Fig 1.
Location of the Lysekil research site on the west coast of Sweden.
(a) The arrow indicates where the sonar platform was deployed. (b) Computer rendering of the subsea environment within the test site.
Table 1.
Common species of interest on the Swedish west coast [35].
Fig 2.
The standalone monitoring platform: (a) The multibeam imaging sonar (MBS) together with split beam sonar (SBS), the three boxes contain batteries, controllers and an on-board computer; (b) The standalone platform deployed at the seabed.
Fig 3.
Lateral view-scheme to facilitate the interpretation of the acquired multibeam data and the resultant acoustic images.
The multibeam sonar (MBS) was located at 24.3 m of depth at a distance of 30 m and 50 m from WECs A and B respectively. The field of view (FOV) opens in 20° vertically and 130° horizontally. The MBS pitch angle (αMBS) was set to 10°.
Table 2.
Technical specifications of sensors and components used in this study.
Fig 4.
Top view of the sonar field of view (FOV).
This scheme facilitates the interpretation of the acquired multibeam (MBS) data and acoustic images. The maximum width of the FOV was 82 m or 130° (θMBS = -66° to 66°) and the maximum range was 100 m. Most of the classified targets were detected within 50 m of range. The MBS was located at a distance of 30 m and 50 m from WECs A and B, respectively.
Fig 5.
Architecture of the data acquisition and processing for detecting, tracking and classifying underwater targets of interest.
The MBS signal is promptly processed by the SDK/ProViwer software running on C++ programming environment. The acoustic images are then analysed and filtered using the SDK/ProViewer and Matlab—Image processing Toolbox [42].
Fig 6.
Total number of observations versus number of observations that resulted in target detection.
For example, on a total of 260 observations 234 contained seals, and 32 did not contain seals.
Fig 7.
Distribution of occurrence of seals in relation to (a) length and (b) detection range. Seals measuring 1.1 m to 2 m were the most frequent and were mostly detected within 6 m to 16 m of range.
Fig 8.
Acoustic image acquired on 26/08/16 at 09:20 UTC showing a group of four seals, measuring between 1.2 m to 2 m of length, detected at 6 m to 11 m of range from MBS and at distances between 20 m and 30 m from WEC A.
Fig 9.
Acoustic image acquired on 28/08/16 at 19:39 UTC showing a large marine mammal (possible orca) located at 15 m of range, with 7 m of length, and a dorsal fin measuring 1.3 m of height.
Fig 10.
Acoustic image acquired on 26/08/16 at 19:26 UTC, showing a school mid-size fish and at least 20 large size fish.
The school measured 12 m of diagonal length and the individual large fish measured between 0.4 m and 0.9 m.
Fig 11.
Time series of the observed targets starting from the time of deployment (23-08-2016) until the time of retrieval (01-09-2016).
Small and mid-size fish were observed and counted in schools. The occurrence of small-size and large fish had diurnal and nocturnal patterns, respectively. Seals occurred with a random trend along the time. The grey shaded areas represent the night-time.
Fig 12.
Temporal variability, occurrence, and behaviour of seals.
(a) Total occurrence of observed seals per hour; (b) Total occurrence of seal swimming in groups, engaged in hunting, and swimming close to WEC A. The standard error is represented by the error bars with caps.
Fig 13.
Temporal variability, occurrence and behaviour of fish.
(a) Total occurrence of large fish (> 40 cm); (b) Total occurrence of schools of mid-size (20 cm to 40 cm) and small fish (< 20 cm). The standard error is represented by the bars with caps.
Fig 14.
Acoustic image acquired on 27/08/16 at 09:22 UTC, showing a school of mid-size fish passing close to WEC A.
A hunting seal measuring 2.5 m of length, was detected hunting and diving into the school.
Fig 15.
Acoustic images showing seals diving into schools of mid-size fish: (a) a school with 2.5 m of diameter and a seal of ca. 1.3 m of length, observed on 27/08/16 at 16:20 UTC; (b) school with ca. 3.5 m of dimeter and a seal of ca.1 m, observed on 27/08/16 at 09:18 UTC; (c) school with ca. 10 m of diameter and seals of ca. 1 m, observed on 27/08/2016 09:20 UTC.