For
all species, the greatest number of sightings is during the summer, when the
waters are getting to their warmest and productivity is at its highest. The
fewest number of sightings for all species is during the winter. Although this
can partly be explained by the poorer weather and therefore less people
watching, it is likely that it is also due to a true reduction in the number of
animals around the island in the winter months. During this time, the waters
are at their coldest and it is not unusual for marine mammal species to move
seasonally due to sea temperature changes.
The
Harbour Porpoise – our
most universal and consistent cetacean
Harbour
porpoise sightings are high also in both spring and autumn, suggesting they are
virtually year round residents in Manx waters. Their distribution around the
island is relatively uniform, indicating they can utilise all Manx waters
throughout the year. The exception to this is the central western coast
(between Peel and Jurby) in which none of the predominant three species were
seen. This is thought to be as a result of the nature of the coast in that
area, leading to a very limited numbers of places that people are able to watch
from. The apparent hotspots at Dalby, Port St Mary and just south west of the
Point of Ayre are areas where regular volunteers carry out frequent watches.
This demonstrates the value of effort based watches – the more you sit and
watch, the more you will see!
The
large number of sightings of harbour porpoise and their ubiquitous seasonal and
geographical distribution shows that Manx waters are clearly very important for
this significant species. Harbour porpoise are top level predators and by
observing their numbers and distribution over time, the status of the whole
marine ecosystem can be monitored.
The
Minke Whale – an Irish
Sea front indicator
Minke
whales show an increased number of sightings in the summer and the autumn, with
fewer numbers in the spring. Baleen whales depend directly on high levels of
primary productivity which, around the Isle of Man, increase during the summer
with the warming of the waters and formation of the Irish Sea Front between the
Isle of Man and Ireland. During the summer therefore, the majority of minke
whale sightings are on the south and west coast of the island. In the autumn,
it is believed that the whales follow the herring round to their east coast
spawning grounds. This is illustrated by the change in distribution of minke
whales in the autumn, with the majority of sightings being on the east coast,
especially around the Laxey Bay area.
At
this point, it is worthwhile to discuss briefly the difference between the
minke whale sightings in 2006 and those in 2007. It is well recognised that the
summer of 2007 was extremely poor and unusual and this had a large impact on
the number of minke whale sightings. The figure below shows the difference in
the number of sightings in the two summers and it can be seen that the number
was significantly reduced in 2007.
 |
 |
Summer 2006
|
Summer 2007
|
The
Risso’s Dolphin – an
elusive, offshore species or Manx resident?
Risso’s
dolphins are seen extremely frequently in both spring and summer, possibly
surprising for a species renowned for its offshore nature. However, due to the
deep water surrounding the Isle of Man, Risso’s dolphins can come very close in
to shore. Needless to say, this is also the reason the minke and other larger
whales can be seen so close to land.
The
distribution of Risso’s dolphins also changes throughout the year. In the
spring, the majority of sightings are on the east coast, in the Douglas
Bay/Marine Drive area. In the summer, the animals appear to move further south,
with the most sightings being between Langness and the Calf. It is likely that
this is due to a change in distribution of the Risso’s dolphin’s diet
(predominantly octopus, squid and cuttlefish), however further study needs to
be done before this can be clarified.
(b)
Month by Month
Alternatively,
the sightings of all species seen each month can be plotted. This gives an
indication of what species are likely to be seen in each month through the
year, and in what locations it would be best to go to watch for them. Over
time, and with the addition of more effort based land watches, this could be
used in the creation of a ‘Whale Trail’. The figures below show which species
have been seen, and where, in the different months of the year. It should be
noted here that sightings in December 2007 have not been included. ‘Unusuals’
include all other species that have been seen, and include Atlantic White Sided
Dolphin, White Beaked Dolphin, Killer Whale, Humpback Whale, Sei Whale, Fin
Whale and unidentified species.
January
February
March
April
May
June
July
August
September
October
November
December
These
distribution maps indicate that, from April through to September, there are a
high number of sightings, from all around the island. The exception to this is
between Peel and Jurby, which, as discussed earlier, is an area where people
cannot readily watch from. In October and November, there are still a moderate
number of sightings, the majority of which are minke whales on the east coast.
Although the numbers of sightings in December through to March are a lot less,
there have still been sightings of at least four different species of cetacean
during this time.
The
number of sightings in each month are summarised in the table below.
|
|
Harbour
Porpoise
|
Minke
Whale
|
Risso’s
Dolphin
|
Bottlenose
Dolphin
|
Common
Dolphin
|
Unusuals
|
|
January
|
1
|
2
|
1
|
4
|
0
|
0
|
|
February
|
10
|
0
|
4
|
7
|
0
|
2
|
|
March
|
10
|
1
|
4
|
0
|
0
|
1
|
|
April
|
38
|
0
|
14
|
2
|
1
|
2
|
|
May
|
39
|
3
|
42
|
3
|
1
|
13
|
|
June
|
88
|
17
|
42
|
2
|
37
|
15
|
|
July
|
93
|
26
|
20
|
0
|
11
|
4
|
|
August
|
40
|
25
|
14
|
5
|
4
|
8
|
|
September
|
27
|
29
|
6
|
0
|
2
|
5
|
|
October
|
20
|
37
|
5
|
0
|
0
|
6
|
|
November
|
32
|
39
|
3
|
0
|
0
|
3
|
|
December
|
1
|
0
|
0
|
0
|
0
|
0
|
This
table highlights the high number of harbour porpoise sightings throughout the
year, the general increase in number of sightings in the spring/summer months,
especially for common and Risso’s dolphins, and the persistence of minke whales
right through till November.
c)
Duration of Sightings
When
considering the significance of different areas around the island for marine
mammals, it is also important to examine the length of time animals spend in
the different regions. The figure below illustrates the main areas of
sightings, each of which are numbered.
The
table below summarises the average duration of sightings of each species, at
each of these numbered areas. It also includes the number of sightings of that
species, as durations based on a small number of sightings are not necessarily
representative and should be treated with caution. Overall, the durations of
sightings are impressive. The fact that animals are spending considerable
amounts of time within an area indicates that they are probably feeding.
Nevertheless,
the table shows that animals do spend longer in some regions when compared with
others. For example, all species spend an average of over 25 minutes per
sighting in region 5 (Douglas Head to Little Ness). In contrast, region 9
(Bradda Head) is utilised for much shorter periods of time. This would have
implications when designing a whale trail, as the longer animals spend in a
region, the more likely people will be able to see them.
|
|
Average
Duration of Sightings (mins)
n
= Number of Sightings
|
|
Area
|
Harbour
Porpoise
|
Minke
Whale
|
Risso’s
Dolphin
|
Others
|
|
1
|
7.1
(n=68)
|
30.7
(n=13)
|
37.4
(n=8)
|
53.3
(n=15)
|
|
2
|
5.6
(n=5)
|
9.3
(n=4)
|
(n=0)
|
15
(n=3)
|
|
3
|
16
(n=43)
|
36.8
(n=80)
|
3.6
(n=4)
|
3.7
(n=3)
|
|
4
|
28
(n=9)
|
8.5
(n=2)
|
53.6
(n=15)
|
6.8
(n=5)
|
|
5
|
27.1
(n=21)
|
29.8
(n=12)
|
35.8
(n=26)
|
37.3
(n=11)
|
|
6
|
11.5
(n=21)
|
14
(n=4)
|
19
(n=38.3)
|
2.4
(n=16)
|
|
7
|
14.9
(n=88)
|
13.9
(n=4)
|
13.8
(n=32)
|
29.8
(n=26)
|
|
8
|
10.9
(n=10)
|
3
(n=3)
|
19.8
(n=13)
|
65
(n=4)
|
|
9
|
5.8
(n=15)
|
8
(n=5)
|
2.2
(n=5)
|
1.7
(n=3)
|
|
10
|
23.5
(n=37)
|
21.2
(n=15)
|
10.1
(n=8)
|
15
(n=10)
|
|
11
|
14.1
(n=39)
|
15.8
(n=26)
|
23.3
(n=4)
|
14.8
(n=7)
|
2.
Effort Based Watches
Effort
based watches allow more accurate comparisons between different sights, as they
take into account the length of time of watching per sighting. Over the two
years, 178 hours of effort based watches have been carried out, resulting in
166 sightings. The breakdown of species of each of these sightings is given in
the table below.
|
Species
|
Number
of Sightings
|
Number
of Individuals
|
|
Harbour
Porpoise
|
114
|
272
|
|
Minke
Whale
|
29
|
36
|
|
Risso’s
Dolphin
|
6
|
20
|
|
Common
Dolphin
|
13
|
32
|
|
Unusuals
|
4
|
7
|
|
Total
|
166
|
367
|
As
with the opportunistic sightings, the harbour porpoise is the most frequently
sighted cetacean, with high numbers of minke whale sightings also, and
surprisingly few Risso’s dolphin sightings. This discrepancy is most likely due
to the fact that a large percentage of Risso’s dolphin opportunistic sightings
are on the east coast, an area where no effort based watches have been carried
out.
Effort
based watches have taken place in 6 different areas round the island; The
Ayres, Maughold Head, Port St Mary, The Calf, Dalby and Peel. Effort is
recorded in 15 minute intervals, so in order to compare the number of sightings
in each location, the percentage of cetacean-positive intervals can be used.
These percentages are given in the table below.
|
Area
|
Effort
Time (hours)
|
Percentage
of Cetacean-Positive Intervals
|
|
The
Ayres
|
3.25
|
84.6
|
|
Maughold
Head
|
23
|
3.2
|
|
Port
St Mary
|
19
|
67.1
|
|
The
Calf
|
20
|
11.2
|
|
Dalby
|
103.25
|
28.3
|
|
Peel
|
7.5
|
13.3
|
Although
The Ayres would initially appear to have an extremely high percentage of
cetacean-positive intervals, it should be noted that this is based on only 3.25
hours of effort and therefore may not be accurately representative. Port St
Mary clearly also has a high rate of cetacean sightings, and Dalby has a
moderately high rate. These percentages indicate that, on average, you would
have 40 minutes of sightings per hour at Port St Mary and seventeen minutes of
sightings per hour at Dalby. Maughold Head has an extremely low percentage of
cetacean positive intervals, and in fact there have only been 3 sightings
there.
Effort
based watches also allow the analysis of the tidal effects on the presence of
cetaceans. From the effort based watches carried out so far, the percentage of
cetacean-positive intervals in the different tidal states has been calculated:
27%
= Cetacean-positive ‘Ebb’ intervals
43%
= Cetacean-positive ‘Flood’ intervals
43%
= Cetacean-positive ‘Slack’ intervals
Although
the data set is not yet large enough to carry out statistical analyses on these
results, initial figures would indicate that there is an increase in cetacean
presence during the flood and slack periods of the tidal cycle.
By
continuing effort based watches, and watching from more sites round the island,
the most reliable areas and times to find cetaceans will become clear. Firstly,
this will give a true indication of cetacean hotspots around the island,
signifying which areas need to be monitored more carefully. Secondly, it could
be used in the creation of a whale trail to distinguish areas which are
consistently good for finding cetaceans.
3.
2007 Manx Cetacean Survey – Boat based.
a)
Effort
Overall,
2588 kilometres were completed during the 2007 boat trips, 1937km on dedicated
surveys and 651km on press and media trips, comprising the following effort
types.
|
Effort
Type
|
Dedicated
Surveys (km)
|
Press
Trips (km)
|
|
Casual
Watch
(no
dedicated observers)
|
35.3
|
55.1
|
|
Dedicated
Search
(2
dedicated observers on fly-bridge, no independent observer)
|
956.1
|
554.9
|
|
Line
Transect
(2
dedicated observers on fly-bridge, 1 independent observer)
|
938.7
|
23.8
|
|
Photo
Identification
|
7.6
|
16.8
|
b)
Sightings
The
poor weather limited the number of boat surveys that could be carried out in
2007 and subsequently has limited the size of the data set that the surveys
produced. Nevertheless, there were still 206 sightings, of 391 individuals.
This equates to a sightings rate of 0.08 sightings per kilometre, or 12
kilometres travelled for every sighting.
However,
sightings rate changes significantly depending on boat speed and sea state.
Therefore, using sightings collected only when on Line Transect Survey is a
better indicator of sightings rate, as this is always done in low sea states
and slow boat speed.
This
results in a sightings rate of 0.10 sightings per kilometre, or one sighting
every 10 kilometres.
The
species breakdown of these sightings is shown in the table below.
|
Species
|
Number
of Sightings
|
Number
of Individuals
|
|
Harbour
Porpoise
|
98
|
168
|
|
Minke
Whale
|
7
|
11
|
|
Risso’s
Dolphin
|
12
|
36
|
|
Atlantic
Gray Seal
|
44
|
119
|
|
Basking
Shark
|
44
|
55
|
|
Unidentified
Cetacean
|
1
|
1
|
The
distribution of the principal cetacean sightings around the island can be seen
in the figures below. Additionally, the distribution of basking shark sightings
is also shown. The blue lines represent the 3 and 12 mile limits around the
island.
Harbour Porpoise
Minke Whale
Risso's Dolphin
Basking Shark
Unfortunately
there are not enough sightings of minke whales or Risso’s dolphins to make any
significant conclusions from this small data set. On the other hand, there are
many more sightings of harbour porpoise and basking sharks and it is therefore
worthwhile to comment on their distribution.
As
with the opportunistic sightings, harbour porpoise are seen in high numbers all
around the island. Interestingly, although there were sightings of porpoises
beyond the 3 mile limit along the North West coast of the island (Peel to
Jurby), there were none close to shore. This seems to substantiate the lack of
opportunistic sightings in this area, although more effort is needed before
this absence can be fully established.
It
can be seen that a huge majority of the basking shark sightings were on the
west coast. There were also some off the south of the island, but only one on
the east coast. This one sighting was unusual for two other reasons; it was
late in November and was of a very small (2-4 metre range) shark. It is also
interesting to note that 80% of sightings were within the 3 mile limit, despite
this representing a small percentage of the area surveyed.
c)
Harbour Porpoise Abundance Estimate
The
high number of porpoise sightings made it possible to derive the first
abundance estimate for harbour porpoise in Manx waters. The statistical program
which calculates this abundance, DISTANCE, requires as minimum the following
information:
·
The total distance travelled on
transects, in kilometres
·
The shortest distance from the
transect line of each sighting, known as perpendicular distance
·
The size of group of each
sighting of porpoise
·
The size of the research area,
in km2
The
total distance covered is derived using a formula that can calculate the
distance between two latitude/longitude positions.
Perpendicular
distance is calculated from the distance each sighting was from the boat and
the angle of the sighting from the boat, using trigonometry.
This
distance is important because there is a statistical relationship between the
likelihood of a sighting and its distance away from the transect line, known as
the probability detection function. Furthermore, bigger groups are more
detectable than smaller groups at greater distances, hence the need to know the
size of groups.
The
DISTANCE statistic uses these two relationships and the distance travelled to
derive a density of animals which, when multiplied by the research area, gives
an abundance within that area.
Therefore,
the density of porpoises within Manx waters (an area of 4492km2) was
0.18 per km2 (Coefficient of Variance = 24.5, 95% intervals =
0.11-0.29).
This
means that there is an average density of 0.18 porpoise per km2,
which statistically may vary by 24.5%, to a confidence level of 95%. This
confidence level has been historically chosen by statisticians to represent
statistical significance, i.e. that the results have not happened purely by
chance.
This
density therefore equates to an estimated abundance of 814 animals (95%
intervals 501-1322). Therefore, there are between 501 and 1322 harbour porpoise
within Manx waters.
This
statistic has been derived using the Half-normal Cosine model and a strip width
of 700 metres. At greater distances than this, porpoise may be missed due to
their size and manner of surfacing, hence these sightings are excluded as
outliers. Furthermore, only data collected during sea states of zero and one
were used to derive the estimate. In sea states two and above, the
detectability of porpoise becomes unreliable, particularly at distances above
three hundred metres. With more kilometres of effort on transect, this number
will become more precise.
d)
Photo Identification of
Risso’s Dolphins
During
boat surveys, we also carried out photo identification (ID) of Risso’s
dolphins, under a licence from the Department of Agriculture, Fisheries and
Forestry. Photo identification is an incredibly useful tool in the long term
study of cetacean populations as it provides information on the number of
different animals using an area, the social associations within a population
and the site fidelity and long term use of an area.
During
the boat based surveys, we had 6 encounters with Risso’s dolphins. From
analysing the photographs taken, it is known that at least 27 different animals
were photographed during boat surveys. Along with photographs from
opportunistic sightings from volunteers, a catalogue of different individuals
has been created. This is split into ‘Well marked’ animals (those which could
be identified from a photo of either side of its fin, i.e. with nicks out of
the edge of the fin) and ‘Lefts’ and ‘Rights’ (those which can only be
identified from one side, by scars on that particular side).
To
date, there are 20 ‘well marked’, 5 ‘left’ and 7 ‘right’ animals in the
catalogue. Fin shots of these identified individuals can be seen in Appendix 1.
Since it is possible that some of the ‘left’ and ‘right’ animals may in fact be
the same individual, it is only possible to say that there are a minimum of 27
different animals that have been photographed (i.e. the number of ‘well marked’
plus the greatest of the ‘left’ or ‘right’). For this reason, the majority of
analysis work on the photo ID is using the ‘well marked’ figures only.
Photo
ID can provide a crude estimate of the size of a population, by using the
number of ‘well marked’ individuals. Since not all individuals are well marked,
it is possible to calculate a percentage of animals that are ‘well
marked’, out of all the animals photographed. Currently, this percentage is
40%. Since there are 20 ‘well marked’ animals in the catalogue, and this
represents 40% of all animals, it can be extrapolated that there are a minimum
of 50 individuals in the population. Whilst this figure does represent
approximately one third of the current estimation of Risso’s dolphins in the
whole Irish Sea, it should be remembered that this is only based on 10
encounters in total.
In
reality, the number of Risso’s dolphins utilising Manx waters is believed to be
much higher. This is because only 5 of the ‘well marked’ individuals have been
re-sighted, and each of these 5 has only been seen twice. This means that
nearly all the animals being photographed are ‘new’ and the catalogue is still
growing. One easy way to chart this over time is to look at a ‘discovery
curve’, as shown below.
Basking Shark
Each
time an individual is identified (whether for the first time or as a re-sight),
the ‘Total No. of ID’s’ increases. The ‘Catalogue Size’ only increases if the
identification is of a new animal which is put in the catalogue. Therefore,
when the curve is still rising, there are still new individuals within the
population being discovered. Only when the curve flattens off can it be
surmised that all individuals within that population have been photographed.
From
this Risso’s dolphin discovery curve, it is clear that we are still
photographing new, different individuals and that the total number of animals
using Manx waters is likely to be a lot higher than the current estimate. Over
time, not only will the true number of individuals using Manx waters become
more refined, but also it can be seen whether the same individuals are seen
year after year, which areas some individuals may favour and whether there are
strong social associations between certain individuals.
We
anticipate that photo ID catalogues will also be created for bottlenose
dolphins and minke whales. Additionally, it is hoped that all the catalogues of
cetaceans using Manx waters can be compared with others from around the British
Isles, to give a broader understanding of the movement of these animals through
British waters.
e)
Sea Surface Temperature
At
the start and end of each transect line, sea surface temperature was measured
to add to the overall database of sea surface temperatures in Manx waters. Over
time this could be used to monitor changes in sea temperature.
f)
Boat Counts
Whilst
on the boat surveys, the number of other boats seen was recorded. All boats
forward of our vessel were counted, whenever a 15 minute effort interval was
recorded. Although the different types of boats were noted, at present the data
set is too small to look at these different types separately. The figure below,
therefore, shows the total numbers of boats in different locations around the
island.

In
general, there were not a large number of boats recorded, with a high number of
instances of no boats at all. There were only two instances of there being more
than 10 boats at one time, and only 8 instances of their being 5-10 boats. It
can also be seen that, as yet, no clear pattern of distribution of boats is
emerging, although there does appear to be slightly more boats off the east
coast than off the west coast. Nevertheless, with the well documented effects
that increasing boat traffic can have on the presence of cetaceans, it is
important to continually monitor the number and distribution of boats around
the island.
Conclusions
The
results presented here demonstrate, for the first time, the extent of the
presence and distribution of cetaceans around the Isle of Man. They show that
there are very high numbers of harbour porpoise all around the island and that
they utilise nearly all Manx waters. There are also significant numbers of
minke whales and Risso’s dolphins, species not often seen so close to land.
These two species appear to have specific patterns of movement throughout Manx
waters during the course of the year. In addition to these, there are also a
large number of sightings of other cetacean species, most notably bottlenose
dolphins and common dolphins. In total, at least 11 different cetacean species
have been seen around the Isle of Man in the last two years; an incredible
number. This indicates the current good health status of Manx waters, that it
is able to support such great biodiversity.
All
these cetacean species are on the IUCN Red list of Threatened species, meaning
that it is vital that they are effectively monitored and conserved in areas
where they are abundant. The results from the last two years have given us an
indication of the significance of Manx waters for these animals, but they also
show that further study is needed to accurately determine their degree of
importance and the most effective management strategies for these charismatic
animals.