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Seismic survey risk assessment for the common dolphin population in the south-western coast of Portugal

G. Spadoni(1), R. Duarte(1), C. Soares(2), M. Fernandez(3) and S.M. Jesus(1)
(1) LARSyS, University of Algarve, 8005-139 Faro, Portugal
(2) Marsensing Lda., Campus de Gambelas, Faro, Portugal
(3) MARE-Madeira/ARDITI, Funchal, Portugal

Comments: (no paper yet)
Ref.: Int. Conf. on the Effects of Noise on Aquatic Life, Berlin (Germany), July 2022

The soundscape of the marine environment suffers from a significant influence of anthropogenic noise sources. While the main source of noise resulting from human activities in the ocean is shipping, there are others such as military sonar, energy harvesting, seismic surveys, etc, that contribute to potentially harmful effects on the surrounding marine life. In the past, several proposals involving seismic exploration were raised along the coast of Portugal, regardless of the fact that the Portuguese south-west coast is of great interest in terms of marine biodiversity. Several methods to assess the potential risk to which species could be exposed were proposed over the years (Erbe et al. 2014; Verling et al. 2021). The objective of this study is to assess the risk areas through the combination of species distribution maps and noise maps in the region of Setúbal as proposed by Erbe et al.,2014.
A seismic survey was simulated in the region of Setúbal, between W[-8.92;-8.87] and N[38.42;38.34], in January and June 2019 using a GEO-Source200 Sparker as reference for the frequency range of 300-1000Hz. Concerning species distribution maps, the population of common dolphins was chosen as a representative of the small cetaceans present in the area. Ecological Niche Models (ENMs) were developed for the Portuguese coast combining observations records from multiple sources and environmental variables through maximum entropy modeling.
The results on noise modelling indicate that sound levels of up to 190 dB are received in some areas, impacting the species hearing perception in a range of 40km around the surveying zone. Additionally, it was also possible to observe the influence of the seasons in the sound propagation. On the other hand, results from the ENM for the common dolphin (Delphinus delphis) showed high habitat suitability values along all the coast of Portugal, with a particularly strong habitat quality near the region of Setúbal, confirming this region as of great interest in terms of risk assessment and protection measures.
The results on risk assessment were obtained by multiplying species distribution and noise maps in a normalized scale between 0 and 1, and by normalizing again the result of this multiplication. The results show the sensitivity of coastal areas, especially until the bathymetric line of 250m. Furthermore, during the summer season, higher levels of risk were found in a broader area than in winter, reflecting a seasonality and consequently indicating periods of the year less suitable for these practices.