• Halichoerus grypus;
  • telemetry;
  • haul-out;
  • fisheries;
  • sandeel;
  • sediment type;
  • Farnes


  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

1. Grey seals Halichoerus grypus Fab. are large, numerous marine top predators. Fears concerning competition with fisheries have prompted calls for control measures. However, little is known about the areas where grey seals forage or the distances they may travel.

2. The movements of 14 grey seals caught at the Farnes in north-east England (12) and Abertay in eastern Scotland (2) between August 1991 and July 1993 were investigated using Argos Satellite Relay Data Loggers (SRDLs). A total of 1461 seal days of location and behavioural data (mean 104·3 days per seal) covered all months of the year except February and March.

3. The seal movements were on two geographical scales: long and distant travel (up to 2100 km away); and local, repeated trips from the Farnes, Abertay and other haul-out sites to discrete offshore areas.

4. Long distance travel included visits to Orkney, Shetland, the Faroes, and far offshore into the Eastern Atlantic and the North Sea. During travel the seals moved at speeds of between 75 and 100 km day–1 (0·87 and 1·16 m s–1). Most of the time, long distance travel was directed to known haul-out sites. The large distances travelled indicate that grey seals that haul out at the Farnes are not ecologically isolated from those at Orkney, Shetland and the Faroes.

5. In 88% of trips to sea, individual seals returned to the same haul-out site from which they departed. The durations of these trips were short (mean 2·33 days) and their destinations at sea were often localized areas characterized by a gravel/sand seabed sediment. This is the preferred burrowing habitat of sandeels, an important part of grey seal diet. This, and the fact that dives in these areas were primarily to the seabed, leads us to conclude that these were foraging areas. The limited extents of return-trips from a haul-out site (mean 39·8 km) suggest that the direct impact of seal predation may be greater on fisheries within this coastal zone, especially those near seal haul-out sites, rather than on fisheries further offshore.

6. An average of 43% of all the seals’ time was spent within 10 km of a haul-out site, although localized foraging areas were identified considerably further offshore. Proximity to a haul-out may provide safety from predation. Alternatively, these periods may be used for rest or social interaction, or we may be underestimating foraging activity near haul-out sites.

7. We suggest that the movement patterns observed in this study may persist through time and across the grey seals which haul-out at the Farnes. We also suggest that a study such as this could be combined with diet studies and haul-out censuses to map foraging intensity. Such information is an essential component of seal–fishery interaction models, upon which management decisions should be based.


  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Grey seals Halichoerus grypus Fab. are large, numerous marine top predators. The British population estimate for 1994 was approximately 108 000 and the population size is increasing by about 5% per year (Hiby et al. 1996). Diet studies, based primarily on scat samples, indicate that grey seals feed predominately on sandeels and gadoids, although there were regional and seasonal differences (Hammond & Prime 1990; Hammond, Hall & Prime 1994a,b). Hammond, Hall & Rothery (1994) estimated that approximately 76 000 tonnes of fish (including 28 000 tonnes of gadoids) may be taken annually by grey seals in the North Sea. However, North Sea spawning stocks of the major gadoid species have decreased markedly over the past 25 years, and some are at their historic minima (Hislop 1996). Although current fishing mortality is high, the influence of other biotic and abiotic factors on the decrease is unclear (Daan, Richardson & Pope 1996).

The increase in grey seal numbers combined with decreasing fish stocks has prompted calls for the UK grey seal population to be controlled under the Conservation of Seals Act 1970. However, management decisions must be based on an understanding of complex seal–fishery interactions (Yodzis 1998). An essential component of modelling these interactions is the temporal and spatial distribution of seal activity (Harwood & Croxall 1988; UNEP 1995).

Grey seals spend a portion of their time hauled out on land. Over the breeding season (October to November in Britain) females aggregate on land at specific breeding sites to give birth, suckle and mate (Bonner 1981). During the remainder of the year, large numbers of grey seals may be seen hauled out on land at a variety of sites, particularly during their annual moult in January to March. Haul-out sites are often in intertidal areas and may vary from discrete areas, where many hundreds of seals may congregate, to stretches of coast where a few or even single seals may haul-out. However, the relationship between where grey seals haul out and where they forage is unknown, except for the Moray Firth, Scotland (Thompson et al. 1996a). Flipper tagging and marking studies have shown that grey seal pups may travel far from their natal sites within their first few months at sea (Hickling 1962; Mansfield & Beck 1977; Baker 1978; SMRU 1984). More recently, telemetry studies have shown that adult grey seals may repeatedly travel hundreds of kilometres from one haul-out site to another (McConnell et al. 1992; Thompson et al. 1991; Hammond et al. 1992; Thompson et al. 1996a). Understanding the geographical relationships between the terrestrial phase of grey seals, when they may be censussed, and their aquatic, feeding phase is essential in assessing the distribution and intensity of foraging.

The Farnes (55°38′ N, 1°37′ W) are a group of small, uninhabited islands 3–6 km off the Northumbrian coast and are designated as a National Nature Reserve. About 1000 grey seal pups are born there annually, representing an all-age population of about 3400 (Hiby et al. 1996). Although this is only about 3% of the total British grey seal pup production, the Farnes represent the largest grey seal breeding site in England. In addition, the islands are used as a major moult and haul-out site for the remainder of the year and haul-out counts can exceed 1000 seals (B.J. McConnell, unpublished data). Abertay (56°27′ N, 2°47′ W), near Dundee, Scotland, consists of tidal, coastal sandbanks. Although over 1000 grey seals may haul-out at Abertay (B.J. McConnell, unpublished data), less than 10 pups are born there each year.

In this paper we describe the movements and foraging areas of 12 grey seals captured at the Farnes between 1991 and 1992, and two captured on the Abertay sandbanks in 1993. Movements and behaviour were studied using Argos Satellite Relay Data Loggers (SRDLs). This satellite telemetry system provided location fixes and detailed dive and haul-out behaviour data. A detailed description of this dive and haul-out data will be presented separately.


  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References


Grey seals were caught near haul-out sites, either at sea in tangle nets or on land using hand nets. The seals were anaesthetized with an intramuscular injection of Zoletil (Virbac, France) at an intramuscular dose rate of approximately 0·8 mg kg–1 body weight (Baker et al. 1990). The seal fur at the site of attachment was dried and cleaned with ethanol and the SRDLs were glued to the fur with a two-part, rapid-setting epoxy resin (Fedak, Anderson & Curry 1983). 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 released into the water once they had regained consciousness. Seal capture was permitted under licences issued by the Home Office and the Scottish Office under the Conservation of Seals Act 1970. All procedures were undertaken in accordance with the Animals (Scientific Procedures) Act 1986.

Telemetry system

The Argos satellite system (Argos 1989) consists of UHF receivers on board two polar-orbiting satellites that pick up signals (uplinks) from Argos transmitters within their field of view. The location of the transmitter is determined from the frequency Doppler shift of the signal. Up to 256 bits of data may be encoded within an uplink, which is relayed via ground stations and made available to the end user via a computer network. Location fixes are calculated at the ground station and are assigned an index of accuracy, termed Location Quality (LQ). LQ values vary from three, highest accuracy, to zero, unguaranteed accuracy.

A detailed description of the Argos Satellite Relay Data Loggers (SRDLs) used in this study is given in McConnell, Chambers & Fedak (1992); a summary is provided here. The SRDLs (Sea Mammal Research Unit, University of St Andrews, Scotland) measured 18 × 10 × 5 cm and consisted of a data logger interfaced to an Argos transmitter unit. Data from a depth sensor and a submergence sensor were used to determine the activity of the seal. The activity was classed as either ‘diving’ (deeper than 6 m for at least 6 s), ‘hauled out’ (dry for at least 240 s), or ‘at surface’. Distance swum was determined by a turbine odometer mounted on the top of the tag. Individual dive records included information on maximum depth, depth profile, distance swum and dive and surface duration. Series of dive and haul-out records were temporarily stored in memory and selected for transmission by a pseudo-random process such that all times of day were adequately represented, irrespective of diurnal satellite availability and animal behaviour.

Data processing

Location filtering and interpolation

The low and irregular uplink rate from an SRDL on a seal to the Argos satellites can result in potentially significant errors in some location fixes. Thus, locations were filtered by the algorithm described by McConnell, Chambers & Fedak (1992), using a ‘maximum speed parameter’ of 2·0 m s–1. The principle of this filter was to reject locations that would require an unrealistic rate of travel to achieve.

The resulting location fixes were distributed irregularly through time, and their rate and quality depended, in part, upon seal behaviour. For example, more uplinks to the satellites (and thus location fixes) 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 overcome this bias, new fixes were estimated at 3-h intervals by interpolation of the original track locations. Interpolation was suspended if the gap between original locations exceeded 24 h. Henceforth, the term ‘locations’ in this paper refers to these interpolated locations, unless explicitly referred to as ‘primary locations’.

Location densities

An area termed the ‘Farnes Box’ was defined as the region of the sea and haul-out sites between 55°N and 56°30′N and 0°W and 3°W. The choice of these boundaries reflected the area for which detailed seabed sediment data were available. The area was gridded into 2 × 2-km cells. For each seal, the density of locations in each cell was expressed as a percentage of all its locations within the Farnes Box. Each seal's grid was then overlaid to produce a grid of mean relative location densities for all seals. This method gave equal weight to all seals, regardless of tracking duration. The calculation was repeated where each seal's distribution was weighted by its tracking duration. However, the resulting density distributions were very similar.

Travel rate

For each location the mean of the speeds of travel from the previous location and to the next location was calculated and was termed travel rate. This provided an index of whether the seal was travelling rapidly or was relatively stationary in a localized area.

Activity classification

In order to construct a time–activity budget, locations were classified into three, mutually exclusive, types of behaviour, based upon a threshold distance from the nearest haul-out site (10 km) and threshold travel rate (0·5 m s–1). The classes were ‘Near Haul-out’ (NH), ‘Fast movement at Sea’ (FAS) and ‘Slow movement at Sea’ (SAS), and the classification scheme is shown in Table 1.

Table 1.  Classification scheme to construct a time-activity budget. Locations were classified into three, mutually exclusive, types of behaviour, based upon a threshold distance from the nearest haul-out site (10 km) and threshold travel rate (0·5 m s–1)
Class nameDescriptionCondition
NHNear Haul-out< 10 km from a haul-out site
FASFast movement At Sea>10 km from a haul-out site and travel rate > 0·5 m s–1
SASSlow movement At Sea> 10 km from a haul-out site and travel rate < 0·5 m s–1

Return-trips and travel-trips

Seal tracks and dive behaviour were reconstructed on the MAMVIS computer visualization system that allows movements to be replayed and animated against a background of coastline and bathymetry (Fedak, Lovell & McConnell 1996). Using this system, the duration and maximum extent of return-trips to sea (when the seal returned to the same haul-out site) and travel-trips (when it travelled to a different haul-out site) were recorded. Due to errors in the location estimates, haul-out and dive records, when available, were also used as independent cues to help determine trip start and end times. These cues allowed us to formulate trip definitions based solely on locations for the periods when the cues were not available. A trip was defined as starting when a seal moved outside a 10-km radius of a haul-out site and ending when it returned to within 10 km of the same haul-out site. A seal was deemed to have crossed the 10 km radius on the basis of a single location with LQ ≥1, otherwise on the basis of two consecutive locations.

Dive data

Approximately 30 000 dive records were obtained. The date and time associated with each dive were used to estimate where each dive took place by interpolation within the track data. Each dive record was then assigned a corresponding seabed depth taken using bathymetry data with 10 m vertical resolution (British Geological Survey 1989). Dive depth percentage was calculated as the maximum depth in each dive expressed as a percentage of the local seabed depth. Thus, a value of 100 indicated that the dive reached the seabed. Due to location fix error, estimates of local seabed depths were unreliable in areas where depth changed rapidly, for example near shore. The majority of unreliable depth estimates were excluded by excluding dives that occurred within 10 km from haul-out sites.


  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Deployment and telemetry performance

Details of SRDL deployment are given in Table 2. A total of 12 SRDLs was deployed on grey seals caught at the Farnes in four catching trips: three seals in August 1991, one in November 1991, three in March 1992 and five in July 1992. In addition, SRDLs were deployed on two seals caught at Abertay in July 1993. All seals appeared healthy with the exception of female F6–92, which was blind in both eyes but otherwise in good condition.

Table 2.  Details of grey seals fitted with SRDLs and duration of tracking. The first letter of the seal reference indicates the catching site: F, the Farnes, Northumbria; A, Abertay, Dundee. The lower part of the Table shows the number of seals tracked in each calendar month. A given month had to have at least two weeks of seal track data before it was included here
SealSexWeight (kg)Date caughtDate of last transmission Duration of tracking (days)
F1–91m18422 Aug 9101 Dec 91101
F2–91m12923 Aug 9115 Jan 92145
F3–91m19024 Aug 9128 Jan 92157
F4–91f16218 Nov 9128 Dec 9140
F1–92m19617 Mar 9217 Jul 92122
F2–92m15320 Mar 9220 May 9261
F3–92m18120 Mar 9212 Aug 92145
F4–92m19306 Jul 9207 Oct 9293
F5–92m9006 Jul 9229 Oct 92115
F6–92f16107 Jul 9209 Oct 9294
F7–92m15209 Jul 9227 Nov 92141
F8–92f18009 Jul 9211 Sep 9264
A1–93m8431 Jul 9326 Dec 93148
A2–93f11531 Jul 9304 Sep 9335
Seals tracked20033

The reproductive status of these seals was only known for female F4–91 which had just given birth to, and weaned, a pup. However, the sexual maturity of the remaining seals may be inferred from capture mass. Pomeroy et al. (1999) reported postpartum mass in grey seal females ranging from 131 to 251 kg. Thus, it is possible that even the smallest of the remaining study females (A2–93, 115 kg in July) could fall within this range by the November breeding period. Physiologically, males become sexually mature at 6 years (Hewer 1960), equivalent to a mass of approximately 120 kg (Anderson & Fedak 1985). Thus, the two smallest study males (F5–92 and A1–93) were probably physiologically immature, and are referred to here as subadult. However, the smallest males observed at a breeding site by Anderson & Fedak (1985) were at least 180 kg. Although three of the study seals (F2–91, F2–92 and F7–92) weighed less than this at the time of capture, they may well have contributed to the haul-outs of males observed at the periphery of breeding sites.

The 14 seals produced a total of 1461 days of data (mean of 104·3 days). The duration of tracking varied between 35 and 157 days. Tracking covered all calendar months of the year except February and March, the time of the annual moult. The summer months were best represented.

The mean daily primary location rates for all seals, grouped by Argos location quality (LQ) and by whether locations passed through the location filter, are given in Table 3. The filtering algorithm rejected 17% of primary locations, most of which were LQ zero. This left a mean of 6·46 primary locations per day, of which 65% were LQ zero. 96% of days during the tracking period had at least one primary location.

Table 3.  Mean number of primary locations per day for all seals, grouped by Argos location quality index, and by whether primary locations passed through the location filter. See Methods for explanation of ‘primary locations’ and the location filter
Argos location- quality indexMean number of prefiltered primary locations per day (% of total)Mean number of postfiltered primary locations per day (% of total)
30·24 (3)0·23 (4)
20·66 (9)0·63 (10)
11·40 (18)1·34 (21)
05·45 (70)4·26 (65)
All7·75 (100)6·46 (100)


The movements of the 14 seals (Fig. 1a,b as individual tracks, and Fig. 2 as combined tracks based on primary locations) can be characterized on two geographical scales. Some seals undertook long and distant travel (up to 2100 km away) to haul-out sites as far afield as Orkney, Shetland, the Faroes, and far offshore into the North Sea and the East Atlantic. At a smaller scale, there were local, repeated return-trips, from the Farnes, Abertay and other haul-out sites to specific areas at sea. The majority of these areas were located 20–60 km eastward and northward of the Farnes.


Figure 1. (a, b. ) Tracks of seals fitted with SRDLs. Six-hourly locations are shown as dots. The minimum map extent is set to the Farnes Box (55°N to 56°30′N and 0°W to 3°W). The limits of the Farnes Box are shown where a seal track went outside it. Details of individual seals are given in Table 2. Sh, Shetland; Fa, Faroes.


Figure 2. Tracks of all seals, based on primary locations, combined. Sites where the study seals hauled out on land are shown as circles. The limits of the Farnes Box are shown. Ab, Abertay; DN, Donna Nook; Fa, Farnes; Ke, Kerry; KL, King's Lynn; MF, Moray Firth; Or, Orkney; PF, Pentland Firth; Sc Scroby Sand.

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Four Farnes-caught seals travelled significant distances to haul-out sites away from the Farnes and these excursions were variable in direction, extent and duration. Travel was terminated at sites where seals were known to haul-out on land. At most of these sites we obtained positive haul-out records from our study animals after their arrival.

The most extensive travel was by female F8–92 over the summer months. It left the Farnes area after 6 days, travelled north for 1 week to Shetland where it spent 10 days at two haul-out sites. It then travelled for 9 days to an area near a haul-out site on the Faroes where it remained 10 days before heading south-west for 24 days. The last location received was 500 km west of Kerry, Eire. While at Shetland and the Faroes it made several, short (1–2 days) return-trips away from the haul-out sites. The total distance travelled over 64 days was 2100 km.

Male F3–92 visited Orkney (400 km from the Farnes) twice. These two excursions from the Farnes lasted 26 and 35 days, and consisted of four travel-trips which lasted 6·3, 7·4, 6·8 and 7·0 days. On the first excursion it visited haul-out sites in the Pentland Firth and then in the northern part of Orkney. On the second excursion, only the sites in the latter area were visited.

Male F3–91 undertook three major excursions between the last week in October and mid-December: 9 days to Kings Lynn (340 km from the Farnes); 11 days to Scroby Sands (400 km away); 18 days to Donna Nook (265 km away). The latter two excursions included eastward travel up to 300 km offshore.

Male F2–91 made a series of four travel-trips (0·7–7·7 days) to and from a haul-out site at the Isle of May (90 km from the Farnes). These excursions lasted between 5 and 14 days.

Most long-distance travel-trips, with the exception of male F3–91, followed direct routes from one haul-out site to another. The tracks of the outward and return travel-trips of male F3–92′s first Orkney excursion were very similar. On its second Orkney excursion, the travel-trips followed a more inshore route, but again the outward and return tracks were similar. The distant travel-trips of male F3–91 were less directed and followed large arcs into the North Sea.

Occasionally, distant travel was initiated only after a seal had spent a period of reduced travel at an offshore site. During distant travel, the mean daily speed (based upon mean daily location to reduce the influence of location error) was between 75 and 100 km day–1 (0·87 and 1·16 m s–1). The mean daily speed of the meandering tracks of male F3–91 was 79·9 km day–1 (0·92 m s–1).

Local movements

Figure 3 shows, for each seal, the mean daily distance from the nearest haul-out site (as shown in Fig. 2) used by the study seals (solid fill), and the Farnes (solid line). When the Farnes was the nearest haul-out site, the graphs coincide. In total, 269 trips were identified from the movement data, and of these 237 (88%) were return-trips (to the same haul-out site) and 32 (12%) were travel-trips (to a different haul-out site). In Orkney (male F3–92), Shetland and the Faroes (female F8–92), the large number of potential haul-out sites made it difficult to distinguish return-trips from travel-trips when a seal was travelling near-shore. These periods (a total of 51 days, equal to 3% of the total tracking period for all seals) were therefore not classified into trips. Details of duration and extent of trips is given in Table 4.


Figure 3. Distances of mean daily locations from the nearest haul-out site used by the study seals (Fig. 2) (solid fill), and the Farnes (line). When the Farnes was the nearest haul-out site, the graphs coincide. Distances over 200 km are truncated to 200 km.

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Table 4.  The number, mean duration and maximum distance of return-trips, and the number of travel-trips for individual seals. A return-trip was defined as starting when a seal moved outside a 10-km radius of a haul-out site and ending when it returned within 10 km of the same haul-out site. Return to a different haul-out site was termed a travel-trip
SealNumber of return-tripsMean return-trip duration (days)Mean return-trip maximum extent (km)Number of travel-trips
All seals2372·3339·832

The mean return-trip duration was 2·33 days (75% were 3 days or less) and the mean maximum extent was 39·8 km. However, there was considerable variability between seals. Female F8–92 made just two short return-trips from the Farnes (mean duration 0·56 days and maximum extent 20·4 km). In contrast, male F2–91 made six return-trips to an offshore area 180 km south-east of the Farnes, in addition to other shorter return-trips to areas 30–45 km from the Farnes. Return-trip duration was positively correlated with maximum extent (Pearson correlation coefficient = 0·42). Occasionally two seals left the Farnes at the same time, and travelled to the same offshore area. However, no persistent synchrony of movements was observed.

All Farnes-caught seals returned to the Farnes at least twice, and in most instances frequently, after initial capture. Five (all Farnes-caught) seals did not travel to other haul-out sites (Table 4). One Abertay-caught seal (female A2–93) was unique in that it did not travel to the Farnes but spent most of its tracking duration (a total of 35 days) within the outer reaches of the Firth of Forth. It spent 8 days at a submerged offshore bank known as the Wee Bankie (cluster 8 in Fig. 5).


Figure 5. Map of slow (travel rate < 0·5 m s–1) locations of all seals within the Farnes Box. Clusters of locations were ranked by their distance from the Farnes and numbered accordingly. The hatching shows areas where gravel was present in the sediment (derived from British Geological Survey 1989).

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Despite being blind in both eyes, the movements of female F6–92 were typical of the other study seals. It used the same localized foraging areas and had similar trip durations. It also carried out a travel-trip to the Isle of May (90 km away) and back.

A seasonal change in behaviour was seen in two seals. Prior to the breeding period (early October), male F3–91 changed from a pattern of frequent return-trips from the Farnes to nearby sites, to a series of long travel-trips out to sea, and then to other breeding sites at Donna Nook and Scroby Sands. Sub-adult male A1–93 made two return-trips (19 and 25 days) from the haul-out at Abertay to the area 20–40 km north-east of the Farnes over the first 60 days of tracking. In mid-October it then travelled to the Farnes from where it made repeated return-trips to the same offshore area.

Temporal and spatial distribution of locations

The time spent within 10, 25 and 50 km of the Farnes was 40%, 62% and 77%, respectively. Figure 4 maps the mean relative location density, based on a 2 × 2-km cell grid, of all seals within the Farnes Box. This map, whose area accounts for 88% of the seals’ time, shows the high activity around the haul-out sites at the Farnes, Isle of May and Abertay, and the locations of activity at sea.


Figure 4. Mean relative location density within 2 × 2-km cells for all seals within the Farnes Box. The grid cells are shaded by the number of standard deviations from the mean density. Ab, Abertay; IM, Isle of May; FF, Firth of Forth.

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SAS (‘Slow movement at sea’, see Table 1) locations offshore were spatially clustered. Eight of these clusters were identified within the Farnes Box and were ranked by their distance from the Farnes and numbered accordingly in Fig. 5. Table 5 shows which location clusters were used by each seal. To varying degrees, all seals contributed to these location clusters. Twelve out of 14 seals visited cluster 1, and eight visited cluster 2. Clusters 6, 7 and 8 were used by only one, different, seal each. Outside the Farnes Box, the area of dense clustering 180 km south-east of the Farnes was used exclusively by male F2–91. This seal also visited cluster 3 within the Farnes Box. In general, the clusters further from the Farnes were shared by fewer seals. Half the seals visited at least three different cluster sites each.

Table 5.  Details of the usage of areas with clusters of SAS locations within the Farnes Box by individual seals. A seal that visited a certain cluster is marked by a ‘ + ’. The locations of the clusters are shown in Fig. 5
Cluster number
Seal reference12345678
F1–91 +     
F2–91  +     
F3–91 +   
F1–92   + 
F2–92   +  
F3–92 +   
F4–92   +   
A1–93+  +   
A2–93       +

The seabed sediment type within the Farnes Box was primarily different ratios of sand and gravel (British Geological Survey 1989). Figure 5 shows SAS location clusters overlaid on a map which groups together all sediment types containing gravel (hatched areas). All location clusters were over a sand and gravel mix or just gravel sediment type [(g)S, gS, sG, G (as defined by Folk 1954)]. Despite the fact that 41% of the area was classified as sand (S), less than 1% of SAS locations were over this bottom type. The southern boundary of location cluster 3 clearly coincided with a local transition from sand and gravel [(g)S, gS] to sand (S). The association was also evident in localized areas used by two seals outside the Farnes Box. The tight cluster of offshore locations of male F2–91, 180 km south-east of the Farnes (Fig. 1a.), was associated with an isolated patch of gravel and sand (gS), surrounded by sand (S). The cluster of locations produced by male F1–92 to the east of the Farnes Box (Fig. 1a.) was also over a localized patch of sand and gravel (sG).

Activity classification

The aim of activity classification was to distinguish foraging at sea (SAS locations), travel to and from foraging areas (FAS locations) and near haul-out activity (NH locations). The frequency distribution of travel rates failed to reveal a discontinuity. However, the chosen SAS/FAS travel rate threshold of 0·5 m s–1 was well below the sustained travel rates (between 0·87 and 1·16 m s–1) observed in long travel-trips (see above). To determine the sensitivity of the activity classification to travel rate threshold, the classification was re-run with values of 0·4 and 0·6 m s–1.

Overall, the times spent in each activity class (with the ranges indicating travel rate thresholds of 0·4 and 0·6 m s–1) were:

NH (Near Haul-out)  43%;

FAS (Fast movement at Sea)  44·5 (40–48·2)%;

SAS (Slow movement at Sea)  12·5 (8·8–17)%.

The temporal variation in the activity classes of individual seals is shown in Fig. 6. The mean percentages of each class over 2-week intervals are plotted against calendar date, starting with March. The mean pupping date at the Farnes [7 November (Coulson 1981)] is also shown. Three adult males were tracked over the breeding season (November/December). All showed an increase in time NH activity between October and December. F1–91 spent more time in NH activity in early October, just prior to the breeding season but then resumed its previous lower value. F3–91 showed a similar pattern in December. F7–92 showed a more gradual rise in NH activity over the three months up to November. This seasonal trend was not apparent in the two subadult males (F2–91 and A1–93) tracked over the breeding season. The only female to be tracked over the breeding season (F4–91), reduced her NH activity from December onwards. During the remainder of the year, NH activity was variable within individual seals, with no apparent seasonal trend. Excluding the breeding season months (October to December) had little effect on the mean relative amount of time spent in the three activities (NH 40·0%, FAS 47·9%, SAS 12·1%). There was variability in NH activity among seals. Female F8–92, which travelled from the Farnes to Shetland, Faroes and then to the west of Ireland (Fig. 1b), spent only 13% of its time in NH activity.


Figure 6. Seasonal activity time budget, within 2-week bins, of individual seals. The activity was classed into one of three, mutually exclusive, types (see Table 1): NH, shown as a black bar (mean 43%); FAS, open bar (mean 44·5%); SAS, shaded bar (mean 12·5%). The triangle shows the mean pupping date at the Farnes (7 November).

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The SAS activity was variable, with no apparent seasonal trend. The largest amount of SAS activity was by female A2–93 which spend part of its time swimming slowly along the southern shore of the Firth of Forth, but without any positive indication that it hauled out on land.

Topography and dive depths

Most dives at deeper, offshore areas were to the seabed. The mean seabed depth within the Farnes Box was 65 m, with 87% of the area between 50 and 90 m deep. Clusters 5, 7 and 8 were over local submerged banks, shallower than the surrounding area (Fig. 5). Figure 7 shows that the frequency distribution of the dive depth percentages of all seals was bimodal. The dive depth percentage may have been subject to two errors: location error may have resulted in a seabed depth value for the wrong place, and the bathymetric data set may not have resolved fine-scale features. However, the higher mode was centred on a value of 100, indicating dives to the seabed, and 40% of dives were between values of 80 and 120. Values greater than 100 were considered as dives to the seabed. The dives in this higher mode group were characteristically longer and occurred in deeper (50–70 m), further offshore areas.


Figure 7. Frequency distribution of dive depth percentages– the maximum depths of individual dives expressed as a percentage of the local water depth (see Methods). A value of 100 indicates the seal dived to the seabed. Dives less than 10 km from haul-out sites were excluded, resulting in a data set of 15 930 dives.

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  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Long-distance travel

The movements of the grey seals in this study (Figs 1 and 2) ranged from large-scale, distant travel to shorter, smaller-scale return-trips to sea from haul-out sites. Long-distance travel (> 100 km) was generally consistent in direction and speed (75–100 km day–1), and the outward and return routes were often very similar (for example, male F3–92). An exception was male F3–91 which, although travelling within the same speed range, travelled in wide arcs out into the North Sea. Travel terminated at sites where other seals were known to haul out on land. Positive haul-out records indicated that most study seals hauled out some time after arrival at such sites. Such direct travel to, and arrival at, a distant haul-out site suggests both navigational ability and knowledge of that site. Although the travels of female F8–92 were, by far, the most distant, there was nothing about her condition or the deployment which singled her out from the other study animals. Indeed, her directed travel to sites where other seals are known to haul-out in Shetland and the Faroes suggests her navigational skills were good. The cues the study seals used to aid navigation are not known. However, the apparent ‘normal’ movements of blind female F6–92 suggest that visual cues are not essential.

The distant travel shown here, and in other studies (McConnell et al. 1992; Thompson et al. 1996a), indicates that grey seals at the Farnes, Orkney, Shetland, the Faroes, and even off the west coast of Ireland, are not ecologically isolated. Such geographical mixing has important consequences in modelling epidemiology, fishery interactions and population management. For example, local population control measures may have a reduced effect due to the interchange of seals from other regions.

Inference of foraging

A temporal view of the tracks (Fig. 3) shows that most movements consisted of a series of trips from haul-out sites to offshore areas. These areas were often very localized and visited repeatedly. We have no direct evidence of when feeding was taking place, but Thompson et al. (1991) directly observed grey seals making similar trips from the Farnes to specific offshore areas. The fact that the seals remained in these localized areas for periods of hours to days, they repeatedly dived to the seabed, together with the presence of pisciverous seabirds led Thompson et al. (1991) to infer foraging. We also infer that the similar behaviour observed in this study represents foraging. We do not, however, exclude the possibility that seals were also foraging near haul-outs or while travelling.

The proposal that local trips represent foraging is further supported by a consideration of the seals’ dive behaviour and diet. The dive depth data shown in Fig. 7 indicate that many dives were to, or near to, the seabed. This is consistent with diet information. Hammond & Prime (1990) and Hammond, Hall & Rothery (1994) analysed grey seal scats collected at the Farnes and estimated that sandeels (Ammodytidae), which spend part of their time burrowed in the seabed (Wheeler 1978), comprised up to 87% of their diet by weight. Further evidence for the role of sandeels when seals are foraging at these offshore areas is based on seabed type. Sandeels have a burrowing preference for a mix of gravel and sand, avoiding pure sand (Reay 1970) and this was the seabed type at all of the areas where we believe foraging was occurring (Fig. 5). Indeed, these foraging areas were often delimited by a transition to a sediment type that did not contain gravel. In this study we do not have any direct evidence of whether the study seals preyed on sandeels themselves (either while the sandeels were shoaling or by the seals disturbing them into the water column) or on other fish species associated with sandeels.

Distribution of foraging effort

The distribution of seal activity within the Farnes Box is shown in Fig. 4. Although the tracks varied with respect to duration, time of year and individual seal, this map does emphasize two key features of the tracks that persisted, to a large degree, across most study seals and through time. These features are the large amount of activity around the haul-outs at the Farnes (and to a lesser extent the Isle of May and Abertay) and the fact that activity at sea was not randomly distributed, but focused within certain localized areas.

We distinguish eight clusters of SAS locations where we believe foraging was taking place within the Farnes Box (Fig. 5). Although all these clusters were within areas containing gravel in the sediment, the gaps between some of the clusters showed no obvious discontinuity in sediment type. This may be due to an insufficient spatial resolution in the sediment type map, an inherent patchiness in sandeel distribution, or to predation on other species (e.g. gadoids) that may not have a preference for specific sediment types. Because the clusters of SAS locations reflect the specific individual foraging areas of the study seals, it is possible that a greater sample of animals would have resulted in a greater spread of clusters.

Return-trips were short (mean 2·33 days) and relatively close to haul-out sites (mean 39·8 km). The positive correlation between return-trip extent and duration was also found in harbour seals Phoca vitulina L. (Thompson et al. 1998). The general pattern of proximity of foraging to haul-out sites suggests that, although grey seals are capable of extended and distant travel, the impact of predation may be greater on inshore fisheries, particularly those close to seal haul-out sites, rather than on fisheries further offshore.

The duration, extent and regularity of return-trips varied considerably, both from seal to seal and through time. Reasons for this variability may be endogenous or external. Endogenous factors include the requirements to spend more time ashore at specific sites, and with sufficient energy reserves, during the breeding and moult seasons. The moult period (February and March) is not covered in this study due to detachment of the SRDLs. However, two seals did change the pattern of their movements towards the start of the breeding season (October). While subadult male A1–93 continued to forage off the Farnes, it changed its haul-out site from Abertay, a non-breeding haul-out site, to the Farnes. Male F3–91 initiated a series of long travel-trips to other breeding sites at Donna Nook and Scroby Sands. There is evidence that grey seals return to the same breeding site each year (Pomeroy et al. 1994; Twiss, Pomeroy & Anderson 1994) and this may be reflected in the changes in movements of adult male F3–93 towards the start of the breeding season.

External factors that may influence foraging patterns include changes in prey availability. This has been suggested (on an interannual scale) by Boyd (1996) for Antarctic fur seals Arctocephalus gazella Peters where the length of foraging trips varied with the changing availability of krill. Thompson et al. (1996b) showed that harbour seal foraging distribution changed with prey availability. In this study we have no direct data on prey availability. However, we suggest that the interruption of foraging trips by long-distance travel, observed in this study, was not due to prey depletion. On different dates, male F3–92 and female F8–92 left the Farnes to travel to Orkney and to Shetland. However, while each was travelling north, concurrently tracked seals continued to forage off the Farnes. The costs and benefits of long-distance travel for grey seals are not clear. In terms of cost, distant travel often took little more time than a return to a local haul-out plus time spent around that haul-out. Potential benefits include the exploration of new foraging areas and the possibility of opportunistic foraging en route.

Understanding the activity budgets of seals is essential in estimating their energy requirements. The activity classification we have presented here is a simple attempt to partition seals’ time into one of three classes: near haul-out (NH), travelling (FAS) and foraging (SAS). In short trips it was difficult satisfactorily to split the time spent travelling and foraging due to the temporal resolution and spatial error of the track data. In addition, the FAS/SAS split was affected by the choice of location travel rate threshold. Failure to distinguish a natural break in the frequency distribution of location travel rates may have been due in part to the smoothing effect of using interpolated 3-hour locations rather than primary locations, and also to the effect of location error. The two factors, spatial and temporal resolution of locations and choice of location travel rate threshold, may account for some of the individual and temporal variability in SAS activity (Fig. 6). The future incorporation of dive-type information, which may be indicative of foraging (Thompson et al. 1991), may increase the ability to identify foraging.

The adult males tracked over the breeding season spent more time in NH activity during this period. However the extent and timing of these increases varied and they may reflect the breeding status achieved during the previous and current season. No such trend was apparent in the subadult males.

It is of interest that the mean NH activity was as high as 40% (excluding the breeding season) while our proposed foraging areas were significantly further than 10-km offshore and only an average of 12% of time was spent in the SAS activity. Why so much time was spent near haul-out sites, and, indeed, why seals haul out on land at all, is a matter of some debate (Brasseur et al. 1996; Watts 1996). A possible explanation is that seals may be safer from predation, for example by killer whales Orcinus orca L., at or near a haul-out site. Alternatively, sufficient food may have been caught during offshore trips and the periods at or near haul-out sites may be periods of rest or social interaction. A third possibility is that we underestimate foraging activity near haul-out sites.

Persistent, localized foraging areas were used by seals that hauled out at the Farnes and Abertay. Can this pattern be quantified and generalized? The number of seals hauling out may be censussed by land or aerial counts and the pattern of haul-out behaviour may be monitored by studies such as this one, conventional VHF telemetry (Thompson 1989; Hammond et al. 1992) or a series of direct counts (Grellier, Thompson & Corpe 1996). In addition, the dynamics of interhaul-out site usage may be estimated by mark–recapture models based on photo-identification data (Hiby 1994). The incorporation of such data (which were not collected during this study) would allow the absolute intensity of foraging of these seals to be mapped. Additional data on diet would allow the predation pressure on different prey species to be mapped. While this may be appropriate in providing a static map of predation pressure, the predictive power of this approach depends on the temporal and spatial predictability of prey abundance and of the prey preference of seals across age, sex, and season. We suggest that this approach may have some predictive power at the Farnes for three reasons. First, the repeated use of the localized foraging areas off the Farnes by all but two study animals (female F8–92 and female A2–93) for most of the tracking periods is a persistent feature across animals and seasons. Secondly, diet studies (Hammond & Prime 1990) have shown that, although the importance of sandeels decreased in spring, the predominance of sandeels in the diet at the Farnes during the pupping season (October to December) persists across years. Thirdly, sandeels have a requirement for specific sediment types, the location of which is stable. Thus, we suggest that the patterns of movement and foraging observed in this study may persist through time and among seals that haul out at the Farnes. This may not, however, be the case in other geographical regions if grey seals forage on prey which have chaotic or transient distributions.


Data on haul-out site usage and foraging patterns also aid interpretation of scat analysis results. Material recovered from scats provides information biased towards the previous 1–2 days’ feeding (Prime & Hammond 1987), the approximate time taken for prey remains to pass through the gut. If foraging trips were long relative to this period, data from scats may not be representative of diet if the prey species consumed changed during a trip. However, most foraging areas identified here were not far in time from a haul-out site; the mean return-trip length was 2·3 days and 75% of return-trips were of three days or less.

The large majority of our sampled trips to sea ended at the Farnes. Five of the 14 seals used the Farnes exclusively for hauling out. Only one seal did not use the Farnes at all, after capture and release. This persistent use of the Farnes as the main haul-out site, combined with the relatively short return-trip duration, gives confidence that inferences about grey seal diet based on scat analysis are justified in this respect.


  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

The movement information presented here represents an insight into grey seal at-sea behaviour that is unique in both its scope and detail. The interpretation of SRDL data, however, does require an appreciation of the temporal resolution and spatial accuracy of locations such that small-scale behaviour can be distinguished from artefact. A rigorous statistical analysis of Argos track data is needed. Caution must also be exercised when inferring population behaviour from a small sample of study animals. The tracking periods in this study varied between 35 and 157 days, from 14 seals of both sexes and of varying weights, from two catching sites, over three seasons and over three years. However, although there was variability in behaviour, certain patterns were repeatedly observed. These included: a high percentage of time spent at or near haul-out sites, short trips to localized offshore areas often with characteristic sediment type, mainly benthic dives and large-scale geographical travel. The persistence of such patterns provides confidence that data of this quality and quantity can be incorporated into the future development of spatially and temporally explicit models of seal–fish interactions, upon which management decisions may be based (Harwood & Croxall 1988; UNEP 1995).


  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

This study was funded in part by contract MF 0503 from the UK Ministry of Agriculture, Fisheries and Food, with additional support from the Natural Environment Research Council. We thank the National Trust for permission to use the Farnes and the many people who helped in the development of the telemetry and data analysis systems, and in deployments in the field. In particular we thank Charlie Chambers, Colin Hunter, Kevin Nicholas and the many National Trust Wardens. We also gratefully thank the RNLI crew of the North Sunderland Lifeboat for special assistance one stormy night.


  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  • Anderson, S. & Fedak, M.A. (1985) Grey seal males: energetic links between size and sexual success. Animal Behaviour, 33, 829838.
  • Argos (1989) Guide To the Argos System. Argos CLS, Toulouse.
  • Baker, R.R. (1978) The Evolutionary Ecology of Animal Migration. Hodder & Stoughton, London.
  • Baker, J.R., Fedak, M.A., Anderson, S.S., Arnbom, T. & Baker, R. (1990) Use of a tiletamine-zolazepam mixture to immobilise wild grey seals and southern elephant seals. Veterinary Record, 126, 7577.
  • Bonner, W.N. (1981) Grey seal. Handbook of Marine Mammals, Vol. 2 (eds S.H.Ridgway & R.J.Harrisson), pp. 111144. Academic Press, London.
  • Boyd, I.L. (1996) Temporal scales of foraging in a marine predator. Ecology, 77, 426434.
  • Brasseur, S., Creuwels, J., V/d Werf, B. & Reijnders, P. (1996) Deprivation indicates necessity for haul-out in harbor seals. Marine Mammal Science, 12, 619623.
  • British Geological Survey (1989) Sheet 55°N-02°W, Farnes, 1: 250,000 series, Seabed Sediment Map. Ordnance Survey for the British Geological Survey, Natural Environment Research Council. Southampton.
  • Coulson, J.C. (1981) A study of the factors influencing the timing of breeding in the grey seal Halichoerus grypus. Journal of Zoology, 194, 553571.
  • Daan, N., Richardson, K. & Pope, J.G. (1996) Changes in the North Sea ecosystem and their causes: Århus 1975 revisited. ICES Journal of Marine Sciences, 53, 879883.
  • Fedak, M.A., Anderson, S.S. & Curry, M.G. (1983) Attachment of a radio tag to the fur of seals. Journal of Zoology, 200, 298300.
  • Fedak, M.A., Lovell, P. & McConnell, B.J. (1996) MAMVIS, A marine mammal behaviour visualisation system. Journal of Visualisation and Computer Animation, 7, 141147.
  • Folk, R.L. (1954) The distinction between grain size and mineral composition in sedimentary rock nomenclature. Journal of Geology, 62, 344359.
  • Grellier, K., Thompson, P.M. & Corpe, H.M. (1996) The effect of weather conditions on harbour seal (Phoca vitulina) haulout behaviour in the Moray Firth, northeast Scotland. Canadian Journal of Zoology, 74, 18061811.
  • Hammond, P.S., Hall, A.J. & Prime, J. (1994a) The diet of grey seals around Orkney and other island and mainland sites in northeastern Scotland. Journal of Applied Ecology, 31, 340350.
  • Hammond, P.S., Hall, A.J. & Prime, J.H. (1994b) The diet of grey seals in the Inner and Outer Hebrides. Journal of Applied Ecology, 31, 737746.
  • Hammond, P.S., Hall, A.J. & Rothery, P. (1994) Consumption of fish prey by grey seals. Grey Seals in the North Sea and their Interactions with Fisheries. Final Report to the Ministry of Agriculture, Fisheries and Food Under Contract MF0503 (eds P.S.Hammond & M.A.Fedak), pp. 3569. Sea Mammal Research Unit, Cambridge.
  • Hammond, P.S., McConnell, B.J., Fedak, M.A. & Nicholas, K.S. (1992) Grey seal activity patterns around the Farne Islands. Environmental Management, Science and Technology. Wildlife Telemetry: Remote Monitoring and Tracking of Animals (eds I.G.Priede & S.M.Swift), pp. 677687. Ellis Horwood Ltd, Chichester.
  • Hammond, P.S. & Prime, J.H. (1990) The diet of British grey seals (Halichoerus grypus). Population Biology of Sealworm (Pseudoterranova decipiens) in Relation to its Intermediate and Seal Hosts (ed. W.D.Bowen), pp. 243254. Canadian Bulletin of Fisheries and Aquatic Sciences, 222.
  • Harwood, J. & Croxall, J.P. (1988) The assessment of competition between seals and commercial fisheries in the North Sea and the Antarctic. Marine Mammal Science, 4, 1333.
  • Hewer, H.R. (1960) Behaviour of the grey seal (Halichoerus grypus Fab.) in the breeding season. Mammalia, 24, 400421.
  • Hiby, A.R. (1994) Abundance estimates for grey seals in summer based on photo-identification. Grey seals in the North Sea and their interactions with fisheries. Final Report to the Ministry of Agriculture, Fisheries and Food Under Contract MF0503 (eds P.S.Hammond & M.A.Fedak), pp. 522. Sea Mammal Research Unit, Cambridge.
  • Hiby, A.R., Duck, C.D., Thompson, D., Hall, A.J. & Harwood, J. (1996) Seal stocks in Great Britain. NERC News, 1996 (January) 20–22.
  • Hickling, G. (1962) Grey Seals and the Farne Islands. Routledge and Kegan Paul, London.
  • Hislop, J.R.G. (1996) Changes in North Sea gadoid stocks. ICES Journal of Marine Sciences, 53, 11461156.
  • Mansfield, A.W. & Beck, B. (1977) The Grey Seal in Eastern Canada. Fisheries and Marine Science Technical Report. Department of the Environment and Fisheries, Canada.
  • McConnell, B.J., Chambers, C. & Fedak, M.A. (1992) Foraging ecology of southern elephant seals in relation to the bathymetry and productivity of the Southern Ocean. Antarctic Science, 4, 393398.
  • McConnell, B.J., Chambers, C., Nicholas, K.S. & Fedak, M.A. (1992) Satellite tracking of grey seals (Halichoerus grypus). Journal of the Zoological Society of London, 226, 271282.
  • Pomeroy, P.P., Anderson, S.S., Twiss, S.D. & McConnell, B.J. (1994) Dispersion and site fidelity of breeding female grey seals (Halichoerus grypus) on North Rona, Scotland. Journal of Zoology, 233, 429448.
  • Pomeroy, P.P., Fedak, M.A., Rothery, P. & Anderson, S. (1999) Consequences of maternal size for reproductive expenditure and pupping success of grey seals at North Rona, Scotland. Journal of Animal Ecology, 68, 235253.
  • Prime, J.H. & Hammond, P.S. (1987) Quantitative assessment of grey seal diet from faecal analysis. Approaches to Marine Mammal Energetics (eds A.C.Huntley, D.P.Costa, G.A.J.Worthy & M.A.Castellini), pp. 161181. Society of Marine Mammalogy Special Publication, 1. Allen Press, Lawrence, Kansas.
  • Reay, P.J. (1970) Synopsis of biological data on north Atlantic sandeels of the genus Ammodytes. FAO Fisheries Synopsis no. 82.
  • SMRU (1984) Interactions Between Grey Seals and UK Fisheries. A report on research conducted 1980–83 for the Department of Agriculture and Fisheries for Scotland. Sea Mammal Research Unit, Cambridge.
  • Thompson, P.M. (1989) Seasonal changes in the distribution and composition of common seal (Phoca vitulina) haul-out groups. Journal of Zoology, 217, 281294.
  • Thompson, D., Hammond, P.S., Nicholas, K.S. & Fedak, M.A. (1991) Movements, diving and foraging behaviour of grey seals, Halichoerus grypus. Journal of Zoology, 224, 223232.
  • Thompson, P.M., MacKay, A., Tollit, D.J., Enderby, S. & Hammond, P.S. (1998) The influence of body size and sex on the characteristics of harbour seal foraging trips. Canadian Journal of Zoology, 76, 10441053.
  • Thompson, P.M., McConnell, B.J., Tollit, D.J., MacKay, A., Hunter, C. & Racey, P. (1996a) Comparative distribution, movements and diet of harbour and grey seals from the Moray Firth, NE Scotland. Journal of Applied Ecology, 33, 15721584.
  • Thompson, P.M., Tollit, D.J., Greenstreet, S.P.R., MacKay, A. & Corpe, H.M. (1996b) Between-year variations in the diet and behaviour of harbour seals in the Moray Firth; causes and consequences. Aquatic Predators and Their Prey (eds S.P.R.Greenstreet & M.L.Tasker), pp. 4452. Fishing News Books, Oxford.
  • Twiss, S.D., Pomeroy, P.P. & Anderson, S.S. (1994) Dispersion and site fidelity of breeding male grey seals (Halichoerus grypus) on North Rona, Scotland. Journal of Zoology, 233, 683693.
  • UNEP (1995) Marine Mammal Fishery Interactions: Analysis of Cull Proposals. UNEP (OCA) /MM.SAC.3/1. United Nations Environment Programme Scientific Advisory Committee on the Marine Mammal Action Plan, Crowborough, UK.
  • Watts, P. (1996) The diel hauling-out cycle of harbour seals in an open marine environment: Correlates and constraints. Journal of Zoology, 240, 175200.
  • Wheeler, A. (1978) Key to the Fishes of Northern Europe. Frederick Warne (Publishers) Ltd., London.
  • Yodzis, P. (1998) Local trophodynamics in the interaction of marine mammals and fisheries in the Benguela ecosystem. Journal of Animal Ecology, 67, 635658.

Received 15 April 1998; revision received 12 May 1999