Interactions between grey seals and fisheries: towards a predictive model for evaluating management options
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- INTRODUCTION
- METHODS Telemetry System
- Relative slow interpolated location density
- RESULTS SRDL deployments
- Proximity to haulout sites
- Telemetry system performance
- Distribution and characterisation
Interactions between grey seals and fisheries:towards a predictive model for evaluating management options.Project MF 0309Report, December 1997Bernie McConnell, Mike Fedak, Colin Hunter & Phil LovellSea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife KY16 8LBINTRODUCTIONThe primary objective of this project was to map UK grey seal foraging areas. It builds upon a previous project (project MF 0503) which concentrated primarily on seals at the Farnes (Northumbria). However, the breeding population there represents less than ten percent of the UK total and so, in the current project, we extend the study to other geographical areas with high concentrations of grey seals. The two sites that were chosen were the Monach Isles (to the west of the Outer Hebrides) and Orkney. Ease of access to deep water from these sites provided an ecological contrast with the previous study in the North Sea. The approach taken here was similar to the previous study. Grey seal movements were obtained using Argos satellite telemetry tags called Satellite Relay Data Logger (SRDLs) which were designed and built by the Sea Mammal Research Unit (SMRU). In addition to providing location fixes with global coverage, the system relayed detailed dive and haulout information. An additional element to the previous study was the inclusion of newly weaned grey seal pups (weaners). In addition to being a numerous component of the population, they are of special interest. After weaning, the pups are deserted by their mothers and have to learn to forage effectively within the first few weeks before they starve. They have no previous foraging experience, and it is possible that chance encounters with prey in their first few months will influence their adult foraging patterns. This weaner study was part of a Strategic Research Project funded by the Natural Environment Research Council (NERC). We present data from three sets of deployments on grey seals: 21 SRDLs on adults at the Monachs in 1995 and 1996, 10 SRDLs on newly weaned pups at the Monachs in 1995, and 15 SRDLs on adults in Orkney in 1993 and 1996. The latter deployment includes historical data from 1993 (funded by the NERC). In addition, we summarise track data obtained from 12 seals caught at the Farnes in 1991and 1992 as part of the previous study (MF 0503). Although a wealth of information about the behaviour of grey seals has been collected in the course of this study, we restrict this report to the primary objective of 'mapping the foraging areas of grey seals'. However, we do discuss some, of the many, questions these data pose, and we suggest some future analyses. The full integration and analysis of the complete data set will be carried out as part of project MF 0515. METHODSTelemetry SystemSeal movements and dive and haulout behaviour data were obtained using the Argos satellite system. Telemetry tags called Satellite Relay Data Loggers (SRDLs) were glued to the fur of seals. SRDLs relayed behavioural data via one of two polar orbiting satellites to an Argos ground station. The location of the SRDL was estimated using the Doppler shift in SRDL transmission frequency as measured by the satellite during an overpass. This information was then made available via the Internet. The accuracy of the locations depends upon the number of successful transmissions (uplinks) to a satellite during an overpass. This, in turn, depends upon the seal being at the surface during those times when satellites are available. The SRDLs, designed by the Sea Mammal Research Unit (SMRU), consisted of a data logger interfaced to an Argos RF unit. Data from a depth sensor and a submergence sensor were used to determine the activity of the seal. The activity was either 'diving' (deeper than 6 m for at least 6 s), 'hauled out' (dry for at least 240 s), or else 'at surface'. Distance swum was determined by a turbine odometer. Individual dive records included information on maximum depth, depth profile, distance swum and dive and previous surface duration. Dive and haulout records stored in memory were selected for transmission such that those times of day when the Argos satellites were not available were adequately represented. DeploymentGrey seals were caught either at sea in tangle nets set near haulout sites, or on land using hand nets. The seals were anaesthetised with an intra-muscular injection of Zoletil at a dose rate of 0.8 - 1.0 mg.kg-1 body weight. The seal fur was dried and cleaned with ethanol and the SRDLs were attached with a two-part rapid-setting epoxy. The SRDLs were placed on the back of the neck just behind the head so that the aerial would emerge when the seal surfaced. The seals were weighed and measured, and then released into the water once they had regained full consciousness. All procedures were undertaken in accordance with Animals (Scientific Procedures) Act 1986 Project Licence number PPL 80/00368. Data processingLocation filteringThe low rate of uplinks to the Argos satellites, due to the high proportion of time grey seals spend underwater, resulted in significant errors in some location estimates. Thus, locations were filtered using an algorithm that rejected locations requiring an unrealistically high swimming speed (McConnell et al. 1992). The maximum sustained swimming speed allowed was 2.0 m.s-1. Temporal interpolation of locations Argos locations were obtained at irregular intervals. The number and quality of location estimates per day depended in part upon seal behaviour. For example, more uplinks to the satellites (and thus locations) may be obtained when a seal is hauled out or during prolonged surface intervals. Thus, raw locations may provide a biased sample of the temporal and spatial distribution of seal activity. To minimise this bias new fixes (termed interpolated locations) were estimated at 6-hour intervals by interpolation of the original track locations. Interpolation was suspended if the gap between original locations exceeded 24 hours. Travel rateFor each interpolated location the mean of the speeds of travel from the previous interpolated location and to the next interpolated location was calculated and termed travel rate. This provided an index of whether the seal was travelling rapidly or was active in a localised area Relative slow interpolated location densityInterpolated locations where the travel rate was less than 0.5 m.s-1 were selected to emphasise the slow moving periods at sea, and the periods when the seal was hauled out on land. We argue below that the former periods represent foraging. In order to quantify the spatial distribution of these slow interpolated locations, for each deployment, a standard area (54 to 61.5 N and 11 to 0 W) was gridded into 10 by 10 km cells. For each seal, the slow interpolated locations were counted in each cell. Each seal’s grid was then overlaid to produce a grid of count totals. The value in each cell was then expressed in units of standard deviations from the mean count of the deployment group. RESULTSSRDL deploymentsData were collected from three sets of deployments of SRDLs on grey seals: 15 SRDLs were fitted to adults at the Monachs in June 1995, which produced 1291 (average 86.1) seal-days of data. A further 6 were fitted in April 1996, which produced 921 (average 107.8) seal-days of data. The combined deployments produced 1938 (average 92.3) seal-days of data (Table 1.). Ten SRDLs were fitted to newly weaned pups at the Monachs in November 1995, which produced 922 (average 92.2) seal-days of data (Table 2.). Three deployments were carried out in Orkney (Table 3.). Four SRDLs were fitted to adults in August 1993, which produced 69 (average 17.3) seal-days of data. Seven SRDLs were fitted to adults in April 1996, which produced 360 (average 51.4) seal-days of data. Four SRDLs were fitted to adults in August 1996, which produced 66 (average 16.5) seal-days of data. The combined deployments produced 495 (average 33.0) seal-days of data. The 1995 Monachs deployment produced the most consistent tracking durations, and none failed before 22 days. In the second Monachs deployment three out of six exceeded 150 days, but two failed after less than six. The durations of the Monach weaners were more variable. Four exceeded 140 days, but three failed before 25 days. The Orkney deployment was less successful, with only seven of the 15 tracks lasting 25 days or more. Seal MovementsFigures 1a-f, 1h-j and 1k-n show the tracks of individual seals, grouped by the three sets of deployments. The depth contours on maps are at 200 and 500m and indicate the continental shelf edge. Argos locations are shown as dots. The combined tracks for each deployment group are shown in Figures 2a-c, and tracks for the Farnes deployment (Project MF 0503) are shown in Figure 2d. The locations are colour coded by individual seal. Place names referred to in the text are shown in Figure 6. Both within and between deployments there was considerable variability between seal movements. However, two types of movement were apparent: trips from a haulout site; and travel, sometimes over large distances, from one haulout site to another. Monachs Adults Movements of Monachs adults extended from the Faroes down to the south west of Ireland, and as far eastwards as Orkney and Shetland. Almost all movements were limited to the continental shelf (< 200 m). There were only three, brief, exceptions. Seal mn_22501_95 (Figure 1d) left the shelf edge off Ireland, travelled for one week out to 12º W, and then returned to St Kilda (Figure 1a). Seal mn_22484_95 travelled over deep water (> 500m) between Shetland and the Faroes. Seal mn_22494_96 (Figure 1e) also travelled over deep water to, and back from, the Faroes. There was very little travel within the Hebridean Minch (between the Mainland and the Outer Hebrides). This was despite the facts that 1. seals crossed over both the northern (from the Butt of Lewis to Cape Wrath) and southern (from Barra Head to Coll and Tiree) entrances of the Minch, and 2. that three seals (mn_22483_95 (Figure 1b), mn_22498_95 (Figure 1c), and mn_26624_96(Figure 1f)) hauled out on the north eastern side of Barra (inside the Hebridean Minch). Monachs Weaners Grey seal weaner movement was more restricted than those of the adults. This may have been due in part to the fact that there was about half the number of seal-days of data from the weaners (922 vs. 1938 seal -days). However, there was little movement north of Gasker (50 km north of the Monachs), and most movements were to the south or west. Four seals visited sites on or off the northern coast of Ireland, and one travelled to the south west tip of Ireland. Similarly to the adults, the continental shelf edge formed the westwards limit (only one seal (mn_1554_b95 (Figure 1i)) travelled briefly into deeper water), and there were no movements into the Hebridean Minch. Orkney Adults Orkney seal movements were primarily to the east of Orkney, and thus on the continental shelf. They travelled north to Shetland and south east to sites off Rattray Head. Two seals (or_17213_96 and or_17216_96 (Figure 1m)), however, used areas 15 km to the west of Orkney. A high proportion of time was spent within the Orkney Isles and the Pentland Firth. However, allowance must be made of the short mean tracking duration of Orkney seals. Proximity to haulout sitesFigure 3 shows the locations of all the sites where seals in this study hauled out on land. Figure 4 shows how far, and for how long, each seal was from a haulout site. For each interpolated location two distances were calculated, a. from the nearest haulout site in Figure 3 (black filled), and b. from the haulout site where the seal was captured (line). The distances were truncated at 200 km. Figure 5 shows the frequency distribution of all the interpolated location distances. Although there was variability among seals, some interesting patterns emerged. A high percentage of seals' time was spent near haulout sites. 69% of Monachs adults' time was spent within 30 km of a haulout site, 46% for Monachs weaners, and 73% for Orkney adults. The time profiles in Figure 4 show that most seals undertook a series of trips to sea from haulout sites. Theses trips were of variable duration and extent. Some were highly regular and to the same offshore location (for example, seal mn_26624_96 (Figures 1f and 4b)) while others were irregular and to a variety of locations (for example, seal mn_22497_95 (Figures 1b and 4a)). In general, trips by adults seldom lasted more than two weeks and often lasted only a few days. Trips to sea by Monachs weaners were generally longer that those by adults. Only one Monachs seal (mn_20918_95 (Figure 1d)) used the Monachs as the only haulout site. The remainder used a variety of other haulout sites. Some sites that were close to the Monachs, such as St Kilda, the Flannan Isles and Barra Head, were used by numerous study seals. Other, more distant sites (from Ireland to the Faroes) were used by fewer study animals. The Monachs weaners visited haulout sites used by the Monachs adults. Only one visited St Kilda, but five of the nine weaners visited haulout sites at either Barra or Barra Head. With the exception of travel to Shetland, seals caught in Orkney were more frequently near the haulout site where they were originally captured. Foraging areas Relative slow interpolated location densities are shown in Figures 6a-d. The intensity of the cell colour is based upon a cell’s total count expressed in units of standard deviations from the mean count. The numbers refer to high densities of interpolated locations offshore that are described in the text and in Table 4. The letters refer to haulout sites that are not readily identifiable as land on the maps, where there were high densities of interpolated locations. The key to these haulout sites is given in the legend to Figure 6. We propose that periods of slow travel (specifically, less than 0.5 m.s-1) offshore represent periods of foraging. Support for this proposal is presented in the Discussion. Distribution and characterisation Slow interpolated locations were clustered in both time and space. We identify here 14 clusters (including one cluster (14) from the Farnes data set) that were not at haulout sites. Some of these offshore areas were used by several seals. For example, area 5 was used by five Monach adults. Areas further from the Monachs were used by fewer Monachs seals. There were varying degrees of overlap between the clusters used by the different deployment groups. Four of the clusters used by the Monachs adults (1, 2, 3 and 13), were also used by Monachs weaners. Two clusters near Orkney (6, and to a lesser extent 7) were used by both Orkney and Monachs adults. The Farnes seals used only cluster 14, and no other study seals used this area. However, a detailed inspection of the Farnes data, revealed that this cluster consisted of several local, discrete clusters (project MF 0503). Some clusters were associated with, rather than limited by, the continental shelf edge. In two seals (mn_22497_95 (Figure 1b) and mn_22500_95 (Figure 1d)) that frequented the shelf edge, foraging was less clustered and was distributed more linearly along the shelf edge. Depth of the foraging areas varied between 60 and 200m. All areas, except for area 6, had a seabed bottom composed of a mixture of sand and gravel. At some areas, (3, 8, 9, 10, 12 and 14) clusters of interpolated locations were associated with local patches of gravel-sand within other bottom types. Diving behaviour Detailed data on over 50,000 dives have been recorded. These data have yet to be fully analysed. However, an initial inspection suggests that most dives to offshore foraging sites were to the seabed. In general, dives in water less than 200m deep were to the seabed. In deeper waters, off the continental shelf edge, dives were in mid water, sometimes to depths greater than 200 m. The maximum dive depth recorded was 350m. DISCUSSIONIn this study we have collected 3629 seal days of location data from 46 seals (excluding the Farnes deployment). In addition, we have recorded haulout events and over 50,000 detailed dive records. These data sets are unique in their scope and detail. However, we restrict our discussion here to the primary objective of 'mapping the foraging areas of grey seals'. The full integration and analysis of these data sets will be carried out in project MF 0515. Telemetry system performanceMost data came from the deployments at the Monachs where an average of approximately 90 seal-days of data was collected. Data from Orkney were sparser, with an average of just 33 seal-days. This was partly due to an unreliable batch of SRDLs. Another factor may have been attachment to the seals' fur before it became fully fast after the annual moult. The imbalance will be redressed in Spring 1998, when 10-15 SRDLs will be deployed in Orkney (and possibly Shetland) as part of project MF 0515. MovementsThe movements of the Monachs seals were varied. Some travelled large distances: south to Ireland, north to the Faroes and eastwards to Orkney and Shetland. Individual seals were not restricted to one direction of movement. For example, the seal that travelled to the north of the Faroes (mn_22494_96 (Figure 1e)) had previously hauled out on the Irish coast. Other seals had a more restricted range and there was a considerable presence within the triangle bounded by the Monachs, St Kilda and the Flannans. There were, however, certain bounds to travel. There was little travel past the edge of the continental shelf edge (> 200 m contour) to the west. To the east, there was also no travel into the Hebridean Minch. This was despite the fact that seals travelled across both entrances to the Minch and hauled out on the eastern side of Barra, facing the Minch. These limits are discussed further below. Two seals from Orkney travelled to southern Shetland, and two to an area off Rattray Head. There was no distant travel to the west of Orkney. The sparseness of data from Orkney limits comparison with other sites. However, an inspection of just the first month of tracking (Figure 4) shows that eight of the 21 Monachs seals moved 200 km or more from the Monachs, compared with two of the 15 seals from Orkney. This is not a powerful comparison, but it does suggest that Orkney-caught seals may travel less far afield. It is evident from the movement data that seals from different areas are not ecologically isolated. Although the geographical distributions of Farnes and the Monachs seals were generally separate, there was a significant overlap. Individuals from both sites travelled to Orkney, Shetland and the Faroes. A number of Monachs adults and weaners travelled south to Ireland. These facts indicate a degree of geographical mixing over a large scale. This has important consequences in the modelling of the foraging ecology, epidemiology and the management of seal populations. Proximity to haulout sitesFigure 4 shows that seal movements consisted of a series of trips to sea interrupted by periods at or close to haulout sites. The extent and duration of these trips were variable. Some seals had regular trips to the same offshore areas, while others travelled irregularly from a variety of haulout sites. The use of multiple haulout sites, and their geographical extent (Figure 3), contrasts with data from the Farnes. Here the Farnes was the primary haulout site for most seals. Although there was distant travel, most trips were from, and returned to, the Farnes. This difference may reflect the lower availability of suitable haulout sites on the east coast. It may also reflect the availability and adequacy of prey close to the Farnes. Why so much time is spent near haulout sites, and, indeed, why seals haul out on land at all, is a matter of conjecture. A possible explanation is that seals may be safer from predation, for example by killer whales (Orcinus orca), at or near a haulout site. Alternatively, sufficient food may have been caught during offshore trips and the periods at or near haulout sites may be periods of rest. A third possibility is that we underestimate foraging activity near haulout sites. Detailed analysis of haulout records, and their relationship to trip duration and extent may provide insight into the motivation and regulation of haulout activity. Foraging areasIdentificationIn this study we have no direct evidence when feeding is taking place. However, we propose that trips to sea interrupted by periods at or near haulout sites represent foraging trips. This pattern is similar that observed at the Farnes (Hammond et al., 1994a). There we argued that the destinations of offshore trips, when the seal reduced its travel rate, probably represented foraging areas. The arguments may be summarised as follows: Real time VHF telemetry has shown that grey seals make directed trips to specific sites, where they remain for periods of hours to days (Thompson et al., 1991). At these sites, the seals were often seen with other top predators, and there were often signs of the presence of prey species. Analysis of faecal samples has indicated a high proportion of sandeels (Ammodytidae) in Farnes seals’ diet (Hammond et al., 1990). Sandeels have a burrowing preference for a mix of gravel and sand and avoid pure sand (Reay, 1970). Data from SRDLs showed that trips showed a similar preference for this type of seabed type, and that dives in these areas were mainly to, or near to, the seabed. Sandeels are also a major component of the diet of grey seals in Orkney (Hammond et al., 1994b) and the Outer Hebrides (Hammond et al., 1994c). In both these areas, sandeels were consumed mainly during the summer. This, and the fact that the movements of seals in this study had a similar pattern of trips from haulout sites, leads us to conclude that seal foraging areas may be similarly identified here. We do not, however, exclude foraging near haulout sites. Indeed, the high density of seals around certain haulout sites suggests that, even if seals only feed opportunistically there, they may still have significant impact on local fish abundance by virtue of their numerous presence. In order to highlight the slow-moving parts of trips, we selected only those interpolated locations with a travel rate of less than 0.5 m.s-1. An inspection of the frequency distribution of travel rates failed to reveal a discontinuity between slow and fast travel rate interpolated locations, and so the threshold of 0.5 m.s-1 was arbitrary. However, changing the threshold to 0.4 and 0.6 m.s-1 did not significantly affect the patterns of clusters of interpolated locations in Figure 6. Analysis of dive data, and classification into dive types, may lead to a better identification of foraging activity. The work will form part of project MF 0515. PresentationSlow interpolated locations are presented in gridded format in Figure 6. For each deployment set the interpolated locations of all seals were summed within grid cells. This has the effect of giving greater weight to seals with more interpolated locations, that is, to seals that were tracked for longer periods. The alternative would be to give equal weight to every seal regardless of tracking duration. This latter method would be appropriate if we assumed that all seals were tracked for a sufficient period for a ‘representative set of behaviours’ to have been recorded. In this presentation the former method is used. The problem of integrating tracks of variable duration into quantitative estimates of foraging distribution and activity will be tackled further in project MF 0515. Distribution and characterisationSlow interpolated locations were not randomly distributed, but formed clusters. Some cluster sites were used repeatedly by one individual, and some were used by more than one individual. The Monachs seals clusters were bounded to the west by the continental shelf edge. It is a matter of conjecture whether increasing depth to the west made benthic foraging ineffective, or whether this edge marked a reduction in prey density. Some seals did dive to depths in excess of 300 m but the maximum depth recorded was 350 m. Two seals foraged along linear areas along the shelf edge, suggesting that the prey items taken by these seals were concentrated along the shelf edge. Some clusters were associated with, and sometimes delimited by, sand–gravel mix sediment type. This is the sediment type preferred by sandeels, an important prey item in Orkney and Outer Hebrides grey seals (Hammond et al., 1994b,c)). However, the association was not apparent in all clusters. Most foraging areas suggested here were not far in time from a haulout site. Few trips were greater than two weeks and most were of a few days. This result gives us greater confidence in using faecal analysis to determine prey items. Faeces only provide information on the most recent (1-2 days) meals. Thus, if there is variability in prey species taken during the duration of a foraging trip, in shorter trips, faeces collected onshore represent a less biased sample of total diet. The purpose of the maps in Figure 6 is to show the broad scale distribution of areas identified as foraging areas. The choice of numbered clusters was subjective, and may underestimate foraging activity in non-localised areas. The map coverage in this figure is gridded into 10 km cells and each map summarises groups of individual seals over a variable tracking periods. However, an inspection of the individual tracks in Figure 1 suggests that interpolated locations were clustered at a higher spatial resolution. Detailed inspection (McConnell et al., 1994) has shown that the area used primarily by Farnes seals (cluster 14 in Figure 6) actually consisted of a set of smaller, discrete areas. At this higher spatial resolution, the association with gravel-sand sediment types became stronger. It is probable that a more detailed analysis of the foraging sites in this study will provide greater insight into the detailed distribution of foraging areas, seasonality, inter-individual separation and specialisation, and association with physical features such as sediment type. We also intend to investigate the track data more fully to test whether there is an association between specific haulout sites and specific foraging areas. If this were the case, foraging distribution and activity could be predicted from regular aerial photographic counts of seals at haulout sites. The movements and foraging areas of adults presented here shows considerable variability from seal to seal. We speculate that some of this variability may be influenced by their early experiences. Newly weaned grey seal pups are deserted by their mothers and so they rely upon their energy-rich blubber stores until they develop effective foraging skills. Although Monach weaner movement was primarily to the south, they did use four foraging areas that were also used by Monachs adults. Finding foraging areas is vital to the pups’ early survival and Figure 5 suggests that more time was spent further away from haulouts sites, possibly searching for food. However, weaners also visited haulout sites used by adults in this study. We further speculate that weaners, having found an established haulout site, may follow adults on their foraging trips. A more detailed exploration of the data will reveal whether the weaners visited a haulout site before they established patterns of foraging trips. REFERENCES Folk, R.L. 1954. The distinction between grain size and mineral composition in sedimentary-rock nomenclature. J. Geol. 62: 344-359. Hammond, P.S., and Fedak, M.A. (Editors) 1994a. Grey seals in the North Sea and their interactions with fisheries. Sea Mammal Research Unit, Cambridge. 157pp. (Final Report to the Ministry of Agriculture, Fisheries and Food under contract MF0503) Hammond, P.S., Hall, A.J., and Prime, J. 1994b. The diet of grey seals around Orkney and other island and mainland sites in northeastern Scotland. J. appl. Ecol. 31: 340-350. Hammond, P.S., Hall, A.J., and Prime, J.H. 1994c. The diet of grey seals in the Inner and Outer Hebrides. J. appl. Ecol. 31: 737-746. Hammond, P.S., and Prime, J.H. 1990. The diet of British grey seals (Halichoerus grypus). In Population biology of sealworm (Pseudoterranova decipiens) in relation to its intermediate and seal hosts. Edited by W.D. Bowen. Canadian Bulletin of Fisheries and Aquatic Sciences, 222: pp. 243-254. McConnell, B.J., Chambers, C., and Fedak, M.A. 1992. Foraging ecology of southern elephant seals in relation to the bathymetry and productivity of the Southern Ocean. Antarctic Sci. 4(4): 393-398. McConnell, B.J., Fedak, M.A., Lovell, P., and Hammond, P.S. 1994. The movements and foraging behaviour of grey seals in the North Sea. In Grey Seals in the North Sea and their Interactions with Fisheries. Edited by P.S. Hammond and M.A. Fedak. . Sea Mammal Research Unit, Cambridge. pp. 88-148., Final Report to the Ministry of Agriculture, Fisheries and Food under contract MF0503. Reay, P.J. 1970. Synopsis of biological data on north Atlantic sandeels of the genus Ammodytes. Fish. Synop. FAO No. 82. Thompson, D., Hammond, P.S., Nicholas, K.S., and Fedak, M.A. 1991. Movements, diving and foraging behaviour of grey seals, Halichoerus grypus. J. Zool., Lond. 224: 223-232. Table1. Tracking and individual information of 21 grey seals fitted with SRDLs at the Monach Isles in 1995 and 1996. Mean tracking duration was 92.3 days. (* data were not received until 22 June 1995 due to SRDL malfunction.)
Table 2. Tracking and individual information of 10 weaned grey seal pups fitted with SRDLs at the Monach Isles in November 1995. Mean tracking duration was 92 days.
Table 3. Tracking and individual information of 15 grey seals fitted with SRDLs at Orkney between 1993 and 1996. Mean tracking duration was 33 days.
Table 4. Description of foraging areas shown in Figures 6a-d. Depth and sediment type taken from British Geological Survey Sea Bed Sediment maps. Sediment classification (based on Falk, 1954): s & S, sand; g & G, gravel; m, mud.
Figures 1a-f, 1h-j and 1k-n. The tracks of individual seals, grouped by the three sets of deployments (a-f, Monachs adults; h-j, Monachs weaners; k-n, Orkney adults). Argos locations are shown as dots. The depth contours are at 200 and 500m and indicate the continental shelf edge. Note: there is no Fig 1g. Figures 2a-c, 2d. The combined seal tracks grouped by the three sets of deployments (a, Monachs adults; b, Monachs weaners; c, Orkney adults) and (d) Farnes adults. The locations are colour coded by individual seal. Figure 3. The locations of all the sites where seals in this study hauled out. Figures 4a-b, 4c and 4d. The direct distance to haulout sites (a-b, Monachs adults; c, Monachs weaners; d, Orkney adults). For each interpolated location two distances were calculated, a. from the nearest haulout site in Figure 3 (black filled), and b. from the haulout site where the seal was captured (line). The distances were truncated at 200 km. Figures 5a-c. The frequency distribution of all the interpolated location distances for each deployment group (a, Monachs adults; b, Monachs weaners; c, Orkney adults). Figures 6a-d. Maps showing the relative slow interpolated location densities (see Methods) grouped by the three sets of deployments (a, Monachs adults; b, Monachs weaners; c, Orkney) and (d) Farnes adults. The intensity of the cell colour is based upon the cell’s total count expressed in units of standard deviations from the mean count. The numbers refer to high densities of interpolated locations offshore, and are described in the text and in Table 4. HM refers to the Hebridean Minch. The letters refer to haulout sites, that are not readily identifiable as land on the maps, where there were high densities of interpolated locations. The key to these haulout sites is:
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