More Statistical Analysis from Cristina

Bore Dda Ffrendiau...

Since the start of our ferry surveys, we have registered more than 500 sightings of common dolphins and more than 8000 individuals. 

We have seen them in small groups, as few as only 1, to hundreds at the same time. But we have also observed that the number of sightings per survey varies between seasons.

It was interesting to find out that, although the number of surveys during the winter is relatively low and the weather conditions tend to be slightly worse when compared to other seasons, the number of sightings per survey during winter was actually similar and even slightly higher than during the spring. Even more interesting was the fact that the number of individuals recorded during this season was at the same level as summer and autumn

The most fascinating aspect is that we did not recall any common dolphins’ sightings during the first few years of our ferry surveys. So, we decided to do the same analysis, but this time, stratifying the data by year. The analysis was very revealing, it showed not only the seasonality that we had observed previously, but also, they seemed to suggest a change in pattern during the study period.
With no individuals recorded in 2004 and 2005, a steep increase in the number of sightings per survey is thereafter captured during the winter, going from around 0.5 sightings per survey in 2006 to over 1 in 2014. In contrast, this number seems to gradually decrease during the summer seems to be displaying the opposite trend, going from over 1.5 sightings/survey in 2004 to below 0.5 in 2014.

I.

These differences are suggesting seasonal and annual movements among the common dolphin population which, at the same time, depend on several aspects such as prey availability. Thus, changes in the seasonal distribution or trends of the common dolphins’ prey will be followed by the corresponding movement of the latter.
Hasta la vista
Cristina!