Disturbances to wild Harbour seals (Phoca vitulina) during the birthing season in the Dollard estuary, The Netherlands.


Harbour Seals (Phoca vitulina) hauling out and breeding in the Dollard area of the Wadden Sea (NL) are monitored annually in terms of adult abundance, pup numbers, and anthropogenic disturbance. In 2011, a wooden viewing platform was constructed on the dyke to reduce disturbance to the colony from human visits. The aim of this research project was to continue the long-term monitoring of the animals, and assess the effectiveness of the platform in reducing disturbance.


Observations were carried out between May and June 2012 at the Dollard Estuary (53.300401N 7.084653W).

Hauled out adult and pup individuals were counted every 15 minutes across all the different sandbanks.

Recording of every potential disturbance event (land, water or air).

Behaviour of seals observed for 2 minutes following each disturbance, recording number of seals visibly reacting under the following categories: no response, heads up, commotion, movement towards water, movement into water.


A maximum of 281 adult seals were recorded hauled out at one time in the area across the whole season, and a maximum of 147 pups.

There were 240 disturbance events during observation hours, 104 of which led to a visible response from the seals. Seals were disturbed on average every 1.2 hours.

By far the most frequent disturbances reactions came from walkers or cyclists (79.8 %). Cars, agricultural vehicles and aircraft were likely to evoke a reaction from the seals, but their occurrence was very low. Sheep nearing the colony also provoked a strong reaction.

9.6 % of actual disturbance events caused seals to move into water.


General increase in seal abundance (adults and pups) hauled out in the Dollard estuary. This fits in with a general increasing trend over previous years of monitoring.

Decrease in potential disturbance events during this year’s study, and less actual disturbed seals than last year, but half of events still caused a reaction and seals the colony was disturbed more often.

The viewing platform did not eliminate disturbance in the colony. Seals were still shown to react to the presence of people, based on noise levels and shadows. However, the reactions were less severe. More restrictions and interpretation could be put in place to increase effectiveness of the platform.

The full paper is available to download here.

Anthropogenic disturbance, Pinnipeds, Breeding, Dollard, The Netherlands, Research

Seal Research and Rehabilitation Centre (SRRC)

August 2013