Project Title: Acoustic investigation of new bycatch mitigation pingers
Chief Investigators: Dr Christine Erbe & Mr Craig McPherson
Pingers, or acoustic alarms, were developed to reduce bycatch of marine mammals in fishing gear. A variety of devices exist emitting pure tones, amplitude-modulated tones, frequency sweeps and broadband pulsed sounds, as well as series of multiple sounds. The Queensland Shark Control Program (QSCP) has been using pingers on shark nets along Queensland's beaches to prevent marine mammal entanglement since 1992. An increase in the occurrence of whale entanglements was seen in 2009. As a result, pingers were replaced with new models in July - August 2010. This replacement rendered all previous studies of pinger characteristics obsolete.
The aim of this project was to measure the acoustic characteristics of the new pingers (Fumunda F3 and F10), to model sound propagation in an environment where QSCP pingers are deployed, to estimate over what ranges pingers might be detectable by marine mammals, to monitor ambient noise in an environment where QSCP pingers are deployed, and to make recommendations to QSCP on pinger deployment to optimise pinger efficacy.
Objective 1: Pinger Characterisation
Three Fumunda F3 and three F10 pingers were measured by mounting them in a purpose-built frame that would keep them at a constant distance of 2 m from the recording hydrophones yet allow 180° rotations in both the vertical and horizontal planes to estimate patterns of sound emission directivity.
F3 Whale Pingers
The F3 pingers emitted a fundamental frequency of on average 2.7 kHz (2.6, 2.7 and 2.8 kHz for the three pingers) with multiple harmonics. The tones were about 400 ms long, occurring once every 6 s. Output levels varied from ping to ping and across pingers. The angular measurements showed some symmetry in the horizontal plane from midpoint to midpoint about the electrode end of the pingers, and in the vertical plane. Patterns were not consistent from pinger to pinger, partly due to the fact that the orientation of the vibrating piezo disk inside the pingers is not consistent but can vary by a few degrees, and partly due to the ping-to-ping variability in output level. The mean source levels over all angles and over five pings at each angle were 98 ± 7, 109 ± 6 and 118 ± 3 dB re 1 μPa2/Hz @ 1 m for the three F3s. Fumanda species an output level of 135 dB re 1 μPa @ 1 m. This level was measured from one of the three pingers at some angles, however, on average, levels were less.
F10 Dolphin Pingers
The three F10 pingers had fundamental frequencies of 9.4, 9.5 and 9.6 kHz, plus multiple harmonics. The tones were about 400 ms long and occurred once every 4 s. The variations in output level from ping to ping, from pinger to pinger, and as a function of angle were larger than for the F3s. The mean fundamental source levels over five pings at all angles for the three F10s were 106 ± 8, 122 ± 2 and 115 ± 7 dB re 1 μPa2/Hz @ 1 m in the vertical plane. The mean broadband source levels were 117 ± 3, 127 ± 2 and 123 ± 4 dB re 1 μPa @ 1 m for the three F10s in the vertical plane. Mean levels in the horizontal plane were on average 10 dB less. Fumunda specifies an output level of 132 dB re 1 μPa @ 1 m, which was reached by one of the three pingers at multiple angles, however, at other angles the levels were less.
Objective 2: Modelling the Pinger Sound Field
JASCO's Marine Operations Noise Model (MONM) was used to predict transmission loss at a site off the Gold Coast where the QSCP has shark nets and pingers installed. Four frequencies were modelled: The F3 fundamental of 2.7 kHz, the first harmonic of 5.4 kHz, the second harmonic of 8.1 kHz, and the nominal F10 fundamental of 10 kHz. Acoustic energy was lost at a rate of 15-20 dB/decade in range over the first 10 m, 10 dB/decade from 10 to 100 m, 15 dB/decade from 100 - 1000 m, and > 20 dB/decade to 10 km in range. The effect of tide on transmission loss was < 2 dB from high tide to low tide over the same modelled ranges.
The results of the transmission loss model have to be combined with ambient noise levels and hearing capabilities of the marine mammals involved in order to determine the pingers' effective ranges.
Objective 3: Predicting Pinger Detectability
The literature was searched and reviewed for hearing abilities of marine mammals encountered along the Queensland coast:
• Humpback whales (Megaptera novaeangliae)
• Indo-Pacific bottlenose dolphins (Tursiops aduncus)
• Bottlenose dolphins (Tursiops truncatus)
• Indo-Pacific humpback dolphins (Sousa chinensis)
• Common dolphins (Delphinus delphis)
• Snubfin dolphins (Orcaella heinsohni)
• Dugongs (Dugong dugon)
Given the lack of species-specific detail on hearing abilities, the dolphins were grouped together.
There is no audiogram for humpback whales. It is assumed that humpback whales are sensitive at the frequencies of their own vocalisations (mostly 100 Hz-4kHz). Anatomical evidence of ears cut out of dead and stranded humpbakc whales confirms such low-frequency hearing specialisation. Humpback whales have been observed to respond to low-to-mid frequency sound from airguns, as well as sonars and pingers. In the absence of hearing thresholds for humback whales, humpback hearing was assumed to be ambient noise limited. A critical ratio of 20 dB was added to model pinger tone detection in broadband ambient noise. For all of the dolphin species, published audiogram and critical ratio data from Tursiops truncatus were used. For dugong, audiogram measurements of manatees were used, and a critical ratio of 20 dB was added.
Using the mean source levels of the fundamental and harmonics of the six pingers measured in Objective 1, applying the transmission loss model from Objective 2, and subtracting the hearing thresholds for the three animal groups, led to a table of pinger detection ranges. Based on this approach the F3s were estimated to be audible to humpback whales and duogong over 210 m in range, and to dolphins over 110 m in range. The F10s were estimated to be audible to humpback and dugong over 130 m in range, and to dolphins over 40 m in range. The output from some of the pingers at specific angles would be audible over much longer ranges. The nominal levels specified by the manufacturer would be audible over a few km in range.
Objective 4: Ambient Noise Monitoring
One of JASCO's autonomous underwater acoustic recorders was deployed four times during the year for up to one month at two locations off the Gold Coast beaches. These instruments recorded underwater ambient sound continuously at a sampling frequency of 32 kHz. Ambient noise below 30 Hz was mostly caused by wind and wave action in shallow water and by fluid flow. Between 100 Hz and 2 kHz a number of sources were identified, including boats, fish, humpback whales, and a sand pump. Above 2 kHz at the January, March and May location, snapping shrimp sound was reduced in the September data, likely due to the different geographic location of the September recorder. Fish sounds were heard throughout, however distinct choruses were not detected. Humpback whales on their southern migration were heard throughout the September deployment. No humpback whales were detected in January and March. A few humpback calls were heard in May/June as they returned on their northern migration. Dolphins were detected in all data sets.
Boat passes were heard throughout the recordings. The September recorder site was about 2.4 km south of a sand pump operating every night and emitting loud sound between 100 Hz and 800 Hz. The sand pump was the strongest and most consistent contributor to ambient noise above 30 Hz during the month of September. For the later three deployments, the recorder was moved 10 km south, away from the sand pump, reducing the received level.
The September recorder site was about 1.5 km from the nearest shark net. At this range, the pingers contributed very little to ambient noise budgets. Neither the F3 nor the F10 fundamental were discernible in the ambient noise spectra; only the first harmonic of the F3 was visible. The January, March and May recorder site was about 500 m from the nearest shark net. The F10 fundamental was clearly visible in all three data sets. No F3 pingers were heard in January and March. The F10 pingers are installed throughout the year, whereas the F3s are only deployed during the humpback migration season and would not have been in operation during the January and March monitoring.
There are currently 3-4 pingers per net of 200 m length. The number of pingers per net has changed over time. Modelling showed this pinger spacing to be more than adquate for humpback whales and dugong even at their top swim speeds. Given the low levels recorded from the F10 fundamental specifically targeted at dolphins, the F10 spacing was sufficient for dolphins at normal travelling speeds, and only inadequate in the case of dolphins swimming at top speed perpendicular to (i.e. directly towards) a net.
Given the small sample size of pingers measured (three per type), it might be useful to test a larger number of units to achieve a better statistical representation of output levels. It would also be useful to measure at what time into a deployment the battery power becomes inadequate to sustain sufficient output levels, in order to advise on recovery times.
For potential future studies on behavioural responses of marine mammals to pingers, the received sound level should be measured in the field at the time, rather than relying on the manufacturers' specifications in combination with a simple (e.g. geometrical) sound propagation model.