A comparison of the number of grey seals (Halichoerus grypus) using two tidal haul-outs in Cornwall, UK. Luke F. Blakemore University of Plymouth, Cornwall College Campus, Falmouth, Cornwall, TR11 3QS This study investigated the abundance of grey seals at two tidal haul-outs in Cornwall. Land based surveys were carried out on twelve days at Gwennap Head and nine days at Pendennis Point within a six week period. A boat based survey took place on three separate days observing Black Rock, Falmouth. Observations took place one hour before, half an hour before, half an hour after, one hour after low tide and at low tide. At each period the number of seals, and whether they were hauled-out or no, were recorded. Where possible the age and sex of each seal were also recorded. Photographs were taken whenever possible and where used to create a photo identification database for grey seals within the Fal. Results showed that there was no significant difference in the number of seals sighted at Black Rock and Gwennap Head but they did indicate that sea state may affect grey seals hauling-out. The data collected may help future projects within this field. The photo identification database will help with future work and can be linked into and used by local and regional organisations and businesses to identify ‘local’ seals within Falmouth and to help track travelling patterns by individuals.
Keywords: numbers, grey seals, Cornwall, Falmouth, Porth Gwarra, Black Rock, Hella Point
INTRODUCTION The worlds grey seal populations can be split into three isolated populations – the East Atlantic, West Atlantic and the Baltic (Boskovic et al. 1996). Around 45% of the world’s population can be found around the British coast (SCOS, 2008), with 0.5% of this found around Cornwall and the Isles of Scilly (ibid). In 2007 it SCOS stated that the total UK population was increasing at around 2.5% per year. With almost half the world’s population of grey seals found around the UK, it is important to monitor numbers and distribution. Whilst spending most of their lives at sea, grey seals regularly ‘haul-out’ on beaches and rocky outcrops. Seals haul-out for a number of reasons including courting, breeding, bathing in sun to warm themselves, to rest and to allow food to digest. Haul-outs are therefore important for the survival of grey seals. Although an important aspect of seal
ecology, there is no true definition as to what classifies a seal haul-out (Sayer, 2010) Although there is no true definition of a haul-out, there are areas that can be identified in which seals often haul out, these are usually deemed to be seal ‘haul-outs’. These are usually areas that are inaccessible by land, such as coves, caves, rocky out-crops and small islands. Due to the need for seals to haul-out, it is therefore important to monitor the usage of haulouts to help indicate if there is a decline or increase in seal numbers. There has been a significant growth to marine mammal tourism worldwide due to their appeal and ease of access from coastal areas (Constantine, 1999) and within Cornish waters the grey seal is the most commonly seen marine mammal (Barnett & Wescott 1999). As such it becomes more important to monitor seal populations and the numerous threats to their well being, such as boating activities, habitat loss (Harwood, 2001), pollution (Wolkers et al, 2004) and fisheries (through by-catch, overfishing or culling).
The purpose of this survey was to find out whether there are any similarities or differences in the amount of seals observed at Black Rock, Falmouth and Gwennap Head, with the hypothesis that there would be more seals at Gwennap because there is less human disturbance at the haul-out than that of Black Rock.
MATERIALS AND METHODS Survey area BLACK ROCK, FALMOUTH AND THE FAL ESTUARY “Falmouth Harbour, including the Carrick Roads, is reputed to be the third largest natural harbour in the world” (Falmouth Harbour Commissioners). Black rock is situated within the centre of the mouth of the Fal. It consists of a fixed concrete marker built on top of a small granite outcrop. Black rock is a tidal ‘haul-out’ for grey seals within the Fal area.
GWENNAP HEAD, PORTHGWARRA Porthgwarra is situated near Lands End, on one of the most southerly tips of Cornwall. The area is “one of the best sea watching sites in England” (Walker and Wilson, 2007). The ‘haul-out’ at Gwennap head is a granite outcrop and is also tidal haul-out for many seals within the surrounding areas. Seals can be seen hauled-out on both Black Rock and at Gwennap Head around low tide. At high tide, the haul-outs become covered by the sea and seals will move off the rocks into the water. Figure 1 shows the haul-outs observed in this study.
Survey method LAND BASED RECORDING
The survey was carried out over six and a half th th weeks, 11 March to 26 April 2011. Each haulout was surveyed on twelve days each and surveys took place at low tide and 1 hour, ½hour before and after low tide. The haul-outs were observed from land based viewing points. Black Rock was observed from Pendennis Point and Gwennap Head haul-out was observed from the cliff top beneath the
coastguard station. Observation were made using Bresser 10 x 50 Bak-4 long eye relief 114m/1000m binoculars. At each time interval recordings were taken of weather conditions, sea state, number of seals hauled out and/or in the water and where possible the age and sex of seals as well as time of low tide, type of tide (neap or spring) and tidal range. Sightings outside the intervals were recorded separately along with time of the sighting. Tides were calculated using the Mylor Yacht Harbour’s 2011 tide time table. Sea state was calculated using the Beaufort scale. BOAT BASED RECORDING
Observations were taken from a RIB (Rigid Inflatable Boat) on three occasions at Black Rock, Falmouth. These observations took place over two hours (1hour either side of low tide). A recording of weather, sea state and seal numbers/age/sex and whether the seals were hauled-out or in the water were taken every half an hour. PHOTO IDENTIFICATION OF GREY SEALS
The pelage patterns of grey seals stay the same throughout their life, and much like the fingerprints of humans, no two seals pelage patterns are the same. It is therefore possible to identify individuals and keep track of them using their patterns. Photographs were taken, where possible, using a DSLR Cannon 450D EOS with a SIGMA DG 70 – 300mm 1:4 – 5.6 lens to produce a data base that can show tracking of individuals. Photographs were taken of the neck and sides of seals where possible, these were then uploaded onto a computer. Once uploaded the photos were compressed, cropped and enlarged. It was then possible to identify patterns on individual seals and create a database showing the names and identifying patterns for individuals. Once the database was created it was/is possible to match historical photos to identified individuals (Sayer, 2011). This can help give an indication as to whether any individual seals re-use the haul-outs or if the seals seen were different.
RESULTS
in the water at Gwennap, but not hauled-out. From the seals observed 5 were identified as adult males, 1 adult female, 1 juvenile and 5 were unable to be identified. Table 1 shows the comparison of seal numbers/age/sex between Black Rock and Gwennap Head.
Observed grey seal numbers A total of 12 seals were observed during the survey, 4 seals were seen at Black Rock (3 from land based survey and 1 from boat based survey) and 8 were seen at Gwennap Head, all from land based surveys. Out of the seals observed 4 were seen hauled-out, 7 were observed in the sea and 1 was seen in the sea and hauled out.
A Mann-Whitney U-test was performed using the collected data to find if there was any significant difference between the numbers of grey seals observed at each site. Figure 1 shows the results of the Mann-Whitney test.
All seals seen at Black Rock were hauled-out for the survey periods. All but one seal were observed
Table 1 - Results showing the number of males/females/juveniles/unknown grey seals and whether they were hauled out or not at each site.
Site Black Rock – Hauled-out Gwennap Head – Hauled-out Gwennap Head – In Sea Total
Survey Number 1 2 3 4 5 6 7 8 9 10 11 12
Black Rock Score Rank A 0 0 0 0 0 0 0 0 0 0 0 0 2 6.5 0 0 0 0 0 0 1 3 1 3
Adult Males 1 1 3 4
Juveniles
Unknown 3
1 1
1 1
2 5
Gwennap Head Score Rank B 0 0 0 0 1 3 0 0 0 0 2 6.5 0 0 0 0 1 3 0 0 1 3 3 8
Ra = 12.5 Xa = 0.3 SDa = 0.65
Adult Females
Total 4 1 8 7
Ua =
NaNb + Na(Na + 1) - Ra 2 Ua = 12*12 + 12(12 +1) - 12.5 2 Ua = 144 +78 - 12.5 Ua = 209.5 Ub =
NaNb +
Nb(Nb + 1) - Rb 2 Ub = 12*12 + 12(12 +1) - 23.5 2 Ub = 144 + 78 - 23.5 Ub = 198.5
Rb = 23.5 Xb = 0.7 SDb = 0.98
Ua= 209.5 Ub = 198.5 198.5 is taken as the U value The Critical Value for 12 samples is 42 The U-value is higher than the Critical value, therefore the alternative hypothesis can be rejected and the null hypothesis can be accepted
Figure 1 - Statistical analysis of data using Mann-Whitney U-test
Photo identification Due to the distance from the haul-out sites only one set of photos, of one seal at black rock were able to be used for photo identification purposes, however photographs of various seals on black rock were obtained from a few sources, and with a total of 12 seals were identified. A catalogue of
identification photographs has been created as well as a log of the dates each seal has been spotted. This catalogue and log are a working progress can be used for further studies and can/will increase as
more photographs are sought out and backlogged. There has already been one seal that has been seen on three separate occasions over a period of
5 months by three different observers. Table 2 shows the log for seal sighting at black rock and surrounding areas.
Table 2 - ID file and log for Halichoerus grypus at Black Rock and surrounding areas
Date
09/09/2010
Surveyor
Trudy Russell
No of seals
3
Luke 19/04/2011 Blakemore
1
20/09/2010
Jason FSS
2
25/09/2010
Jason FSS
1
29/07/2010
Trudy Russell
5
13/06/2008
Bioblitz
1
18/04/2010 Simon Bone
1
Number of Number of AM:AF:U seals ID'd J:JM:JF: from New List of ID Low tide W:WCP catalogue seals ID'd codes time
02:01:00: 00:00:00: 00 01:00:00: 00:00:00: 00 00:02:00: 00:00:00: 00 00:01:00: 00:00:00: 00 00:04:00: 00:00:00: 00 01:00:00: 00:00:00: 00 00:01:00: 00:00:00: 0
DISCUSSION The observations made indicate that there may be more grey seals within the Gwennap head area than there are around Falmouth Bay. This is what would be expected when looking at historical seal censuses and previous data sets (Wescott, 1997; Leeney et al, 2010). However the statistical analysis of the data revealed that there was in fact no significant difference between the numbers of grey seals at each site. The survey took place between March and April; previous research shows that this is the key time of year for grey seals to moult (McConnell et al, 1999; Sayer per’s comms, 2011). When moulting, grey seals become more constrained to haul outs and will generally spend less time travelling from site to site, tending to stay close to a single haul-out (Sayer per’s comms, 2011).
0
3
BRM1 BRF2 BRM3 is BRM4
0
1
BRM5
0
2
BRF6 BRF7
0
1
1
3
BRF8 BRF8 BFF9 BRF10 BRF11
0
1
MAEM12
1
0
BRF8
Time of survey
Going in or out Sea force
Wind force
There are many factors that influence whether or not a seal will haul-out and whether or not it will be observed. Effecting factors include sea state, weather, visibility and human interactions. These variables may have caused errors within the data collected; many of the sightings a Gwennap Head were in sea states 3 or 4 and were too rough for the grey seals to haul-out, whereas at Black rock, sea conditions were generally fairer, 1 or 2, and all seals observed were hauled out throughout the survey periods. The photo identification of grey seals at Black Rock and surrounding area was more successful than originally hoped. Twelve individual seals were identified using past and present photographs and a database has been created to display each of the individuals identifying patterns. One seal placed onto the
Wind direction
S e a ls disturbe d Y e s/ No, c a use a nd c ode (1= se a ls look round; 2 = se a ls move but not to se a ; 3 = se a ls into se a . )
Ot infor on com s
data base was photographed on three separate dates within 5 months of each other, indicating that grey seals will re-use the same haul-outs; unfortunately it does not indicate whether this seal was just passing by on three separate occasions or whether it is a ‘local’ seal. This database is the baseline of a working progress and can be added to as and when more surveys take place. It is also possible to back date sightings using photographs to help identify new seals and/or build-up a time line of individuals returning to Falmouth.
ACKNOWLEDGEMENTS The author would like to acknowledge the funding support of Falmouth Marine School and Falmouth Harbour Commissioners. The author would like to thank Sue Sayer for seal photo identification training as well as local knowledge and guidance; thanks to Tom Blakemore and Jason Gallaway (Falmouth School of Sailing) for use of boats, photographs and local information; thanks to Trudy Russell for photographs; thanks to Mylor Yacht Harbour for tide time table; thanks to Lisa Allen for data and to Lauren Clegg for help with collecting data.
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