Final report harbour seal tagging campaign 2025

Seals depend on both terrestrial and aquatic environments to feed, breed and rest. In the Netherlands, they mostly rest and breed on tidal flats while feeding in the adjacent North Sea. In search of new resources, the marine environment is being developed to accommodate off shore windfarms among other growing human activities. It is of importance to ensure the seal population is not affected by these activities. The Ecology and Cumulation Framework (KEC) is designed to estimate the effects of offshore wind farms on the population of protected species, including harbour and grey seals. This is done by estimating the potential overlap between construction sounds and seal distribution based on monitoring data combined with existing tracking data of the movement of these animals at sea. The existing tracking data used to create the distribution maps originate from various unrelated projects, carried out between 2006 and 2019. The current project aims at acquiring new information on the distribution of seals in the North Sea, with specific attention to juvenile harbour seals, as there have been demonstrable changes in the survival of the young harbour seals, and the population has decreased. This study concerns the tagging of 28 harbour seals. The report provides an overview of the tagging activities, presents the resulting seal tracking data. The long-term aim is to collect data that enables its potential use in follow-up projects aimed at addressing the following questions: 1. What is the distribution of harbour seals (modelled based on transmitter data and habitat requirements) on the northwest of the Dutch Continental Shelf (NCP) in particular? - 2. What is the behaviour of juvenile harbour seals? For example, do they swim further than adult harbour seals? Is the distribution of rehabilitated juvenile harbour seals observed in 2021-2022 representative for juvenile harbour seals in the wild? - 3. Do the harbour seals use the Dutch wind farms? Do they swim into these farms and how do they use these wind farms (to swim through them alone or also to forage there)? How can this use be compared to areas outside the wind farms with similar habitat characteristics? A total of 28 seals were tagged in spring and autumn 2025: six adult males, six adult females and six sub- adults, and ten young of the year. All seals were captured at Balgzand east of Den Helder and equipped with a SMRU GPS/GSM tracker. These devices collect GPS locations and detailed diving data and convey data using the GSM network. The individual tracks of the seals are presented showing potential differences between the age and sex groups. Also the results show that some of the seals cross or even use the windfarms. The tag data collected in this project supplemented the existing data set substantially, specially creating insight in the current distribution of seals using the western Wadden Sea to haul out and include especially young of the year. Thus providing recent information of movements along the northwestern Dutch coast of the seals tagged in the western Wadden Sea. As such, the data collected is estimated to contribute significantly to answering the questions above. In evaluating the data and questions above, gaps in some more fundamental knowledge became apparent. The most important gaps concerned the effects of wind farms on the seal populations and the (natural) annual and seasonal changes in behaviour and movements of seals, especially how this is related to age and sex. More detailed analysis of the existing data (some of which is already planned) but also additional tracking data is needed to fill these gaps. Additional tracking will be needed on one hand to compare periods without wind farms and current situation, on the other to obtain regular data to follow the current and future changes in the environment. Given the decreasing trend in the Wadden Sea harbour seal population the additional data will be of great importance to assure best possible information for the management of their habitat.