Pelagic Ecology Mitigation and Monitoring
Independent Expert Appraisal
The consultancy services of marine mammal behaviour and ecology expert Prof. Bernd Würsig of Texas A&M University,
USA, were engaged in starting April 2008 to conduct a peer review of marine mammal activity / sensitivity to the Project,
and to advise on monitoring and management requirements of the proposed Project area.
Mitigation Measures
Referring to Thomsen et al (2006), if less noisy construction methods to percussive piling exist, these should
preferentially be used. For the proposed project, considerable effort has been taken to investigate the feasibility of the
suction caisson foundation option, with this construction method selected as preferred to avoid adverse impacts.
Controls on dredging and jetting activities as referred in the water quality assessment shall ensure that impacts on
pelagic species are not signifcant
Monitoring
It has been well-documented for all but Dall’s porpoises (Phocoenoides dalli), that porpoises of the family
phocoenidae (as opposed to most dolphins, delphinidae) tend to be cryptic while they surface to breathe, and are
therefore difficult to see. This is exacerbated in the finless porpoise, as it is small and even more difficult to see than
other species due to the lack of a dorsal fin, as well as a muted gray coloration that often makes the porpoise blend in
with a slightly choppy water surface (Jefferson and Hung, 2004).
Furthermore, following literature review and the 2006/07 field survey event for this Project, it is evident that there are
factors that amplify limited data in areas with an already low encounter rate. These include:
For such reasons, cetacean studies for international offshore wind farm developments, such as the monitoring
conducted at Nysted Offshore Wind Farm, have modified the approach by deploying acoustic devices which are less
weather-dependent and can allow for continuous monitoring. Such devices, or passive acoustic monitors (PAMs), are
invaluable for detecting the high frequency clicks of porpoises that are easily-distinguished from sounds of other
marine animals.
Most monitoring work of this type has been carried out on European harbour porpoises, Phocoena phocoena
(Villadsgaard et al., 2007; Evans, 2008), although the technique has also been experimentally demonstrated locally for
finless porpoises using towed recording devices jointly engaged during line transects. (Jefferson et al. 2002: Goold
and Jefferson, 2002).
One type of PAM is the T-POD (for, “timing porpoise detectors”) which is used specifically for monitoring porpoise
clicks and can be mounted on the seabed to give a 24-hour-day record of marine mammal presence within a
detection range of 75 to up to about 200m. A more recent development is the C-POD, for “cetacean porpoise
detector”, that more accurately records vocalizations of porpoises plus all other echolocating toothed whales and
dolphins (www.chelonia.co.uk).
Given the above, we do not suggest further stand-alone use of the visual observation method to obtain more
information on porpoise occurrence patterns in the general area. We have also explored the possibility of using visual
surveys in conjunction with towed acoustic sensing devices (T-PODs) as were used by Jefferson et al. (2002) for
detecting finless porpoises. While this double-system of evaluation was useful for corroboration of visual sightings, it
is apparent that porpoises are at times shutting off their active acoustics due to the presence of the line-transect
research vessel, and we have received expert advice (Nick Tregenza, Chelonia Ltd.) that as a result, it is unlikely that
towed systems enhance visual surveys for finless porpoises, at least with present resolution and capabilities.
Based upon the above, we recommend the use of C-PODs to monitor the activity of finless porpoises both day and
night and in all weather conditions due to their greater reliability relative to sensing porpoises as well as other marine
mammals. Further details of the proposed monitoring programme shall be provided in the Project’s stand-alone
Marine Environment Monitoring Plan (MEMP) to be developed in parallel with the engineering design, although the two
core elements shall involve:
The C-PODs deployed will need to be serviced every three to four months to download accumulated data and replace
batteries. It is proposed that the C-PODs be installed after installation of the turbines when security from trawling
damage / loss can be afforded, and for a sufficient period of time to obtain a robust record of marine mammal usage of
the area, especially due to the great inter-seasonal and inter-year differences already known for finless porpoises.
The MEMP shall detail the integrated monitoring requirements associated with pelagic ecology, benthic ecology and
fisheries resources within the Project area, and development and implementation of the MEMP shall form a condition
of the Environmental Permit.
Independent Expert Appraisal
The consultancy services of marine mammal behaviour and ecology expert Prof. Bernd Würsig of Texas A&M University,
USA, were engaged in starting April 2008 to conduct a peer review of marine mammal activity / sensitivity to the Project,
and to advise on monitoring and management requirements of the proposed Project area.
Mitigation Measures
Referring to Thomsen et al (2006), if less noisy construction methods to percussive piling exist, these should
preferentially be used. For the proposed project, considerable effort has been taken to investigate the feasibility of the
suction caisson foundation option, with this construction method selected as preferred to avoid adverse impacts.
Controls on dredging and jetting activities as referred in the water quality assessment shall ensure that impacts on
pelagic species are not signifcant
Monitoring
It has been well-documented for all but Dall’s porpoises (Phocoenoides dalli), that porpoises of the family
phocoenidae (as opposed to most dolphins, delphinidae) tend to be cryptic while they surface to breathe, and are
therefore difficult to see. This is exacerbated in the finless porpoise, as it is small and even more difficult to see than
other species due to the lack of a dorsal fin, as well as a muted gray coloration that often makes the porpoise blend in
with a slightly choppy water surface (Jefferson and Hung, 2004).
Furthermore, following literature review and the 2006/07 field survey event for this Project, it is evident that there are
factors that amplify limited data in areas with an already low encounter rate. These include:
- Changes in local distribution pattern: fluctuating porpoise utilization rate has been recorded across eastern
waters (Figure 6.6 refers). - The survey methodology: Studies at Nysted Offshore Wind Farm in Denmark for example concluded that as
porpoises were mostly active at night-time, there were fundamental limitations in visual only (i.e., day-time)
surveys. - Weather conditions: the vessel-based observation method is highly weather dependent and can be
compromised greatly by, for example, low sunlight intensity (Evans, 2008). Locally, Jefferson (AFCD, 2000)
concluded that the overall abundance, which is a function of sighting rate and probability density function, could
drop by as much as 42% when survey observations were conducted in unfavourable environmental condition.
For such reasons, cetacean studies for international offshore wind farm developments, such as the monitoring
conducted at Nysted Offshore Wind Farm, have modified the approach by deploying acoustic devices which are less
weather-dependent and can allow for continuous monitoring. Such devices, or passive acoustic monitors (PAMs), are
invaluable for detecting the high frequency clicks of porpoises that are easily-distinguished from sounds of other
marine animals.
Most monitoring work of this type has been carried out on European harbour porpoises, Phocoena phocoena
(Villadsgaard et al., 2007; Evans, 2008), although the technique has also been experimentally demonstrated locally for
finless porpoises using towed recording devices jointly engaged during line transects. (Jefferson et al. 2002: Goold
and Jefferson, 2002).
One type of PAM is the T-POD (for, “timing porpoise detectors”) which is used specifically for monitoring porpoise
clicks and can be mounted on the seabed to give a 24-hour-day record of marine mammal presence within a
detection range of 75 to up to about 200m. A more recent development is the C-POD, for “cetacean porpoise
detector”, that more accurately records vocalizations of porpoises plus all other echolocating toothed whales and
dolphins (www.chelonia.co.uk).
Given the above, we do not suggest further stand-alone use of the visual observation method to obtain more
information on porpoise occurrence patterns in the general area. We have also explored the possibility of using visual
surveys in conjunction with towed acoustic sensing devices (T-PODs) as were used by Jefferson et al. (2002) for
detecting finless porpoises. While this double-system of evaluation was useful for corroboration of visual sightings, it
is apparent that porpoises are at times shutting off their active acoustics due to the presence of the line-transect
research vessel, and we have received expert advice (Nick Tregenza, Chelonia Ltd.) that as a result, it is unlikely that
towed systems enhance visual surveys for finless porpoises, at least with present resolution and capabilities.
Based upon the above, we recommend the use of C-PODs to monitor the activity of finless porpoises both day and
night and in all weather conditions due to their greater reliability relative to sensing porpoises as well as other marine
mammals. Further details of the proposed monitoring programme shall be provided in the Project’s stand-alone
Marine Environment Monitoring Plan (MEMP) to be developed in parallel with the engineering design, although the two
core elements shall involve:
- Joint visual / C-POD Calibration survey: As bottom-mounted acoustic monitoring devices have not previously
been used for detecting finless porpoises, it is important that these be calibrated relative to visual surveys. It is
thus proposed that a line transect survey be conducted in conjunction with placement of C-PODs using
standard approved line transect methodology (as per AFCD, 2005, etc.) for calibration purposes. Given the low
level of sightings (and, hence potentially, acoustic contacts), the line transects and C-PODs should be engaged
in for sufficient time for statistical robustness relative to inter-calibration. - One option for this work, to be discussed and agreed with AFCD prior to commencement, is to conduct the joint
acoustic / visual survey in HKSAR western waters where finless porpoise activity is relatively high (compared
with offshore eastern waters), and hence with greater potential for good quality calibration data. Under this
scenario, for example, deployment of 2-3 C-PODs combined with a 3-month visual survey would likely yield
good quality data for C-POD calibration. Ultimately the number of C-PODs to be deployed and the necessary
duration of visual transect survey for calibration would depend on exactly where this part of the survey
programme was to be conducted. - Placement of C-PODs: After inter-calibration of acoustic and visual data to support generation of an accurate
estimate of finless porpoises by the C-PODs, the second aspect of the programme shall involve placement of
these devices within and just outside the wind farm area (for example, Teilmann et al., 2006). As Porpoise
clicks are substantially above 100 kHz in frequency, not very loud (Goold and Jefferson, 2002) the detection
distance is likely to be on the order of low 100’s of meters from the bottom-mounted C-PODs. This will
influence the number and configuration of C-PODs deployed, with exact details of the number and positioning
of the devices to be presented in the MEMP.
The C-PODs deployed will need to be serviced every three to four months to download accumulated data and replace
batteries. It is proposed that the C-PODs be installed after installation of the turbines when security from trawling
damage / loss can be afforded, and for a sufficient period of time to obtain a robust record of marine mammal usage of
the area, especially due to the great inter-seasonal and inter-year differences already known for finless porpoises.
The MEMP shall detail the integrated monitoring requirements associated with pelagic ecology, benthic ecology and
fisheries resources within the Project area, and development and implementation of the MEMP shall form a condition
of the Environmental Permit.
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