (e.g. Polychaeta and Mysidacea). Items within each taxon were blotted with Multivariate analysis Empty stomachs were numerically important (2186 empty stomachs; frequency of occurrence=5.9%) and provided an indication of feeding intensity (Junquera, 1995). Therefore, percent of total catch and percent empty stomachs were plotted along capture depth, predator standard length, and latitude scales. Occurrences of rare prey (frequency of occurrence <2%) were removed from the data prior to analysis in order to avoid derivation of erroneous relationships (Hill, 1979a; Gauch, 1983; Lilly and Rice, 1983; Graham and Vrijenhoek, 1988; Kanapathippillai et al., 1994). Taxa were not pooled since pooling may result in a loss of information (Rice, 1988). The analyses made use of three complementary strategies: scattergram analysis, ordination and hierarchical classification. Mass was chosen as the analysis variable. Scattergrams (prey mass vs. predator total length, bottom temperature and capture depth) were used to identify multivariate patterns, suggest ordination methodologies and aid in the interpretation of the ordination and classification results. If prey were concentrated along environmental variables then weighted average ordination methods would have been appropriate. If linear trends had been evident then linear methods would have been used. Ordination was

A multivariate analysis of food and feeding trends among Greenland halibut 821 West Greenland 7 N 1A Median predator Total length cm 1 A 2 3 4 1B 5 6+ 65 Baffin Island 1C Depth = 1 m Depth = 2 m Depth = 4 m Depth = 8 m Depth = 1 m 2 mile limit 1D B 1E 65 W 6 55 5 Figure 1. Median total length of Greenland halibut caught, per set, in Davis Strait during the summer of 1986, by the Canadian research vessel Gadus Atlantica. 45

822 D. C. Orr and W. R. Bowering used to reduce variability, determine the relative influence of each variable upon distribution patterns, and produce a species/environment biplot. Hierarchical classification was conducted to determine associations between taxa. FORTRAN 77 versions of DEtrended CORrespondence ANAlysis (DECORANA) (Hill, 1979a), Two- Way INdicator SPecies ANalysis (TWINSPAN) (Hill, 1979b), and CANOnical Community Ordination (CANOCO) (ter Braak, 1987) were redimensioned to accept up to 5 samples. Prey and environmental data matrices were prepared in CORNELL condensed format (Singer and Gauch, 1979; ter Braak, 1987) using programs written for SAS Release 5.18 (SAS, 1989). Empty stomachs were deleted from the input data sets since their high frequencies and stable scores (mass=. gm) would have provided too great a contrast to the less frequent and more variable masses for prey taxa (Graham and Vrijenhoek, 1988). DECORANA was used to determine the range of weighted average sample scores. The range was in standard deviation (S.D.) units. Within the present report, standard deviation refers to the rate of species turnover along an environmental variable (Gauch, 1983). If the range was greater than 1.5 S.D. units then it was assumed that there was at least one species turnover and linear ordination techniques such as principal components analysis would not be appropriate. Similarily multivariate classification techniques such as discriminant analysis would not be appropriate as the data do not meet the requirements of multivariate normality with homogeneous covariance matrices over groups. Factor analysis also requires multivariate normality (Graham and Vrijenhoek, 1988). Scattergrams indicated that certain organisms were concentrated along environmental variables; DECO- RANA results indicated an environment range of 4.89 S.D. units. Therefore, weighted average ordination techniques were appropriate. Canonical Correspondence Analysis (CCA), within the CANOCO program, was chosen because it determines the dominant community distribution patterns and relates them to environmental variables (ter Braak, 1986, 1987). Taxonomic scores were modes within prey response curves (ter Braak, 1987) and were calculated from weighted averages of eigenvector sample scores. This technique resulted in prey and environmental biplot scores that were correlated with the ordination axes, therefore it was possible to create a single graph by overlaying environmental biplot scores upon prey scores. Quantitative interpretation of the biplots was therefore possible (ter Braak, 199). CCA was run iteratively, with a stepwise removal of environmental variables that had variance inflation factors greater than 2. Next, variables were removed on the basis of their canonical coefficients and t-values. Canonical coefficients have higher variances than regression coefficients, therefore, canonical coefficients and their respective t-values were used in an exploratory sense rather than for statistical significance. If a variable had a relatively low canonical coefficient and a t-value below 2.1, it was treated as a covariable and Monte Carlo simulations were run to determine whether it had a significant effect upon diet (α=.1). Environmental cross products (temperature class predator size class, temperature class depth class, predator size class depth class and temperature class predator size class depth class) were created when testing for interaction effects. In order to create meaningful permutation classes within the Monte Carlo simulations, it was necessary to create cross products from class (temperature class width=1 C, predator size class width=1 cm, depth class width=1 m) data. Cross products were treated as covariables. A Monte Carlo simulation was run for each cross product; a cross product was added to the biplot only if the Monte Carlo simulation indicated that it significantly influenced distribution patterns. Classification was conducted by running TWIN- SPAN. The resultant two-way classification table indicated associations between taxa. To determine whether the results from these analyses were due to inter- or intra-station variation, the DECO- RANA, CCA and TWINSPAN analyses were repeated using within station mean data (W. Warren, pers. comm.). Results Greenland halibut were sampled over a broad geographic area and from various depths (Fig. 1). Median Greenland halibut sizes tended to increase toward the south where water depths were greatest. Relative importance A total of 83 food types were identified, of which 12 were present within at least 2% of the stomachs (Boreogadus saida, Cephalopoda, Euphausiacea, Gammaridea, Gonatus spp., Hyperiidae, Liparidae, Mysidacea, Natantia, Pandalus borealis, Reinhardtius hippoglossoides and Sebastes mentella). Relative importance was dependent upon both index and predator size (Fig. 2). Hyperiidae, Boreogadus saida and Pandalus borealis were important within the diets of fish smaller than 6 cm. Cephalopoda and Liparidae were commonly eaten by a wide size range of fish, whereas Sebastes mentella and Reinhardtius hippoglossoides were important components of the diet among Greenland halibut that were larger than 75 cm. Figure 2 thus confirms that

A multivariate analysis of food and feeding trends among Greenland halibut 823 Percent frequency of occurrence 8 7 6 5 4 3 2 1 15 Size class no. of fish <= 15 cm 99 16 3 cm 748 31 45 cm 177 46 6 cm 1588 61 75 cm 55 76 9 cm 152 => 91 cm 8 3 45 6 75 9 15 2.5 2. Mean PFI 1.5 1..5 15 3 45 6 75 9 15 Percent weight 1 9 8 7 6 5 4 3 2 1 15 3 45 6 75 9 15 Greenland halibut total length (cm) Figure 2. The relative importances of various prey items in the diets of Greenland halibut (Reinhardtius hippoglossoides) as determined by three complementary indices plotted against Greenland halibut total length. Reinhardtius hippoglossoides, Sebastes mentella, Liparidae, Cephalopoda, Pandalus borealis, Boreogadus saida, Hyperiidae. taxonomic turnover occurred as predator length increased. Please note that Euphausiacea, Gonatus spp., Mysidacea and Natantia had relatively low scores for each of the indices, and were therefore not included in Figure 2. Environmental analyses Figure 3 indicates that the percent of empty stomachs increased toward the south (b=.12; r 2 =.865; F<.1) as sampling depth (b=14.8; r 2 =.792; F<.1)

824 D. C. Orr and W. R. Bowering Percent 1 8 6 4 2 15 1 8 6 4 2 25 25 35 45 55 65 75 85 Predator total length (cm) 45 65 85 15 Depth (m) 125 1 8 6 4 2 61.5 62.5 63.5 64.5 65.5 66.5 67.5 68.5 69.5 Latitude ( ) Figure 3. Percent of total catch ( ) and percent empty stomachs ( ) vs. various environmental variables. increased. The percent of empty stomachs did not significantly increase with size (b=1.6; r 2 =.617; F=.66). Ordination results Raw data CCA results are illustrated within the species/ environment biplot (Fig. 4). Predator total length [longest vector, highest absolute canonical coefficient (1.31), t-value (3.14) and absolute inter set correlation with axis 1 (.63)] and capture depth [second longest vector, highest absolute canonical coefficient (1.9), t-value (2.28) and absolute inter set correlation with axis 2 (.546)] were significant variables (p<.1) accounting for 85.2% of the variance in diet. Bottom temperature [shortest vector, highest absolute canonical coefficient (.81), t-value (1.19) and absolute inter set correlation along axis 3 (.288)] did not have a significant effect upon species dispersion when used as a covariable within a Monte Carlo simulation, but was included within the biplot because it accounted for approximately.6 of the explained inertia, which in this case equalled 1.11. Eigenvalues for these axes were.553,.393 and.164 respectively. The biplot is interpreted as follows: Hyperiidae and Boreogadus saida are at the upper right of the biplot. Therefore, these organisms were mainly preyed upon by a relatively narrow size range of Greenland halibut. 95 Figure 2 confirms that Hyperiidae and Boreogadus saida were eaten mainly by Greenland halibut that were smaller than 6 cm. Conversely, Sebastes mentella and Reinhardtius hippoglossoides are at the extreme left and were eaten by all sizes of Greenland halibut. The abscissa divides Figure 4 into top and bottom halves allowing information along the depth vector to be interpreted similarly. Shrimp, Hyperiidae, Sebastes mentella and Boreogadus saida were from relatively narrow depth ranges while Cephalopoda, Gammaridae, Liparidae, Mysidacea, Euphausiacea and Reinhardtius hippoglossoides were collected from various depths. Scattergrams confirmed the low importance of bottom temperature upon prey distributions in stomachs. Most organisms were found in stomachs collected from sites within relatively narrow ranges of bottom temperatures ( 4 C). Many prey distribution patterns were bimodal, and no clear relationship existed between prey distributions and bottom temperature. TWINSPAN organized taxa into groups of associated organisms. Circles are drawn around taxa that were grouped by TWINSPAN (Fig. 4). TWINSPAN associated Gonatus spp. with taxa found within the lower right quadrant of the biplot. Organisms within this quadrant were identified to the highest taxonomic level and were collected from a wide range of depths but a narrow range of predator sizes (most were collected from 3 7 cm Greenland halibut). Since Gonatus spp. is close to the origin, its distribution was not greatly influenced by bottom temperature, predator length or capture depth. Natantia, Hyperiidae and Boreogadus saida were collected from relatively small Greenland halibut that were caught at depths of less than 6 m. The association between Pandalus borealis and Reinhardtius hippoglossoides was not obvious. Northern shrimp were mainly eaten by Greenland halibut that were less than 6 cm in total length (Fig. 2), and were usually taken from depths of less than 6 m. Reinhardtius hippoglossoides were, however, eaten by a wide size range of predators and were collected from depths as great as 11 m. Mean station data DECORANA resulted in a gradient length of 4.26 S.D. units. It was therefore appropriate to use canonical correspondence analysis when analysing this data set. Figure 5 is the taxa/environmental biplot prepared by conducting CCA upon mean station data. Monte Carlo simulations indicated that bottom temperature, capture depth and mean predator length were significant (p<.1) when latitude, longitude and each of the cross products were treated as covariables. The eigenvalues for the first three axes were.391,.346 and.35 respectively. Bottom temperature was the most influential variable [canonical coefficient along axis ( 1.5), t-value ( 1.9), absolute inter-set correlation with axis

A multivariate analysis of food and feeding trends among Greenland halibut 825 2 Pandalus borealis Natantia Hyperiidae 1 Boreogadus saida Sebastes mentella Gonatus spp. Axis 2 (35.4%) Reinhardtius hippoglossoides Temperature Total length Mysidacea Cephalopoda Depth Euphausiacea Liparidae 1 Gammaridea 2 1 1 Axis 1 (49.8%) Figure 4. Prey (frequency of occurrence 2%)/environment biplot as determined by Canonical Correspondence Analysis (CCA), within CANOCO. The data matrix consisted of raw mass values within each stomach. 1 (.65)]. Capture depth was associated with axis 2 [canonical coefficient along axis 2 (.97), t-value ( 6.6), absolute inter-set correlation with axis 2 (.67)], while mean predator length was associated with axis 3 [canonical coefficient with axis 3 (1.27), t-value (3.3), absolute inter-set correlation with axis 3 (.89)]. The percent of variance accounted for by each axis was 5.6, 44.9 and 4.6 respectively.

826 D. C. Orr and W. R. Bowering 2 Pandalus borealis Hyperiidae 1 Natantia Boreogadus saida Sebastes mentella Axis 2 (44.9%) Temperature Gonatus spp. Mysidacea Cephalopoda 1 Reinhardtius hippoglossoides Mean length Depth Euphausiacea Liparidae Gammaridea 2 1 1 Axis 1 (5.6%) Figure 5. Prey (frequency of occurrence 2%)/environmental biplot as determined by CCA. The data matrix consisted of mean mass values within each station. A visual comparison between Figures 4 and 5 indicate very minor differences between analyses of raw and mean station data. The placement of taxa and vectors are very similar, with the exception that Pandalus borealis, Gonatus spp., Natantia, Cephalopoda and Reinhardtius hippoglossoides are much closer to the origin in Figure 5 than in Figure 4. The proximity between these taxa and the ordinate axis confirms that

A multivariate analysis of food and feeding trends among Greenland halibut 827 the importance of predator length has decreased. Pandalus borealis, Natantia and Cephalopoda are on right half of Figure 4 but are on the left half of Figure 5. The primary split in Figure 4 separated Sebastes mentella from all other prey. The primary split in Figure 5 includes Sebastes mentella with Gonatus spp., Cephalopoda, Euphausiacea, Gammaridea, Liparidae and Mysidacea. Gonatus spp., Cephalopoda, Euphausiacea, Gammaridea, Liparidae and Mysidacea are associated with each other in both figures. Similarly, shrimp, Boreogadus saida and Hyperiidae are grouped together in both figures. Pandalus borealis is a member of the Natantia. It is impossible to know whether it is important that Pandalus borealis is associated with Reinhardtius hippoglossoides in Figure 4, while Natantia is associated with Reinhardtius hippoglossoides in Figure 5. Undoubtedly, many of the Natantia were well digested Pandalus borealis. The fact that ordination and classification results were not dependent upon raw or mean station data indicates that the solutions were due to inter-station rather than intra-station variation. Discussion Ordination and classification provided objective, complementary and robust means of exploring multivariate feeding data. Each technique illustrated species/ environment relationships, and associations between prey taxa. As indicated below, these relationships are confirmed by published evidence, and explain dietary changes over broad geographic areas. Percent variances accounted for within the species/ environment relationships indicated that Greenland halibut size had the greatest influence upon prey choice, while capture depth and bottom temperature were of lesser importance. Other studies confirm the influence of Greenland halibut size upon diet (Yang and Livingston, 1988; Bowering and Lilly, 1992; Pedersen and Riget, 1993; and Rodríguez-Marin et al., 1995). Predator size is probably the most important factor because Greenland halibut make extensive diurnal vertical migrations (Bowering and Parsons, 1986; Pedersen, 1994; Bowering and Brodie, 1995), which was confirmed by the fact that a wide range of predator sizes were captured at all depths (Atkinson and Bowering, 1987). Ontogenetic changes in diet may also be the result of horizontal migrations undertaken by Greenland halibut as they grow. Many of the small Greenland halibut were collected from shallow water, north of 67. The shallow water surrounding Disko Island is known to be an important nursery area for the species (Smidt, 1969; Atkinson et al., 1982). Conversely, the largest fish were collected south of 65, at depths greater than 5 m. This is in agreement with Riget and Boje (1989) who note that older fish move toward deeper water within the fjords and down the continental slope. As they migrate, the availability of various prey probably changes. This hypothesis was tested by Bowering and Lilly (1992) who found that medium sized NAFO Division 2J3K Greenland halibut (2 64 cm) were on the continental shelf where capelin (Mallotus villosus) were abundant. In this situation, capelin were the main component of Greenland halibut diet. However, large Greenland halibut (>69 cm) were more abundant in deeper water. Capelin were not abundant in these areas and consequently Greenland halibut fed mainly upon demersal fish. However, it should be noted that large Greenland halibut do not usually feed on capelin even when the latter are available (Bowering and Lilly, 1992). It is important to note that ordination and classification techniques separated organisms according to levels of identification. Clear distribution patterns existed for species, although conversely, diffuse patterns existed for organisms identified to higher levels. There is no way of knowing whether broad distribution patterns are the result of combining several species, thus high level classification resulted in a loss of information. Comparisons with other studies indicate location related differences in diet. Boreogadus saida, Sebastes spp., Reinhardtius hippoglossoides, Cephalopoda, Hyperiidae, Mysidacea, Euphausiacea and Pandalus borealis were relatively common and constituted much of the food within stomachs collected for virtually every northwestern Atlantic feeding study cited within the present report. These animals were important over a broad geographic range and throughout 23 years of study. However, capelin had a more localized importance. They were common within stomachs collected along the western Greenland coast (Smidt, 1969; Pedersen and Riget, 1993) but were not mentioned by Pedersen (1994), who describes deepwater samples from Davis Strait. Neither Yatsu and Jørgenson (1989) nor the present authors found capelin in Davis Strait deepwater samples. Capelin were of minor importance within Hopedale and Cartwright Channels (Bowering et al., 1983), but were a major dietary component within Trinity Bay (Lear, 197) and upon the continental shelf off southern Labrador and northeastern Newfoundland (Chumakov and Podrazhanskaya, 1986; Bowering and Lilly, 1992; Rodríguez-Marin et al., 1995). Atlantic cod formed a minor dietary component off the coasts of West Greenland (Smidt, 1969; Pedersen, 1994; the present study), southern Labrador and northeastern Newfoundland (Bowering and Lilly, 1992; Rodríguez- Marin et al., 1995). Within the present study, latitude and longitude may not have had a significant influence upon diet because collections were taken from a relatively small geographic

828 D. C. Orr and W. R. Bowering area and because there are several confounding factors at any location. For instance, a range of depths may be found along any latitude or longitude, and both temperature and predator size are positively related to depth (Atkinson and Bowering, 1987). Also, two water masses influence the physical and chemical oceanography of the study area. Northward flowing, warm, highly saline water from the Labrador Sea is found along Greenland s continental slope, whereas water along the Canadian side of Davis Strait is characterized by cold, fresh water flowing southward from the Arctic (Atkinson and Bowering, 1987; Ross, 1992). When analysing raw data, each stomach represented a separate sampling unit. In this case, CCA determined that predator length was the most important explanatory variable. Sebastes mentella was a common food item, but was not often found in stomachs with other prey. TWINSPAN therefore separated Sebastes mentella from all other taxa. However, when DECARANA, TWINSPAN and CCA were performed upon mean prey mass and mean length data, the importance of predator length decreased because each station was used as a sampling unit. This reduced the resolution of the analysis. In this case, Sebastes mentella, Gonatus spp., Cephalopoda, Euphausiacea, Gammaridea, Liparidae and Mysidacea were often collected at the same station, hence TWINSPAN grouped these organisms together. The fact that Pandalus borealis, Cephalopoda and Natantia moved from the right side of the biplot (Fig. 4) to the left (Fig. 5) is immaterial because the increased proximity to the ordinate axis confirms that their mean mass distributions are not strongly influenced by predator length. In the present study 5.9% of the stomachs were empty. The highest percentages of empty stomachs were found in the southeastern portion of the study area. The greatest capture depths, highest bottom temperatures, and largest Greenland halibut were taken in this area. Such high percentages of empty stomachs are common (Bowering and Lilly, 1992; Pedersen and Riget, 1993; Rodríguez-Marin et al., 1995; Bowering and Brodie, 1995), and the depth related feeding intensity trends are in agreement with observations in the Barents Sea (Nizovtsev cited in Chumakov and Podrazhanskaya, 1986), Davis Strait (Yatsu and Jørgensen, 1989), and NAFO Division 2J3K (Bowering and Lilly, 1992). However, research within NAFO Division 3LM (Junquera, 1995; Rodríguez-Marin et al., 1995) indicated that percentage of empty stomachs decreased as depth increased. Ordination and classification indicated that predator size and capture depth influenced diet. As Greenland halibut grew, they moved into deeper water where prey availability probably changed. Feeding intensity decreased toward the south as capture depth also increased. Additionally, comparisons with previous studies provided an indication of spatial differences in importance of various prey. Such knowledge of connections between fish and their environment is essential if we are to understand dynamics within the Greenland halibut population. References Anon 1994. Scientific Council Reports. Fifteenth issue. Dartmouth, Nova Scotia. Atkinson, D. B., Bowering, W. R., Parsons, D. G., Horsted, Sv. Aa., and Minet, J. P. 1982. A review of the biology and fisheries for roundnose grenadier, Greenland halibut and northern shrimp in Davis Strait. Northwest Atlantic Fisheries Organization Science Council Studies, 3: 7 27. Atkinson, D. B. and Bowering, W. R. 1987. The distribution and abundance of Greenland halibut, deepwater redfish, golden redfish, roundnose grenadier and roughhead grenadier in Davis Strait. Canadian Technical Report of Fisheries and Aquatic Science, No. 1578: v+29 p. Bowering, W. R., Parsons, D. G., and Lilly, G. R. 1983. Predation on shrimp (Pandalus borealis) by Greenland halibut (Reinhardtius hippoglossoides) and Atlantic cod (Gadus morhua) offlabrador (Div. 2H and 2J). Northwest Atlantic Fisheries Organization Science Council Research Document, 83/IX/88: 26 p. Bowering, W. R. and Chumakov, A. K. 1989. Distribution and relative abundance of Greenland halibut (Reinhardtius hippoglossoides (Walbaum)) in the Canadian northwest Atlantic from Davis Strait to the Northern Grand Bank. Fisheries Research, 7: 31 327. Bowering, W. R. and Lilly, G. R. 1992. Greenland halibut (Reinhardtius hippoglossoides) off southern Labrador and northeastern Newfoundland (northwest Atlantic) feed primarily on capelin (Mallotus villosus). Netherlands Journal of Sea Research, 29: 211 222. Bowering, W. R. and Brodie, W. B. 1995. Greenland halibut (Reinhardtius hippoglossoides). A review of the dynamics of its distribution and fisheries off eastern Canada and West Greenland. 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