In the North Sea, harbor porpoises play a prominent role as top predators that contribute to ecosystem health and functioning. Their population declined in the 1960s in the southern part of the North Sea but have slowly recovered in numbers in the recent years. They are directly threatened by anthropogenic activities such as bycatch, noise pollution, overfishing of prey species, toxic chemicals and climate change. Several conservation measures of porpoises are implemented through the EU Habitats Directive, the EU Marine Strategy Framework Directive (MSFD) and the OSPAR convention. Within the context of these measures, long-term biodiversity observations over space and time are vital to understand ecological shifts in a rapidly changing environment.

The Belgian cetacean passive acoustic network (BCPAN) within the LifeWatch observatory was established to collect data on the presence of harbor porpoises (Phocoena phocoena) in the Belgian waters in 2016 through Passive Acoustic Monitoring (PAM) loggers. PAM is a widely used non-intrusive method of monitoring cetaceans by means of recording their echolocation clicks, or vocalization, without interfering with the animals’ activities. These echolocation clicks can be classified to the cetacean species and give clues of their habitat use, foraging, social structure and behavior. PAM is widely used to monitor distributions, observe behaviors and estimate absolute abundances over time and space. Until 2017, PAM loggers were either moored onto a navigation buoy or anchored to shipwrecks or artificial hard structures. Since the summer of 2018, all PAM loggers were deployed on a multi-use platform, an in-house developed tripod which can be moored on the seafloor. The multi-use platform remarkably improved the stability of the logger which led to the reduction of unusable noisy data.

The BCPAN is presently established across eight locations in the Belgian part of the North sea (BPNS), contributing long-term high-quality data series on porpoise presence. As of August 2024, there have been a total of 164 deployments which generated a sum of 26,238,798 recorded minutes across all stations in the BPNS. Porpoises were present for up to 8% of the time recorded and were detected the most during the colder seasons, with higher proportions along mid-shore stations and interestingly, around the offshore wind farms. 

Porpoise presence and absence data complement long-term data on fish movement and behavior, plankton dynamics and abiotic parameters such as temperature, salinity and nutrients. The LifeWatch RShiny application allows visualization and selection of a variety of parameters such as the timeframe, level of processing and a sample period. Through the lwdataexplorer package in RStudio, a wealth of data can be accessed from the LifeWatch observatory. The function getCpodData in lwdataexplorer allows the retrieval of PAM data. Minute- and hour-resolution datasets are also published annually in a standardized format in the Integrated Marine Information System (IMIS) and several public repositories such as the European Marine Observation and Data Network (EMODnet) and the Ocean Biodiversity Information System (OBIS). The data is accompanied by its metadata description and is under a CC-BY license, allowing the use of data under the condition of providing reference to the original source.

Acknowledgements

This work demonstrated LifeWatch observation data and its data pipeline, which made use of infrastructure (the cetacean passive acoustic network) provided by the Flanders Marine Institute (VLIZ) and funded by Research Foundation - Flanders (FWO) as part of the Belgian contribution to LifeWatch. A lot of hands contributed to the maintenance of the network— we thank the Marine Observation Center and Infrastructure department of VLIZ and the crew of RV Simon Stevin for their continuous involvement and support at sea. We also acknowledge the teams of Data Center and IT at VLIZ for their hard work in developing and maintaining ETN and LifeWatch data explorer and RShiny applications.