Cetacean monitoring is essential in determining the status of a population. Different monitoring methods should reflect the real trends in abundance and patterns in distribution, and results should therefore ideally be independent of the selected method. Here, we compare two independent methods of describing harbour porpoise (Phocoena phocoena) relative distribution pattern in the western Baltic Sea. Satellite locations from 13 tagged harbour porpoises were used to build a Maximum Entropy (MaxEnt) model of suitable habitats. The data set was subsampled to one location every second day, which were sufficient to make reliable models over the summer (Jun-Aug) and autumn (Sep-Nov) seasons. The modelled results were compared to harbour porpoise acoustic activity obtained from 36 static acoustic monitoring stations (C-PODs) covering the same area. The C-POD data was expressed as the percentage of porpoise positive days/hours (the number of days/hours per day with porpoise detections) by season. The MaxEnt model and C-POD data showed a significant linear relationship with a strong decline in porpoise occurrence from west to east. This study shows that two very different methods provide comparable information on relative distribution patterns of harbour porpoises even in a low density area.

The BIAS Implementation Plan describes a regional programme for monitoring underwater ambient noise in the Baltic Sea. The programme combined measurements and modelling and was successfully implemented in 2012-2016 within the EU LIFE+ project Baltic Sea Information on the Acoustic Soundscape (BIAS) by Sweden, Denmark, Germany, Poland, Estonia and Finland.

In 2014, one year of sound measurements was performed at 36 locations with the purpose of establishing the baseline state of ambient noise levels in the Baltic Sea. The measurements, as well as the post-survey processing of the data, were subject to standardized field procedures, quality control and signal processing routines were all developed within BIAS.

BIAS dealt exclusively with monitoring continuous low frequency sound as referred to by the Marine Strategy Framework Directive indicator 11.2.1. Therefore, the study focused on the 1/3 octave frequency bands of 63 and 125 Hz, as specified by the indicator, and a third frequency band (2 kHz) which was added to strengthen the ecological relevance of the BIAS results.

The measured sound data were used to model the soundscape for the entire project area, providing the first results for the Baltic Sea on a monthly basis. Soundscape maps were produced for the three targeted frequency bands, and three depth intervals: the surface layer (0 to 15m deep), the deep layer (30m to the bottom), and the full water column of the Baltic Sea.

A large number of soundscape maps were produced constituting the base for future management of noise in the Baltic Sea. To facilitate an efficient handling of these, and future, results a GIS-based soundscape planning tool was created for visualizing the measured data and the modelled maps in a management friendly concept. BIAS identified two useful statistical measures for characterizing the soundscape which were incorporated into the soundscape planning tool. The year-by-year change of these measures directly relate to the current definition of the indicator.

Based on the experiences made in the project, the BIAS implementation plan also outlines a plausible strategy for the future monitoring and the elements needed for maintaining a joint implementation for underwater ambient noise in the Baltic Sea region.