The abundance and distribution of harbour porpoises is monitored using an internationally standardised methodology. Distribution feeds into information on possible effects of offshore construction and where needed and feasible, appropriate preventive or mitigative measures are taken. The cause of death of stranded marine mammals (including bycatch) is being investigated. There are only few Belgian fishing vessels using static gear, the types of net where bycatch mainly occurs, so on-board monitoring does not contribute significantly to the estimation of bycatch at population level (CFP – DCF). Fishermen are encouraged to report bycatch. Abundance of harbour seals at the more or less permanently used haul-out sites is monitored since 2019. Since no colonies are present in Belgian waters, this monitoring is not relevant for population size estimation but gives an indication of occurrence and suitability of resting places. Regional cooperation exists for the assessment of abundance and distribution of harbour porpoise (SCANS projects and SCANS-type national surveys) and seals (ICES database).

Implementation of the 1992 EU Habitats Directive (92/43/EEC) requires Member States to undertake surveillance (i.e. monitoring) of a wide range of Annex-listed flora, fauna and habitat types that are earmarked for conservation. The status of these features is then subject to detailed re-assessment and reporting once every six years, in accordance with Article 17 of the Directive. In this regard, the fundamental parameters underlying individual species assessments are the Range, Habitat for the species, Population (including trend information) and Future Prospects (including evaluation of pressures, threats and conservation measures).There are nine survey campaigns within the Monitoring Programme for Habitats Directive & Birds Directive species, four of these collect data relating to D1-Biodiversity-mammals.

The principal purpose of the MSFD monitoring programme is to review the progress that has been made towards achieving good environmental status prescribed for each criterion in the Marine Strategy Part I (2018). This review is based on established indicators. The monitoring can also be used to evaluate the environmental targets defined for each descriptor. The environmental targets are operational in nature and are linked to specific actions and/or measures in the Marine Strategy Part 3. The effects of individual measures cannot generally be linked directly to environmental status or the criteria. Monitoring data can, however, indirectly give an indication of the effectiveness of measures. The monitoring (methods, spatial and temporal coverage) aims to achieve sufficient statistical confidence in the assessment. The risk of not achieving GES or deterioration from GES is addressed in the Marine Strategy, Parts 1 and 3. The European Commission requests that the electronic reports explain how the DPSIR cycle is monitored and to which part of the cycle the monitoring surveys are linked. The MSFD monitoring programme helps to generate better insight into the relationships between the use of the sea and the marine ecosystem. This can be accomplished by monitoring pressures and the underlying activities (D1C1: incidental bycatch, D2: non-indigenous species, D3C1: fish mortality, D5: eutrophication, D6C1/D6C4: physical loss of seabed/habitats and D6C2: fisheries intensity and sand extraction, D8 and D9: pollutants, D10: litter, D11: underwater noise), and by monitoring species and habitats (D1: biodiversity (with the exception of D1C1), D3C2: spawning stock biomass, D4: food web, D6C3/D6C5: habitats ) and hydrographical characteristics (D7). The numerous relationships between the various elements of the marine ecosystem are complex, and many are still not known. Consequently, it is often only possible to give an indication of the impact of specific activities on the marine ecosystem. Experts generally derive DPSIR relationships from the monitoring of pressures/activities and of species and habitats (from the MSFD monitoring programme), in combination with data derived from permits and research programmes. However, some surveys have been established to measure pressures and their effects and/or to learn more about the effectiveness of measures. In designing the monitoring survey for benthic animals (habitats), the Netherlands explicitly took account of the need to

Marine mammals are found high up in the marine food web and are affected both directly and indirectly by human activities. Monitoring of seals (gray seals, harbor seals and ringed seals) and porpoises provides a basis for assessing environmental conditions, based on population size and growth rate, as well as trends in population distribution and health status. These parameters can be affected by a variety of human activities. Hazardous substances can be enriched in the food chain and lead to lower growth rates and reduced population sizes. Underwater noise and other physical disturbance can have both direct effects at the individual level and diffuse effects at the population level. Top predators can also be affected by changes down the food chain that have effects on, for example, different fish species that make up the seals' prey, so-called bottom-up effect as a result of overfishing or other antrophogenic pressures. The monitoring of by-catch and hunting provides a basis for assessing direct impact, while other human activities and pressures that may affect marine mammals and therefore are concidered when assessing mammals, such as underwater noise, fishing activities and hazardous substances, are included in other monitoring strategies. Except for E.2, there are no targets in place specifically linked to marine mammals, however, the targets for hazardous substances, eutrophication, marine litter and fishing activities are indirectly relevant to acheive good status also for mammals. The related measures may cause positive changes in the marine mammals populations, but just as for the targets the linkage between the human activities and pressures and how they may impact the populations are not always crystal clear.