Post 2. Ferry surveys – I have the data, now what?...

And so we continue with some more of the science and statistic's Cristina has harvested from all the data that so many of us have contributed to on our many years of gathering data and recording the cetaceans that inhabit the St Georges Channel in the southern Irish Sea...

"Stena Europe" leaving Fishguard.

Post 2. Ferry surveys – I have the data, now what?...

Bore da ffrindiau!

Sea Trust survey team just visible on Starboard (right.hand) bridge wing.

In this week’s post we continue investigating the data we have gathered on our ferry surveys. Do you want to know which species we have recorded in St George’s Channel? And how many of them? Then this post is for you!

Sea Trust started the Ferry surveys aboard the Stena Europe covering the route Fishguard-Rosslare back in 2004. In order to make accurate inferences, we want to work with the cleanest and most consistent datasets therefore we have excluded surveys where the conditions were sub-optimal.

Still, even with those exclusions, Sea Trust has carried out a total 491 surveys, this equates to 1780 hours of effort and 55,800 km covered during the period 2004/2017!! And all of these with favourable weather and survey conditions, not just in the warmer months when most survey work is done, but all year round!

We have already examined the importance of this data to understand and expand our knowledge on the different species, their behavior and their habitat preferences. But how do we carry out this analysis? What do we look for when we look at these datasets? Do we need to have a background knowledge on stats, math's, data analysis, biology? Needless to say, all these helps, but the most important skill we need is an analytical mind!

So, let’s start by having a look at the ferry surveys. We already have an idea of the surveys that have taken place, the average number of observers, the weather conditions. Now, let’s put all these records in a map. The first map includes all the sightings for the period 2004-2017. The different colours in the map indicate the species as shown in the legend and the size of the bubbles indicates the number of individuals seen in each group. The bigger the bubble, the higher the number of individuals. What we can see here is a total of 3549 sightings and 15343 individuals!

But if we have a closer look, we will see that there are too many bubbles here and it is difficult to tell what is really going on. So, let’s concentrate on the four most common species encountered on our surveys. I have generated a separate map each, for Harbour porpoises, Common dolphins, Risso’s dolphins and Minke whales. 

Possibly the most obvious aspect in these individual maps is that the number of sightings differ substantially from one species to another. And in this sense the highest number of sightings that we have had corresponds to the Harbour Porpoise

If we put both the number of sightings and the number of individuals per species into a plot, we can appreciate that although the number of sightings for Harbour Porpoise is the highest, the species for which we have actually recorded the most individuals is the Common dolphin with more than 8000 individuals registered. In contrast to the Harbour porpoise and other species, Common dolphins tend to hang out in bigger groups. In some cases, we have encountered and been able to record what we class as super pods, with more than 500 individuals seen at the same time!
This simple comparison between the number of sightings and individuals gives us a lot of information about their social behavior. For example, the usual group size for Harbour porpoises 2-3 individuals, around 15 for Common dolphins, 4-5 individuals for the Risso’s dolphin and finally only 1 individual is the norm for the Minke whale.

This is only an example of the characteristics that can be interpreted from using the maps that I have produced. However there are more! We will talk about them in next week’s post.

But meanwhile… can you guess some other characteristics that can be discovered? Let us know what you think! Do you see something in these maps that intrigues you? Do you have an analytical mind?

All maps diagrams text and images © Copyright Sea Trust/Cristina Munilla unless other wise stated. Permission to use any of these Materials must be requested- email: seatrustwales@gmail.com