Response of common seals (Phoca vitulina) to human disturbances in the Dollard estuary of the Wadden Sea

Abstract

The Dollard area is a core breeding area of common seals, Phoca vitulina, in the Dutch Wadden Sea. Seals mainly use this area in the birth season, and it is at this time that mother and pup pairs are prone to disturbance. Seals in the Dollard lie on sandbanks close to the dyke area which exposes them to various human activities on land and in the coastal waters.

The colony of common seals on the sandbanks of the Dollard has increased from 77 individuals in 1993 to 332 in 2010. Observations were made during the pupping and lactation seasons in 2007, 2008, 2009 and 2010. In total, 692h of observations were made over the last 4 years, and a total of 1329 potential disturbances were recorded. Human activities were more frequent on land than on water or in the air. In total, most disturbances were also recorded as arising from land. Actual disturbances of seals were recorded 344 times; seals were alerted 249 times and seals escaped into the water 95 times. An escape response was observed for 81 of the 1037 (7.8%) recorded land activities, 6 of the 92 (6.5%) recorded water activities, and 8 of the 200 (4%) air activities. These percentages of escape responses did not differ significantly between the land, water and air activities.

The construction of a culvert in the dyke in 2001, and the building up of sand ridges along the water flow towards the culvert, provided the seals with an extra place to haul out. Land activities only affected seals resting on these sand ridges which are at a distance of 50–200 m of the dyke. Boats have the potential to disturb seals on all sandbanks of the study area. They affected the highest number of seals per disturbance, with up to 117 animals fleeing into the water. Flying at lower altitudes appeared to cause more disturbances of seals. Therefore, the effect of flying at low altitudes (150–300 m) warrants further investigation.

Disturbances which lead to fleeing into the water create the potential risk of separating pups from their mothers. This may contribute to the high incidence of orphaned pups found in the Dollard region. Yearly numbers fluctuated between 13 and 24 orphans representing a substantial proportion of the seals born in this area (with highest pup counts of ca. 100). In addition to separation, disturbances may also impact the condition of pups if they occur repeatedly during suckling.

The results of this study demonstrate that disturbance is caused by various human activities to a colony of seals using sandbanks close to the mainland. As these disturbances pose a risk that pups become orphaned or are impacted in terms of their body condition, efforts should be made to minimise the effects of human presence.