Fig. 1
![Fig. 1](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs41200-018-0160-3/MediaObjects/41200_2018_160_Fig1_HTML.png)
Monitoring site of the harbour porpoise at the mouth of the Douro River. The land-based point of observation is marked with a pin
Monitoring site of the harbour porpoise at the mouth of the Douro River. The land-based point of observation is marked with a pin