Behavioral responses of herring (Clupea harengus) to 1–2 and sonar signals and killer whale feeding sounds
Typical example of transmission loss (given in figure as TL) from the sonar source to the observation point of the echosounders as a function of time. Transmission started away from the observation point (the echosounder), and the source ship took to pass the echosounder. The herring layer was usually distributed at depths between 10 and . Data are therefore presented for the individual depths; 15, 25, 35, 45, and . The source levels were 209 and (re at ) for F1 and F2 transmissions, respectively, and received levels can be calculated as the difference between source level and TL. Transmission loss were calculated using the acoustic model LYBIN, with input parameters being the measured sound speed profiles and sonar source characteristics. The upper panel shows the received sound pressure levels at the echosounder being passed by the source ship, and the lower panel shows the received levels at the second echosounder, located further south.
Spectogram and waveform of the transmitted sonar signals: F1 and F2. The left panel shows the spectrogram for F1 (lower curve) and F2 (upper curve), with frequency as a function of time. The scale on the left indicates intensity (dB). The right panel shows the corresponding waveforms, with relative amplitude as a function of time. The transmitted signals were hyperbolic frequency modulated waveforms from for F1, for F2, both with duration of .
Waveform (top panel) and spectrogram (bottom panel) of a representative segment of the killer whale feeding sounds recorded during playback to herring. Note that the signals from the echosounder used to monitor the behavior of the herring are apparent at . The killer whale sounds include a number of calls, and a tail-slap sound (starting at ). The call around is a typical example of a call resembling the sonar signals in duration and frequency content (see Fig. 2).
Typical echogram examples. Responses to the sounds were measured as herring density, and herring vertical distribution, depth, and are presented as a function of time. Thick lines represent the particular experiment that the echogram is taken from, while thin lines are the average of all passages of this transmission type. The vessel wash from the passing vessel as well as the towed body sonar can be seen as strong distinct echoes around . (a) Control; passage with vessel and sonar source without any transmission. The upper line is experiment, followed by depth experiment, depth average, and average on the bottom. (b) Playback of killer whale feeding sounds. The vertical lines indicate start and stop of playback. The two upper lines indicate depth experiment and depth average, respectively, the lower ones indicate average and experiment, respectively. (c) F2 transmission . The upper line is depth experiment, followed by average, depth average, and experiment on the bottom. (d) F1 transmission . The upper line is depth experiment, followed by depth average, average, and experiment on the bottom. No clear differences between sonar transmission (F1/F2) and the control can be detected by inspecting the echograms. A small vertical drop in the herring layer is seen at the point in time when the towed body sonar passes, but this reaction is similar for all types of transmission, and hence probably an avoidance to the source rather than the sound. In response to the passage involving playback of killer whale feeding sounds (b), there is a reduction in density that starts before passage of the source, almost immediately after onset of the sound, indicating that this reaction is to the sound. Echograms (a), (c), and (d) are from November 12, 2006, while (b) is from November 22, 2006.
Estimates and 95% confidence bounds for the predicted average density response, (left bars in black), and vertical response, depth (right bars in gray), for the following factors: (a) experiment, (b) block within experiment, (c) transmission type, and (d) order of the types of transmission. (a) There were significant differences between the experiments. Experiment 4 produced significantly lower values than the other days regarding both and depth. Experiment 3 had significantly higher than the other days. (b) Block 3 had a significantly lower than blocks 1 and 2 within an experiment, but no significant differences were found with respect to depth. (c) There were no significant differences between the two sonar transmission types (F1 and F2) and the control either for or depth. Playback of killer whale feeding sounds (Orca), however, had significantly lower values than F2 and control. (d) There was no significant effect of the order of the types of transmission, nor for nor depth.
Controlled exposure experiments carried out with herring. Experiments 1, 2, 3, and 6 consisted of three blocks each, and herring were exposed to F1 and F2 frequency sonar signals as well as a control run without transmission. Experiments 4 and 5 consisted of one block each, consisting of playback of killer whale feeding sounds (Orca), F1, and a control run. The order of transmission types within each block was randomized.
Significant Tukey comparisons of the four factors included in the statistical model (experiment, block, transmission type, and order of transmissions), three factors had significant effect in explaining the average horizontal and vertical (depth) response of herring; experiment (significant for depth and ), block, and transmission (significant for ).
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