Tidal influence on the occurrence of the harbour porpoise Phocoena
phocoena in the Marsdiep area, The Netherlands
Lonneke L. IJsseldijk1, Kees C.J. Camphuysen2, Janine Nauw2 and Geert Aarts1
1
IMARES Wageningen UR, Institute for Marine Resources and Ecosystem Studies, PO Box 167, 1790
AD Den Burg, The Netherlands
E-mail: lonneke.ijsseldijk@gmail.com
2
Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, The
Netherlands
Abstract: Tide is one of the most important factors explaining the distribution and behaviour of
coastal marine mammals. In the Marsdiep area, the largest tidal inlet between the North Sea and the
Dutch Wadden Sea in the northwestern part of the Netherlands, the presence of harbour porpoises
Phocoena phocoena has been studied as a function of time. However, tidal forces instigate a large
number of primary and secondary processes, such as changes in water depth, salinity, temperature,
current velocity and direction. Historic studies suggest that porpoises are most abundant prior and
during high tide. However, currently it is unknown which of the different tidal processes is most
important.
To study the importance of this for the Marsdiep inlet, observations were done in early spring from a
ferry crossing the area on an hourly basis. Environmental and sightings data were collected by one
observer, while an Acoustic Doppler Current Device (ADCP) recorded velocity profiles during each
passage. Sightings (n=134) were then linked to the tidal elevation, the geographical position, local
depth-averaged water velocity, water temperature (with and without seasonal trend correction) and
salinity. Most porpoises (86%) were observed during high tide (n= 65). A correlation with water
velocity occurred, but appeared to be an inferior indicator compared with the tidal phase. However,
variation in sighting rates was best described by trend-corrected tidal changes in temperature.
The tide dependent sighting rates confirmed that porpoises enter the area during flood phase of the tide
and leave during ebb. The effect of temperature suggests that porpoises enter the area in bodies of
(warmer) North Sea water. Porpoises seemed to concentrate in areas with relatively low current
velocities. The tide dependent movement could be a passive tidally driven influx, or, alternatively a
phase in which foraging was most profitable.
Key words: harbour porpoise, phocoena phocoena, tidal cycle, habitat use