| Summary: | One of the most important factors explaining the distribution and behaviour of coastal marine mammals aretides. Tidal forces drive a large number of primary and secondary processes, such as changes in water depth, salinity,temperature, current velocity and direction. Unravelling which tidal process is the most influential for acertain species is often challenging, due to a lack of observations of all tide related covariates, strong correlationbetween them, and the elusive nature of most marine organisms which often hampers their detection.In the Marsdiep area, a tidal inlet between the North Sea and the Dutch Wadden Sea, the presence of harbourporpoises (Phocoena phocoena) was studied as a function of tide related covariates. Observations were carriedout in early spring from a ferry crossing the inlet on a half hourly basis. Environmental and sightings data werecollected by one observer, while an on-board Acoustic Doppler Current Profiler (ADCP) and temperature sensorcontinuously recorded current velocity profiles and temperature, respectively. Sea surface temperature and salinitywere measured at a nearby jetty. Sightings (n = 134) were linked to tidal elevation, geographical position,local depth-averaged current velocity, water temperature (with and without trend correction) and salinity.Variation in sighting rate was best described by salinity, with highest sighting rate at high levels of salinity(N30 g kg-1), indicating that porpoises enter the area in bodies of (more saline) North Sea water. Second bestvariable was time of day, with the highest sighting rate early morning, and decreasing during the day. However,surveys in the morning happened to coincide more often with high water and hence, the apparent time of dayeffect could be due to collinearity.Most porpoiseswere present in the northern part of theMarsdiep, particularlyduring high tide.Tide dependent sighting rates confirmed that porpoises reside in the North Sea, and enter the westernWaddenSea during the flood and leave during ebb. This tidal influx is most likely related to prey availability, whichcorresponds to other recent studies in this area showing higher fish abundance during high tide. Documentinginformation on tide related patterns could be used in practice, when e.g. planning anthropogenic activities orassessing critical habitats for this species.
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