Analysis of all recorded Cetacean Data

from around the Isle of Man, 2006 and 2007,

including

Analysis of the Data from the

2007 Manx Cetacean Survey

By Tom Felce and Eleanor Stone

 

Introduction

 

Since 2006, the Manx Whale and Dolphin Watch website has been collecting opportunistic sightings of cetaceans from members of the public. This has resulted in 369 recorded sightings in 2006 and 506 in 2007 (up to the end of November). Through raising awareness of cetaceans around the island the number of people watching the sea has increased, leading to the increased number of sightings in 2007. At least eleven different cetacean species have been seen over the two years. All of these species are classed as at least ‘threatened’ on the IUCN Red List of endangered species. In addition to the opportunistic sightings, several volunteers have been doing effort based watches, at various locations around the island. This has resulted in a total of 178 hours of watching and 166 sightings.

 

In 2007 the Manx Cetacean Surveys were started, carrying out dedicated cetacean boat surveys, throughout Manx waters, for the first time. Despite the poor weather, 2588.4 kilometres were travelled, with 118 sightings of cetaceans, 44 of Atlantic Gray seals and 44 of basking sharks. This report presents and discusses more in-depth results of these three aspects of data collection, to give a more detailed description of the distribution of mega-fauna around the Isle of Man. Alongside this, these results can be used to develop the concept of a ‘Whale Trail’ around the Island.

 

1. Opportunistic Sightings

 

The number of sightings reported by members of the public over the two years has resulted in a large data set, charting the movements of the different species throughout the year. This can be looked at either on a species by species level or on a month by month level.

 

Whilst this type of data is very useful for qualitative representation of the presence and distribution of species, because it is not effort related, a couple of points need to be taken into consideration during analysis. Firstly, a lack of sightings could just as easily be because no one was watching, rather than an actual lack of animals. Secondly, the sea conditions are not taken into account so periods of bad weather (e.g. during winter) could mean animals were missed, rather than not being present. However, the sheer number of sightings means that patterns of movement are beginning to emerge, as can be seen from the figures below. Nevertheless, it should also be remembered that the more data that are collected, over the years, the clearer these patterns will become.

 

a)    Species by Species

 

By plotting the sightings of each of the main species individually, the patterns of movement of these species throughout the year can start to be understood. The predominantly sighted species have been Harbour Porpoise, Minke Whale and Risso’s Dolphin.

 

In the figures below, the year has been divided into ‘Spring’ (March-May), ‘Summer’ (June-August), ‘Autumn’ (September-November) and ‘Winter’ (December-February).

 

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.

 

 

 

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.

 

 

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.

 

 

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.

  

 

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