ron-t kayaking blog

04 June 2016 ............ Ganavan to Eilean Musdile - the sequel

After my sighting of what I assumed to be one or more dolphins, I was a bit curious as to whether it had been a dolphin or dolphins that I saw, or whether it was some other kind of sea mammal.

The more I dug around on Google, the more curious I became, as I started to find pictures that were very similar to what I had seen, but they were pictures of Minke whales.

So did I see a Minke whale ? They are known to exist around the west coast of Scotland.

I didn`t ever see the classic sort of image of a dolphin leaping out of the water, all I saw was a long rounded back and a fin - it was all dark grey or black, no other colours.

At this point I really can`t say whether the fin was in the middle of the back, or whether it was near the rear end of the back - on Minke whales the fin is towards the rear end of the back.

I also can`t say whether the fin was hooked or not - another indication of a Minke whale.

A couple of weeks later I was back in the Oban area, and went to visit the Ocean Explorer Centre to see if they could shed any light on it - the Ocean Explorer Centre is down the road from Dunbeg, and next to SAMS and Dunstaffnage Castle.

They suggested that it might also have been a Basking Shark, as they are known to exist around Mull.

Back to the internet - and it appears that sometimes with Basking Sharks you see a snout some distance in front of the back and the fin as it cruises along with its mouth open sucking in huge volumes of sea water which it expels through the gillrakers and gills collecting all the plankton, and you also sometimes see a second fin behind the first one - this is actually its tail. Now I didn`t see either of these, I am pretty certain of this.

However a slow moving back and fin is quite a common type of sighting of Basking Sharks, without the snout and tail.

So yes, a Basking Shark is another possibility.

I am not sure I can really do much more - my significant lack of observational skills means there are just too many unknowns and I am not any closer to knowing what I saw. Maybe Dolphin, maybe Minke whale, maybe Basking Shark.

But fascinating to see, all the same.

-----------------------------------

Whilst I was at the Ocean Explorer Centre, I had a look around at the various exhibits all about marine research - it is well worth a visit.

One of the exhibits that attracted my attention was a large interactive touch screen monitor that shows a satellite image of the area around the Firth of Lorne, and in part of it there is a superimposed colour graphic showing the depth of the sea bed - the graphic extends from the north end of Jura up to the south end of Lismore.

The graphic was based on data obtained by the INIS Hydro project partnership between 2011 and 2014 - the INIS Project was a project to map the seabed of various specific areas around Scotland and Ireland, and the Firth of Lorne was one of these areas.

The graphic shows the sea depth as a progression of colour from red at the shallow end, through orange, yellow, green, blue, violet, black at the deep end.

The thing I found fascinating about it is the way that the depth of the sea bed correlates with the locality of the tidal races around the Firth of Lorne.

So to start with, there is a very deep trench between Jura and Scarba, with a more moderate depth either side - so the Corryvrecken has got a much larger cross sectional area than its width would suggest, and during the flood tide a huge volume of water flows through it, meets the shallower seabed away from the Corryvrecken, and all the water is forced upwards as it flows west, and hence the Great Race.

I am not familiar with the tidal currents around the Garvellachs, so don`t know how this correlates, but there is another deep trench down the east side of the Garvellachs, between the Garvellachs and Eilean Dubh Mór and Eilean Dubh Beag.

All around Scarba and Lunga and across to the south end of Luing there is a moderate depth, however the Grey Dogs between Scarba and Lunga is very shallow - so the tidal flow coming up between Luing and Scarba at a moderate depth is forced through the much shallower Grey Dogs, hence the tide race through the Grey Dogs.

The Grey Dogs are sometimes called Little Corryvrecken, however the water flow is the exact opposite - in the Corryvrecken there is a huge volume of very deep water flowing, but in the Grey Dogs there is a much smaller flow being forced up over the shallow seabed.

Another one that really stands out is the very shallow seabed around the north end of Lunga, Belnahua, and Fladda.

So not only is there a restriction in the width of the channel between the north end of Lunga and Luing, but the tidal current is also being forced up and over a shallow sea bed - which explains the very fast tidal currents that exist around the north end of Lunga, Belnahua, and Fladda.

There is another deep trench up the east side of Luing and Torsa which will carry a large volume of water up the east side of Luing and Torsa - but it is a lot shallower in Cuan Sound than it is on either side, so again, there is a restriction in width and a shallower sea bed which combine to create the very fast currents that flow through Cuan Sound.

Another one that stands out is around the south end of Lismore.

All the way up the northeast corner of Mull there is a deep and wide sea bed, so a lot of water can be flowing up that. It then meets two restrictions.

Firstly, there is an underwater continuation of Lismore and Eilean Musdile that goes right across to Mull - an underwater ridge with little depth of water over it.

Secondly, there is a shallow plateau that stretches right across from the south end of Lismore across to the north end of Kerrera.

So the big volume of water flowing up the wide and deep area to the east of the northeast corner of Mull is going to meet these restrictions, and pile up to the south of Lismore - one of the escape routes for all this water is round the south end of Lismore and Eilean Musdile - so strong currents and races are a result.

All in all it is a fascinating graphic, and fascinating to see what is causing the many tide races that exist around the Firth of Lorne.

Sadly the graphic doesn`t extend quite far enough southeast to see the seabed around the Dorus Mor. During the early part of a flood tide there can be a very distinct wave train out towards the middle of the Dorus Mor, which suggests that it could be fairly shallow.

If it isn`t already there, all the depth information will be available in the public domain at some point in time. I was able to take a photograph of the graphic so can sit and look at it at home, however I am not sure if I can put it on a public website, so didn`t.

However it is in the Ocean Explorer Centre if you are interested in tide races.

website design by ron-t

website hosting by freevirtualservers.com

+ +