MARINE MAMMAL RISK ASSESSMENT OF PROPOSED DUBLIN PORT COMPANY MAINTENANCE DREDGING AND DISPOSAL AT SEA
Prepared by Dr Simon Berrow
IWDG Consulting, Merchants Quay, Kilrush, Co Clare
1 | INTRODUCTION
The Irish Whale and Dolphin Group (IWDG) were contracted by RPS to carry out a Marine Mammal Risk Assessment of the proposed maintenance dredging campaign in Dublin Port during 2020 and 2021 and subsequent disposal of dredge material at Burford Bank. The proposed works will involve the removal of approximately 600,000 m3 of dredge material.
Proposed works
Dublin Port is required to undertake regular maintenance dredging of its berths, basins and navigation channel to maintain advertised depths for the safe passage of vessels
Maintenance dredging is required within the berths, basins and navigation channel within the inner Liffey channel (Dublin Harbour) and the navigation channel within Dublin Bay. Areas within Dublin Harbour containing Class 2 sediments (slightly contaminated) and Class 3 sediments (highly contaminated) are excluded from the proposed dredging works. The areas excluded are Alexandra Basin West and the navigation channel between East Link Bridge and the Western Oil Jetty.
The material to be dredged within Dublin Harbour is mostly silt. The sediment chemistry of the material to be dredged has been tested and the Marine Institute has confirmed its suitability for disposal at sea. Around 300,000 m3 of material is required to be dredged from Dublin Harbour over the 2020 – 2021 period using a Trailing Suction Hopper Dredger (TSHD). No overspill of silts will be permitted within Dublin Harbour to avoid significant elevated levels of suspended solids during the maintenance dredging operations. The dredger’s hopper will be filled to a maximum of 4,100 m3 to avoid any potential accumulation of suspended solids at the proposed offshore dump site.
The material to be dredged within Dublin Bay is mostly fine sand. The sediment chemistry of the material to be dredged has been tested and the Marine Institute has confirmed its suitability for disposal at sea. Around 300,000 m3 of material is required to be dredged from Dublin Bay over the 2020 – 2021 period using a Trailing Suction Hopper Dredger (TSHD). Overspill of fine sands will be permitted within Dublin Bay during the maintenance dredging operations because the fine sand released drops rapidly to the seabed and does not give rise to
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significant elevated levels of suspended solids. No restriction on the dredger’s hopper fill capacity is required for the dredging of fine sands within Dublin Bay.
The maintenance dredging operations will be restricted to the period April – September during 2020 and 2021. This is to avoid overlap with the Alexandra Basin Redevelopment (ABR) Project capital dredging works which are scheduled to take place over the winter seasons (October 2019 – March 2020) and (October 2020 – March 2021). Dredging will commence during daylight hours and suitable sea conditions and once started will operate on a 24/7 hour basis, provided there are no breaks in dredging operations.
The total volume of material to be dredged from both Dublin Harbour and Dublin Bay during the 2020 and 2021 maintenance dredging campaigns is around 600,000 m3. It is proposed to dispose of the dredge spoil at the licenced offshore dump site located at the approaches to Dublin Bay to the west of the Burford Bank. The proposed dump site is within Rockabill to Dalkey Island Special Areas of Conservation (SAC) with harbour porpoise as a qualifying interest.
Figure 1: Navigation channels to be dredged and the site of the proposed spoil ground.
2 | METHODS
The risk assessment was based on a review of the available literature and original data collected during the ABR Project (Russell et al. 2018). The marine mammal community in, and adjacent to, the dredge and dumping sites is well known following extensive survey work over the past three years carried out on behalf of Dublin Port as part of the ABR Project work.
3 | LEGAL STATUS
Irish cetaceans and pinnipeds are protected under national legislation and under a number of international directives and agreements which Ireland is signatory to. All cetaceans as well as grey and harbour seals are protected under the Wildlife Act (1976) and amendments (2000, 2005, 2010 and 2012). Under the act and its amendments it is an offence to hunt, injure or wilfully interfere with, disturb or destroy the resting or breeding
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place of a protected species (except under license or permit). The act applies out to the 12 nm limit of Irish territorial waters.
All cetaceans and pinnipeds are protected under the EU Habitats Directive. All cetaceans are included in Annex IV of the Directive as species ‘in need of strict protection’. Under this Directive, the harbour porpoise (Phocoena phocoena), bottlenose dolphin (Tursiops truncatus), grey seal (Halichoerus grypus) and harbour seal (Phoca vitulina) are designated Annex II species which are of community interest and whose conservation requires the designation of special areas of conservation.
Ireland is also signatory to conservation agreements such as the Bonn Convention on Migratory Species (1983), the OSPAR Convention for the Protection of the Marine Environment of the northeast Atlantic (1992) and the Berne Convention on Conservation of European Wildlife and Natural Habitats (1979).
Under the EU Marine Strategy Framework Directive with respect to maintaining good environmental status (GES), “human activities should occur at levels that do not adversely affect the harbour porpoise community at the site” and “proposed activities or operations should not introduce man-made energy at levels that could result in a significant negative impact on individuals and/or the community of harbour porpoise within the site”. This refers to the “aquatic habitats used by the species in addition to important natural behaviours during the species annual cycle”.
In 2007, the National Parks and Wildlife Service (NPWS) of the Department of Arts, Heritage and the Gaeltacht produced a ‘Code of Practice for the Protection of Marine Mammals during Acoustic Seafloor Surveys in Irish Waters (NPWS, 2007). These were subsequently reviewed and amended to produce ‘Guidance to manage the risk to marine mammals from man-made sound sources in Irish waters’ (NPWS, 2014) which include mitigation measures specific to dredging. The guidelines recommend that listed coastal and marine activities (including dredging) be subject to a risk assessment for anthropogenic sound-related impacts on relevant protected marine mammal species to address any area-specific sensitivities, both in timing and spatial extent, and to inform the consenting process.
Once the listed activity has been subject to a risk assessment, the regulator may decide to refuse consent, to grant consent with no requirement for mitigation, or to grant consent subject to specified mitigation measures.
4 | BASELINE ENVIRONMENT
4.1 | Ambient Noise Levels
Ambient, or background noise, is defined as any sound other than the sound being monitored (primary sound) and, in the marine environment, is a combination of naturally occurring biologically and physical sound sources including sediment transfer, waves and rain and that of a biological origin including fish, crustaceans and from marine mammals. The impact of noise created by human activity is strongly influenced by background or ambient noise, the impact is less in a noisy environment compared to a quiet environment and it’s the intensity and frequency of this increased noise compared to the ambient levels at a site which defines its impact. As ambient noise levels increase, the ability to detect a biologically important sound decreases. The point at which a sound is no longer detectable over ambient noise is known as acoustic masking. The range at which an animal is able to detect these signals reduces with increasing levels of ambient noise (Richardson et al. 1995). This is important when considering the impact of sound sources on marine mammals by the proposed works.
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Ambient noise levels worldwide have been on the rise in recent decades with developments in industry and, in particular, in commercial shipping. In the North Pacific, low frequency background noise has approximately doubled in each of the past four decades (Andrew et al. 2002), resulting in at least a 15- to 20-dB increase in ambient noise. In recent years, interest has grown in the effects of anthropogenic noise on marine life. Ambient noise in Dublin Bay has been estimated at around 113 db by Beck et al. (2013) and by McKeown (2014). This level is higher than that reported from Galway Bay and the Shannon Estuary and reflects the greater vessel traffic at this site.
4.2 | Cetaceans
A number of dredging campaigns have been carried out since 2016 which provides site specific information on the receiving environment and interactions with dredging operations. O’Dwyer et al. (2016) reported 134 sightings of marine mammals during 42 days from 20 June to 31 July 2016. The most frequently reported species was grey seal with 76 sightings (57% of the total sightings) followed by harbour porpoise with 56 sightings (42%). There was one sighting of a single harbour seal (Figure 2).
a)
b)
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Figure 2: Grey seal (a) and Harbour porpoise (b) sightings during the 2016 maintenance dredging programme (from O’Dwyer et al. 2016)
Keogh et al. (2017) reported a total of total of 187 marine mammal sightings between 14 and 30 September 2017. Grey seal was the most frequently recorded species, accounting for 70% of the total number of individual marine mammals sighted. Harbour porpoise accounted for 22%, and harbour seal for 6% (Figure 3). Twenty-five marine mammals were observed when loading operations were underway, three during dumping operations, four while the vessel was docked and 155 during transit to and from the loading and dump sites.
Figure 3: All marine mammal sightings during the 2017 Maintenance Dredging Campaign (from Keogh et al. 2017) Keogh et al. (2018) reported a total of 111 marine mammal sightings between 9 and 22 April 2018. There were 110 sightings of marine mammals on days where loading and dumping took place with grey seal the most frequently recorded species, with 65 sightings, accounting for 58% of the total number of marine mammals sighted. Harbour porpoise were recorded on 35 occasions (32%), and harbour seal with 11 sightings (10%) (Figure 4). Nine marine mammals were observed when loading operations were underway, four during dumping operations, one while the vessel was docked and 93 during transit to and from the loading areas and spoil grounds.
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Figure 4: Marine mammal sightings during the 2018 maintenance dredging programme (from Keogh et al. 2018)
Harbour porpoise (Phocoena phocoena)
Harbour porpoise are the most widespread and abundant cetacean in inshore Irish waters, with highest abundances in the Irish Sea (Berrow et al. 2010). Harbour porpoise were the most frequently recorded cetacean species during dredging operations, with between 22 and 42% of all sightings being of harbour porpoise (O’Dwyer et al. 2016; Keogh et al. 2017; 2018). O’Dwyer et al. (2016) reported three sightings within Dublin Harbour. Most sightings were outside of Dublin Harbour, along shipping channel and at the dump site.
Static acoustic monitoring (SAM) has been ongoing at the offshore dump site and at a control site since 2016 (Russell et al. 2018). SAM is particularly effective at logging the presence of harbour porpoise but can detect dolphins too, though not identify to species level. The results from SAM show harbour porpoise to be present on most days (98-100%) (Table 2), with highest detections per day at the control site to the south (SAM 4).
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Figure 5. Figure PAM and SAM positions within the spoil grounds and Dublin Bay (from Russell et al. 2018)
Table 2. Summary of results from SAM at each of four locations
Location Location No. Dates CPOD Porpoise Dolphin Total % days Mean number days no detected DPM/Day SAM 1 North Burford 102 07/07-15/10/2018 2874 3725 962 4687 100 36.5 SAM 2 Mid Burford 81 07/07-24/09/2018 2875 3061 10 3071 100 15.5 SAM 3 South Burford 102 07/07-15/10/2018 2876 2610 18 2628 98 25.5 SAM 4 South Burford 92 07/07-05/10/2018 2879 7279 16 7295 100 79.1
Location Location No. Dates CPOD Porpoise Dolphin Total % days Mean number days no detected PPM/Day SAM 1 North Burford 52 15/10-4/12/2018 2873 2943 0 2943 100 58 SAM 2 Mid Burford 51 15/10-4/12/2018 2877 4181 0 4181 100 83
SAM 3 South Burford 52 15/10-4/12/2018 2881 3054 14 3068 100 60
SAM 4 South Burford 52 15/10-4/12/2018 2882 6164 7 6171 100 121
Generalized linear mixed-effect models (GLMM) were carried out by Russell et al. (2018) using tide, time of day and tidal phase as factors to explore the fine scale use of the spoil site by harbor porpoises. Most detections were recorded at the control site (SAM 4) outside the spoil ground, but of the three locations monitored at the spoil ground, more detections were recorded in the middle (SAM 2), with significantly more detections during winter, and during the hours of darkness. The neap phase had the greatest number of detections in comparison with spring and transitional, as well as during times of high tide (Figure 6).
The site north of the spoil ground (SAM 1) had the second highest number of detections with significant trends similar to SAM 2; more detections during winter and during the hours of darkness. Additionally, more detections were recorded during neap and transitional tidal phases and at high tides (Figure 7) (Russell et al. (2018).
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Figure 6: Predicted proportion of detection positive hours, in the narrow band high frequency channel at SAM 2 (middle SG) across the variables of season; diel, where D = day, E = evening, M = morning and N = night; tidal phase, where Trans. = transitional phase, NT = neap tide and ST = spring tide; and tidal cycle, where E = ebb, L = slack low, F = flood and H = slack high (from Russell et al. 2018).
To the south of the spoil ground (SAM 3), trends contrasted to those recorded at SAM 1 and 2, but there were significantly more detections logged during autumn. Diel showed some significant but different trends compared to SAM 1 and 2, whereby significantly more detections were logged during day and night-time hours. The transitional phases of the tidal phase had significantly more detections, as did the ebbing stage of the tidal cycle (Figure 8) (Russell et al. (2018).
This demonstrates different uses of the spoil ground by harbour porpoises over very fine spatial scales.
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Figure 7: Predicted proportion of detection positive hours, in the narrow band high frequency channel at the SAM 1 (north SG) across the variables, season; diel, where D = day, E = evening, M = morning and N = night; tidal phase, where Trans. = transitional phase, NT = neap tide and ST = spring tide; and tidal cycle, where E = ebb, L = slack low, F = flood and H = slack high (from Russell et al. 2018).
Figure 8: Predicted proportion of detection positive hours, in the narrow band high frequency channel at SAM 3 (south SG), across the four variables of season; diel, where D = day, E = evening, M = morning and N = night; tidal phase, where Trans. = transitional phase, NT = neap tide and ST = spring tide; and tidal cycle, where E = ebb, L = slack low, F = flood and H = slack high (from Russell et al. 2018).
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At the control site (SAM 4), data collection continued only until the end of May 2018 but data were analysed similar to other locations and results showed significantly more detections were logged during summer, across the evening hours, during transitional tides, especially at times of low tide (Figure 9).
Figure 9: Predicted proportion of detection positive hours, in the narrow band high frequency channel at the control site SAM 4 (control) across the four variables of season; diel, where D = day, E = evening, M = morning and N = night; tidal phase, where Trans. = transitional phase, NT = neap tide and ST = spring tide; and tidal cycle, where E = ebb, L = slack low, F = flood and H = slack high (from Russell et al. 2018).
Table 3: Significant results from the long-term dataset at each site (from Russell et al. 2018) Buoy 1=North SG, Buoy 2=Middle SG, Buoy 3= South SG and Buoy 4=control off Dalkey (*no significance)
Significant factors SAM 1 SAM 2 SAM 3 SAM 4
Season Winter Winter Autumn Summer
Diel Night Night Night Evening
Tidal phase Neap/Trans Neap Trans Trans
Tidal cycle High High Ebb Low
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In summary, results across all days monitored at each of the sites showed porpoises to be present on average 97- 100% of days monitored. Presence was highest inside the spoil ground during autumn at SAM 3 and during the winter months at SAM 1 and SAM 2 and during the hours of darkness (incl. dawn and dusk) and a range of tidal cycles and phases (Table 3).
Bottlenose dolphin (Tursiops truncatus)
Bottlenose dolphins are infrequently recorded off Co Dublin. Bottlenose dolphins are widespread and relatively abundant off the Irish coast with most sightings along the western seaboard (Berrow et al. 2010). Recent genetic evidence (Mirimin et al. 2011) suggests the existence of three discrete populations of bottlenose dolphins in Ireland: the Shannon Estuary, an inshore population and an offshore population that ranges from the Bay of Biscay and the Azores (Louis et al. 2014). The inshore population is highly mobile and photo-identification has shown individuals recorded off Co Dublin to be part of this population (O’Brien et al. 2009). No dolphins have been observed at or near the dredge or dump site by marine mammal observers during dredging operations.
Dolphins were occasionally detected by SAM especially during summer months (Table 3). These were most likely bottlenose dolphins.
Minke whale (Balaenoptera acutorostrata)
Minke whales are widespread and abundant in inshore Irish waters from May to October (Berrow et al. 2000). They are regularly reported in the Irish Sea and adjacent to Dublin but have not been recorded in or adjacent to the dredging or dump site.
4.3 | Pinnipeds
Grey and harbour seals are distributed around the entire Irish coast with grey seals being more abundant along the western seaboard (Cronin et al. 2004; O’Cadhla et al. 2007; O’Cadhla and Strong 2007).
Grey Seal (Halichoerus grypus)
Grey seals are regularly and frequently recorded within Dublin Harbour and up the River Liffey into Dublin city. They were the most frequently recorded marine mammal during dredging operations from 2017 to 2019 with between 57 and 70% of all sightings being of grey seals, usually single individuals (O’Dwyer et al. 2017; Keogh et al. 2017; 2018). Grey seals forage locally but may also range long distances but it is likely seals encountered during dredging are the same individuals with dredging providing foraging opportunities.
Harbour Seal (Phoca vitulina)
Harbour (or Common) seals are much less frequently recorded within Dublin Harbour and at the dump site but have been recorded at both locations and along the shipping channel. They were the least frequently recorded marine mammal species during dredging operations from 2017 to 2019 with between <1 to 10% of all sightings, always of single individuals (O’Dwyer et al. 2017; Keogh et al. 2017; 2018).
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Monitoring at Bull Island Haul-out Site
Long term monitoring at a well-known haul out site on Bull Island (Russell et al. 2018) conducted as part of Dublin Port marine mammal monitoring programme shows the regular presence of both grey and harbor seals (Figure 10). Grey seals tend to haul out between May to October/November with peak counts in June while harbour seals occurring throughout the year with peak counts from December through to February/March. These peaks coincide with moulting periods rather than pupping periods.
Figure 10: Number of grey and harbour seals hauled out at Bull Island 2016-2019 (from Russell et al. 2018 and unpubl. data)
5 | IMPACT ASSESSMENT
The potential effects of dredging and dumping on marine mammals was addressed by assessing the likelihood that marine mammals would be exposed, or interact, with the activity. Impacts assessed include likelihood of collision, dumping of dredge material on top of individuals and disturbance especially from noise emitted during dredging and dumping and from the dredge vessel. Acoustic disturbance includes the ability of the individual to detect increased noise levels over ambient levels, masking, Temporary Threshold Shift (TTS) and Permanente Threshold Shift (PTS) and behavioural impacts, i.e. resulting in a behavioural change by individuals.
5.1 | Description of Activities
As part of the proposed site works the activities likely to impact on marine mammals include:
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5.1. Dredging
The material to be dredged within Dublin Harbour is mostly silt. Circa 300,000 m3 of material is required to be dredged from Dublin Harbour over the 2020 – 2021 period using a Trailing Suction Hopper Dredger (TSHD).
The material to be dredged within Dublin Bay is mostly fine sand. Circa 300,000 m3 of material is required to be dredged from Dublin Bay over the 2020 – 2021 period using a Trailing Suction Hopper Dredger (TSHD). The outer reaches of the navigation channel within Dublin Bay extends into the Rockabill to Dalkey Island Special Area of Conservation (SAC).
The maintenance dredging operations will be restricted to the period April – September during 2020 and 2021. This is to avoid overlap with the ABR Project capital dredging works which are scheduled to take place over the winter seasons (October 2019 – March 2020) and (October 2020 – March 2021). Dredging will commence during daylight hours and suitable sea conditions and once started will operate on a 24/7 hour basis, provided there are no breaks in dredging operations.
The time to take on a full load from a TSHD depends on the area and the type of material being dredged but based on recent experience during dredging campaigns at Dublin Port it takes an average of 300 minutes (2hrs 40mins) to fill a TSHD with a capacity of circa 4,500m3 in the channel. However when dredging occurs within the berths it can take an average of only 60 minutes (1 hour) to fill the hopper. Transit time to the dump site takes 20-30 minutes depending on traffic and sea state and releasing spoil from the dredger can take 10-20 minutes, depending on the material.
5.2 Dumping
DPC’s current practice is to dispose of the dredge material at the existing licensed offshore disposal site located at the entrance to Dublin Bay and to the west of the Burford Bank. The spoil grounds have been in operation since 1996 and is 8.8km from the Poolbeg Lighthouse at the entrance to Dublin Port. It is roughly 1.25km from east to west and about 1.5km from north to south and has an average depth of 20m. The licensed offshore spoil ground for dredge spoil also lies within the Rockabill to Dalkey Island SAC, therefore, it is essential that construction activity does not impact on the site or its conservation objectives. Figure 11: Rockabill to Dalkey Island SAC, Dublin Bay shown by hatched area (NPWS, 2013)
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The likely vessel is a TSHD similar to the one shown below which, has a maximum capacity of 4,500m3 of dredge material. Therefore it is anticipated that dredge spoil will be transported to the dump site at a rate of around 7- 10 loads per day
Dredger used during 2018 Maintenance Dredging Campaign. Photo: Hannah Keogh
5.3 Vessel noise
The TSHD once filled with dredged material will transit to the dump site. It will take around 30 minutes for a round trip back to the loading area. This increase in vessel noise relative to the daily traffic accessing Dublin Port is very low and is unlikely to cause any significant disturbance as ferries, freighter and fishing and other vessels regularly use this area.
5.2 | Literature Review of Impacts and Mitigation
The NPWS ‘Guidance to manage the risk to marine mammals from man-made sound sources in Irish waters – January 2014’ recommends that listed coastal and marine activities, undergo a risk assessment for anthropogenic sound-related impacts on relevant protected marine mammal species to address any area-specific sensitivities, both in timing and spatial extent, and to inform the consenting process. It is required that such an assessment
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must competently identify the risks according to the available evidence and consider (i) direct, (ii) indirect and (iii) cumulative effects of anthropogenic sound (NPWS, 2014).
Dredging Impacts
There have only been a few studies on the effects of marine dredging (Thomsen et al. 2006; Nowacek et al. 2007). Todd et al. (2017) provide a very useful review of the state of current knowledge and potential impacts of dredging on marine mammals.
Dredging produces continuous, broadband, low frequency sound, below 1 kHz, with sound pressure levels between 168dB and 186dB re 1μPa at 1m (Todd et al. 2015). In most cases the noise is continuous in nature. Robinson et al. (2011) measured noise levels of six TSHDs, stating that operating dredgers are similar in noise level to that of merchant vessels, and that the primary source of noise is produced as material passes through the drag head, suction pipe and pump. Previous studies on sound production by TSHDs in silt/mud substrates have found that maximum source levels from different activities (loading, transit, direct dumping, rainbowing or pumping ashore) associated with TSHD dredging did not produce louder sounds than the dredger during transit (De Jong et al. 2010).
Baleen whales
Of the baleen whales in Irish waters, only the minke whale is potentially exposed to dredging activity and even this is a very low risk as minke whales have not been recorded during dredging operations over the past three years. Richardson et al. (1995) reported on a controlled exposure experiment on Bowhead whales which received broadband levels of <113 – 131 dB re 1 µPa (<11 – 30 dB above ambient) from a suction dredger which lead to weak and inconspicuous avoidance, however he considered the low frequency components were under- represented. Off the southeast coast of the US Northern Right whales exposed to intensive dredging by noisy hopper dredges apparently show some tolerance of this noise (cited in Richardson et al. 1995). The best documented case of long-term change by baleen whales is from Baja California where Gray whales breeding in lagoons subjected to industrial activities, including dredging were virtually absent during years with shipping which led to the suggestion that the constant dredging may have been the main source of disturbance (cited in Richardson et al. 1995).
Odontocetes
The effects of dredging on dolphins and porpoise have been poorly studied. Belugas showed less reaction to stationary dredges than moving barges in the Mackenzie estuary, Canada and it was concluded that passage of belugas along a shoreline was temporarily blocked by a dredging operation involving frequent barge traffic but not by a dredging operation with little barge traffic (cited in Richardson et al. 1995).
Recently Pirotta et al. (2013) carried out the most comprehensive study of the potential effects of dredging on bottlenose dolphins using static acoustic monitoring before, during and after maintenance and capital dredging of Aberdeen Harbour off NE Scotland, where 400,000m3 of spoil was removed. The Moray Firth is home to a resident group of bottlenose dolphins and they demonstrated a clear avoidance response to dredging at a foraging area despite it being a highly urbanised site. Dolphins spent less time in the harbour as the intensity of dredging increased. Visual monitoring also showed a lower probability of observing dolphins occurred when dredging boats were present. Group size was not affected suggesting that all individuals in a group were affected equally and were likely to leave the area (Pirotta et al. 2013). The mechanism leading to displacement was not clear. The response may have been due to the discontinuous and rarely occurring stimulus, not regularly experienced by
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dolphins, or due to masking and impacting on communication or foraging. The effect may have been indirect by effecting the dolphins prey within this prey patch.
Diederichs et al. (2010) through the use of acoustic monitoring with click detectors, showed that porpoises temporarily avoided an area where sand extraction took place off the Island of Sylt in Germany. The authors found that when the dredging vessel was closer than 600m to the monitoring location, it took three times longer before a porpoise was again detected compared with times without sand extraction. However, all of these studies only considered dredging and not the dumping of dredged material. Tougaard et al. (2015) recently reviewed proposed noise exposure limits for harbour porpoises. TTS was previously induced at 164 dB at 4kHz with a single pulse or 164-175 if exposed for longer periods and a range of frequencies. Tougaard et al. (2015) suggested TTS could be elicited at SEL of 100-110 dB but this work was really aimed at pulse sounds from pile driving and not continuous sound produced by dredging and shipping. It is clear that of all the odontocetes, harbour porpoise are likely to be most affected by anthropogenic noise due to their high foraging rates as they tend to prey on small fish (Wisniewska et al. 2016).
Seals
Although there are fewer studies on pinnipeds or odontocetes these animals do tolerate considerable noise from such sources (Richardson et al. 1995). Elevated noise from dredging could also affect seals which are sensitive to a lower frequency range (Todd et al. 2015). Todd et al. (2015) reported on observations of dredging operations in Geraldton, Western Australia between 2002 and 2003, reported that New Zealand fur seals and Australian sea lions showed no sign of disturbance reactions, despite the relative closeness of dredging to popular haul-out sights. Similarly, Hawaiian monk seals showed no adverse reactions to bucket dredgers around Tern Island. Anderwald et al. (2013) found that grey seals showed some level of avoidance to high construction vessel traffic in Ireland, although it should be noted that observations were undertaken from a cliff, so animals possibly taking advantage of increased food close to operating dredgers may have been missed by observers.
Odontocetes are often even more tolerant of shipping noise, being repeatedly exposed to many vessels, small and large. Pinnipeds also exhibit much tolerance and often haul out on man-made structures where there is considerable human activity. This exposure may lead to some chronic exposure to man-made noise, with which they tolerate. Ecological or physiological requirements may leave some marine mammals with no choice but to remain in these areas and continue to become chronically exposed to the effects of noise. In areas with repeated exposure, mammals may become habituated with a decline in avoidance responses and thus become less sensitive to noise and disturbance (Richardson et al. 1995). Thus dredging seems to have less effect on marine mammals than moving sound sources although avoidance behaviour of whales exposed to high levels of activity have been documented. Reactions, when measured have only occurred when received sound levels are well above ambient levels.
Despite these references to the potential effects of dredging on marine mammals there is little consideration of the impact of the actual dumping of dredge material as opposed to removal of material from the site to be dredged. This is either an oversight, or more likely reflects the extremely low impact of the dumping of dredged material on marine mammals, compared to the effects of dredging, which are considered low down the spectrum of impacts of coastal activities on marine mammals. OSPAR (2008) suggested that the dumping of dredge materials are largely irrelevant with respect to environmental impact and the issue is confined to disturbance due to underwater noise emission during the dumping process and during the transport (ship noise).
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Acoustic monitoring of dredging and dumping operations in Dublin Bay
As part of the ABR Project in Dublin Port, long term static acoustic monitoring is carried out to record the occurrence of harbour porpoise at the dump site and at a control site (Russell et al. 2018).
Monitoring during September 2017 to March 2018 and in April 2018 was carried out before, during and after dredging and dumping at the spoil ground. Generalized linear mixed-effect models (GLMM) were carried out to assess significant differences between monitoring locations, allowing for a detailed but preliminary assessment of fine scale use of the survey area during the dredging campaign by harbour porpoise. Harbour porpoises were present on 97-100% of days monitored. The highest presence was detected at all locations during the winter months, during the hours of darkness (incl. dawn and dusk) and a range of tidal cycles and phases.
700
600 CD1 CD2 CD3 CD4 CD5
500
400
300
200
100
0 18/09/2017 18/10/2017 18/11/2017 18/12/2017 18/01/2018 18/02/2018 18/03/2018
B1 B2 B3 B4
Figure 12: Number of harbour porpoise detections per day recorded across all locations (B1=SAM 1, B2=SAM 2, B3=SAM 3, B4=SAM 4) with the grey transparency box indicating the dates of the first (22-31 Oct), second (23 Nov- 09 Dec 2017), third (06-17 Jan 2018), fourth (15-23 Feb) and fifth (04-14 Mar) capital dredging (CD) campaign
Results from SAM carried out during the capital dredging campaign during 2018-2019 are reported in Russell et al. (2018) and summarized below (Figure 12). The number of harbour porpoise detections per day on the spoil ground peaked during dumping activity suggesting that, in some instances, harbour porpoise were more active in the spoil ground during dumping and did not appear to perceive the activity as a threat or disturbance (Fig 13 a-
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Marine Mammal Risk Assessment for DPC Maintenance Dredging and Disposal at Sea • • • c). This may be due to a new feeding opportunity associated with dumping, that outweighed any potential negative impacts.
Figure 13a. Number of harbour porpoise detections per day at SAM 1, north of the spoil grounds. Grey boxes indicate the periods in which capital dredging took place, and the blue box for the maintenance dredging campaign (09-22 April) 2018) (from Russell et al. 2018)
Figure 13b. Number of harbour porpoise detections per day at SAM 2, middle of the spoil grounds. Grey boxes indicate the periods in which capital dredging took place, and the blue box for the maintenance dredging campaign (09-22 April 2018) (from Russell et al. 2018)
Figure 13c. Number of harbour porpoise and dolphin detections per day at SAM 4, the control site off Dalkey. Grey boxes indicate the periods in which capital dredging took place, and the blue box for the maintenance dredging campaign (09- 22 April 2018) (from Russell et al. 2018)
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5.3 Risk Assessment
The total amount to be dredged and dumped is estimated at 600,000 m3 over a two year period with 300,000 m3 in each year (2020 and 2021) to be dredged between April and September. The dump site is on the Burford Bank and within the Rockabill to Dalkey Island SAC which lists harbor porpoise as a qualifying interest.
5.3.1 Acoustic disturbance
Noise associated with dredging
The potential for disturbance to marine mammals is greatest when elevated levels of underwater noise are considered. Marine mammals, especially cetaceans, have well developed acoustic capabilities and are sensitive to sound at much higher frequencies than humans (Richardson et al. 1995). They are less sensitive to the lower frequencies but there is still great uncertainty over the effects of sound pressure levels on marine mammals and thus the assessment of its impact. Sources of noise include that generated by the vessel during dredging and transiting to and from the dump site, the noise generated by dredging and that generated during dumping.
Received levels of dredging noise by marine mammals can exceed ambient levels to considerable distances depending on the type of dredger used (Richardson et al. 1995). Hopper dredges produced broadband sound between 20-1000 Hz and the highest levels occurred during loading. Evans (2000) suggested dredging activities produce sounds varying from 172-185 db re 1 ųPa at 1 metre over the broadband range 45 Hz to 7 kHz but there have been no studies examining the reaction of odontocetes to this activity. Audiograms for bottlenose dolphins show peak sensitivity between 50-60 kHz and no sensitivity below 2 kHz and above around 130 Khz (Richardson et al. 1995). Because of rapid attenuation of low frequencies in shallow water dredge noise normally is undetectable underwater at ranges beyond 20-25km (Richardson et al. 1995). The effects of low frequency (4-8 kHz) noise level and duration in causing threshold shifts in bottlenose dolphins were predicted by Mooney et al. (2009). They found that if the Sound Exposure Level was kept constant significant shifts were induced by longer duration exposures but not for shorter exposures.
Figure 14. Noise exposure experiments from maintenance dredging in Dublin Bay (from McKeown 2016)
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NPWS (2014) identify increased sound pressure levels above ambient do occur due to dredging which could be detected up to 10km from shore. These levels are thought to potentially cause masking or behavioural effects but are not thought to cause injury to a marine mammal. There is no guidance on the effects of noise generated by dumping of dredge material on marine mammals.
McKeown (2016) carried out underwater noise measurements during the 2016 maintenance dredging campaign. The PSD plots of the dredging operation show some lower frequency tonal components between 200 Hz and 2 kHz are attributed to the pump. The dredging operation has a higher frequency signal in comparison to the dumping operation.
Sound levels for the dredging operations at ranges of 213 and 268 m were below the disturbance threshold for harbour porpoise of 140 dB re 1 μPa SPLRMS and 140 dB re 1μPa² s SEL. The sound level of 142.7 dB re 1 μPa SPLRMS for the dumping operation at a range of 90 m were 2.7 dB re 1 μPa SPLRMS above the disturbance threshold for harbour porpoise, suggesting porpoise may react <100m of the dredger during dumping. However, this level is still below the NOAA general behavioural threshold for marine mammals of 160 dB re 1 μPa SPLRMS.
Increased noise is restricted to <100m from the dredger during dredging (McKeown 2016), thus there will be no sound pressure associated with dredging within the SAC so sound exposure levels will be at or below ambient noise levels at Burford Bank for dredging activity. The outer reaches of the navigation channel within Dublin Bay extends into the Rockabill to Dalkey Island SAC. Maintenance dredging, if required, within this area is expected to be limited to less than circa one day per annum, which will expose porpoises within the SAC to increased noise and disturbance but only for a day which will not lead to any significant impact.
5.3.2 Noise associated with shipping
Shipping produces low broadband and “tonal” narrowband sounds. The primary sources are propeller cavitation and singing and propulsion of other machinery (Richardson et al. 1995). For large and medium vessels tones dominate up to around 50Hz and broadband components may extend to 100Hz.
Many odontocetes show considerable tolerance to vessel traffic. Sini et al. (2005) showed bottlenose dolphins resident in the Moray Firth generally exhibited a positive reaction to medium (16-30m) and large vessels (>30m) and showed some evidence of habituation. Buckstaff (2004) suggested an exposure level of 110-120 dB from vessel noise solicited no observable effect on bottlenose dolphins. A similar exposure level solicited minor changes in orientation behaviour and locomotion changes in minke whales (Palka and Hammond 2001). Harbour porpoise are frequently observed near vessels but tend to change behaviour and move away and this avoidance may occur up to 1-1.5km from a ship but is stronger with 400m (cited from Richardson et al. 1995). Seals show considerable tolerance to vessel activity but this does not exclude the possibility that it has an effect.
5.3.3 Disturbance during transit
The presence of a dredger in the area will lead to increased vessel traffic and associated noise. Large vessels produce low frequency sounds and TSHD are large (e.g. MV Freeway is 92m in length). However given the busy nature of Dublin Port and shipping lane and increased ambient noise already experienced at this site (Beck et al. 2013) the presence of an additional vessel and associated noise, is extremely unlikely to be significant. The increased noise above ambient levels generated by the dump vessel will be of relatively short duration (maximum of 4-6 weeks per annum).
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5.3.4 Disturbance during dumping
The disposal site has been routinely used for the dumping of dredged material, with approximately eight million tonnes of material dumped at this site between 1997 and 2012 at an average rate of around 550,000 m3 per annum. The current license for capital dredging (Reference number: SOO24-01) permits a maximum of 8,760,000 tonnes (equivalent to 5,300,000 m3) of dredged material may be loaded and dumped at sea up until and including March 2021.
Increased noise is restricted to <100m from the dredger during disposal (McKeown 2016), thus increased sound pressure associated with spoil disposal within the SAC will be above ambient noise levels at Burford Bank within a very small area (radius <100m). The outer reaches of the navigation channel within Dublin Bay extends into the Rockabill to Dalkey Island SAC.
5.3.5 Physical Disturbance
The risk of injury or mortality is considered extremely low as marine mammals are exposed to considerable vessel traffic on a daily basis and would be aware of their presence. The dredge vessel is slow moving and not able to turn quickly thus any animals in the area would have sufficient time to avoid any collisions and thus injury or mortality. The chance of actually releasing dredged material on top of a marine mammal is extremely unlikely. The duration of the release of dredged material last around 10-20 minutes and the vessel slows down during spoil release.
5.3.6 Collision Risk
Collisions are unlikely due to the slow speed of the TSHD. Dredging is unlikely to cause damage to marine mammal auditory systems, but masking and behavioural changes are possible (Todd et al. 2015). Sediment disturbance and any increases in turbidity are unlikely to affect marine mammals that use echolocation, or pinnipeds since research indicates that vision is not essential to pinnipeds’ survival or ability to forage (McConnell et al. 1999). Static acoustic monitoring of harbor porpoises recorded a significant increase in acoustic detections during dumping. Whether this is due to increased occurrence, increased click rate due to increased foraging opportunities or a decrease in visibility due to increased turbidity is unclear (Russell et al. 2018) and requires further exploration but clearly there is no evidence of an aversive reaction. During maintenance dredging of the Port of Cork, grey seals approached the TSHD after commencement of loading operations which did not appear to cause any disturbance to them. They came very close to the TSHD to investigate on a number of occasions, possibly using it as a feeding opportunity (Russell and Levesque, 2014).
5.3.7 Potential disturbance to life-cycle
The dumping of dredged material will not cause any adverse effects on cetaceans or seals in the area providing mitigation measures are in place but may affect prey availability. Small shoaling fish that occur regularly in the diet of seals and porpoises (Rogan 2008) and are likely to be affected during operations. However, with the benthos and demersal fish species subject to periodic smothering over the last 15 years, together with an increase in acoustic detections of harbor porpoise during dumping (Russell et al. 2018), there is no evidence of an aversive reaction leading to impacts on species life cycle. Any displacement resulting from indirect impacts on available prey will be short-term and local, with fish returning to the area at the completion of dumping activity.
Increased turbidity will result from dumping spoil within the dump site which is located within the Rockabill to Dalkey Special Island SAC. Turbidity is monitored and maintained at or below permitted levels. Increased turbidity
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is unlikely to have a direct effect of marine mammals but may have an indirect effect through impacts on prey (Todd et al. 2015). There is limited evidence for an effect of increased turbidity on marine mammals. Harbour porpoise use echolocation to navigate and locate prey and thus would not be affected by increased turbidity. Even when increased turbidity has been shown to substantially reduce visual acuity in seals, which are not known to use sonar for prey detection, there is no evidence of reduced foraging efficiency (Todd et al. 2015).
6 | Mitigation Measures
Potential mitigation measures during the dumping operation are limited. Similar activities both nationally and internationally have been monitored through the provision of a Marine Mammal Observer (MMO) who ensures that there are no marine mammals within a pre-agreed distance prior to dumping during daylight hours. The MMO can also record any reaction to the dumping operation. However, this mitigation measure will only be effective during daylight hours and in favourable weather conditions.
The National Parks and Wildlife Service recommend a distance of 500m radial distance of the dredging sound source in water depths of <200m (NPWS 2014) on commencement. Noise measurements by McKeown (2016) suggests an exclusion zone of 100m is sufficient, beyond which marine mammals are unlikely to detect the activity over ambient noise. If a significant negative change in behaviour is recorded such as rapid movement away from vessel or distress then the MMO should have the authority to cease operations. If marine mammals enter the buffer zone during dredging. Marine mammals should not be within 50m of the dredger when it is dumping.
It is expected that animals would habituate to vessels during dredging and dumping, and would return to foraging in the affected areas when operations area completed. However, given the volumes of material to be dumped, and the long time-scale of these operations, mitigation measures to reduce and avoid the potential impact of dredging and dumping on harbour porpoise (and other marine mammals) are recommended. Long term static acoustic monitoring is currently underway in Dublin Port and will be maintained throughout the life of this current application. These data can be explored to ensure there are no long term impacts on harbour porpoise from maintenance dredging.
6.1 Disturbance
The most effective way of mitigating the potential effects of disturbance is through the provision of an MMO ensuring no marine mammals are present within an agreed buffer zone.
6.2 Collision, injury and mortality
The most effective way of mitigating the potential effects of collision, injury and mortality is through the provision of an MMO ensuring no marine mammals are present within an agreed mitigation zone.
6.3 Disruption of normal behaviour
Dredging activity is of short duration and displacement will be short term. Pre, during and post dredge monitoring would allow for an assessment of any disruption and if it is evident then the level can be quantified. Post-dredge monitoring would also provide a means to establish if disruption occurred and how long it takes for animals to return to an area and resume site usage. Static acoustic monitoring is in place at the dump site and control site to assess any effects on harbor porpoise and monthly seal count sat a haul out site on Bull Island are underway.
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Provision of MMOs during maintenance will provide opportunities to record sightings of marine mammal during dredging, on transit and while disposing at the spoil ground.
While sound exposure levels from such operations are thought to be below that expected to cause injury to a marine mammal, disturbance, from the noise generated by dredging, from the physical presence of the dredger, and possibly from the increased water turbidity in the area of operations have the potential to cause lower level disturbance, masking or behavioural impacts, for example (NPWS, 2014). The presence of an additional vessel and the associated noise produced, is very unlikely to have a significant impact on marine mammals, though it may lead to short term displacement of seals from the dump site.
7 | NPWS Assessment Criteria
1. Do individuals or populations of marine mammal species occur within the proposed area?
Grey seals are the most frequently observed marine mammal species at the dredge and dump site followed by Harbour porpoise and to a lesser extent harbor seal. Bottlenose dolphins may occasionally occur near the dump site and minke whale in the wider area. There is an important haul out site for grey and harbor seals nearby on Bull Island. All marine mammals are part of a larger population and very mobile.
2. Is the plan or project likely to result in death, injury or disturbance of individuals?
The project will not cause injury or death but could lead to local disturbance, from noise associated with the project.
Noise Impact
The activities proposed during this project consist of dredging and dumping operations. It is extremely unlikely any noise generated will be capable of causing permanent or temporary hearing injury to a marine mammal. Localised disturbance to marine mammals in the works area may occur during operations, but current evidence from recent dredging operations suggests no disturbance occurs and indeed dumping may provide increased foraging opportunities.
Physical Impact
The risk of injury or mortality is considered low as marine mammals in the in the immediate vicinity of the site are exposed to human activity on a daily basis and would be accommodated. The dump vessel is slow moving and thus any animals in the area would have sufficient time to avoid any collisions and thus injury or mortality.
3. Is it possible to estimate the number of individuals of each species that are likely to be affected?
No abundance estimates for marine mammals exposed to the proposed activity are available. Abundance estimates are available for grey and harbour seals from a haul out site at Bull Island near to the dredging site. The numbers of grey seals range from 2-19 between May and November and harbour seals from 2-22 and occur throughout the year. Abundance of grey seals at nearby Lambay Island was estimated at 49 pups, with small numbers of pup’s present (<3) at Irelands Eye, Dalkey Island and St. Patricks Island during a national census in 2005 (Ó Cadhla et al. 2007). A further Island. Further surveys 77 pups were recorded on Lambay
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Island and Ireland’s Eye in 2009 (Ó Cadhla et al. 2013). Abundance of harbour seals during at nearby Lambay Island, was estimated at approximately 30 during a national census in 2003 (Cronin et al. 2004) and 2012 (Duck and Morris, 2012).
Harbour porpoise density and abundance estimates are available from the Rockabill to Dalkey Island SAC and range from 1.44 to 1.62 porpoises km-1 during dedicated surveys between 2013 and 2016 (O’Brien and Berrow, 2016). Berrow et al. (2014) calculated a density estimate of 1.19 porpoise km-1 in the summer of 2008. These equate to abundance estimates of between 391 and 424 in Rockabill to Dalkey Island SAC and 138 in Dublin Bay.
4. Will individuals be disturbed at a sensitive location or sensitive time during their life cycle?
The proposed works are recommended to be carried out between April and September. This avoids the grey seal pupping and breeding season but coincides with harbour seal pupping period. As harbour seals are only known to pup at Lambay Island which is >15km from the dredging and dump site it is unlikely to have any effect as harbour seal pups remain close to shore and females forage close to pupping sites during this period. Harbour porpoise occur throughout the year at the site and there is no evidence the period April to September is a sensitive period for this species. Harbour porpoise adults with calves have been recorded at the dump site during summer months.
5. Are the impacts likely to focus on a particular section of the species’ population, e.g., adults vs. juveniles, males vs. females?
There are no data to suggest that any particular gender or age group for seals or cetaceans predominates in the area. As breeding and haul out sites for seals occur in the broader area of the dredging and dumping site it is likely all age groups and both gender occur. Similarly for harbour porpoise both adults and juveniles have been recorded at the site and adults with calves during summer months.
6. Will the plan or project cause displacement from key functional areas, e.g., for breeding, foraging, resting or migration?
As harbour porpoise and seals (especially grey seals) frequently and regularly occur at both the dredging and dump sites and in the broader adjacent areas, there may be temporary disturbance to individuals. However, they are accommodated to human activities and are likely to not be affected, indeed there is some evidence dredging and dumping may provide foraging opportunities for both seals and harbour porpoise.
7. How quickly is the affected population likely to recover once the plan or project has ceased?
While there may be temporary disturbance of marine mammals in the area, they are accommodated to human activities and are likely to recover from any temporary disturbance within hours or days.
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8| Mitigation
Timing of Dredging and Disposal at Sea
Both grey seals and harbour porpoise can potentially be affected by proposed operations and are listed on Annex II of the EU Habitats Directive. Harbour porpoise are considered as being particularly sensitive species to anthropogenic noise.
Dredging during summer months will expose grey and harbour seal and harbour porpoise which at worse may lead to temporary disturbance. The breeding and pupping season for harbour seals occurs from June to July but given that the nearest known pupping site is >15km from the dredging operation it is unlikely to have any effect. Harbour porpoise occur throughout the year with calves present from July to December but as they are highly mobile and are likely to forage over a wide area any impacts from disturbance are not confined to the proposed dredging period.
To minimise any disturbance effects on seals and harbour porpoise we recommend adoption of the NPWS Guidelines for minimising impacts of man –made sounds in Irish waters.
Guidance to Manage the Risk to Marine Mammals from Man-made Sound Sources in Irish Waters
The mitigation measures recommended by the NPWS are for the presence of a trained and experienced Marine Observer (MMO) and the use of “ramp up” procedures for noise and vibration emitting operations. The proposed mitigation measures (Guidance to Manage the Risk to Marine Mammals from Man-made Sound Sources in Irish Waters) recommended by the Department of Arts, Heritage and the Gaeltacht in 2014 are designed to mitigate any possible effects.
The following mitigation measures are proposed to minimise the potential impacts on marine mammals and to allow animals move away from the area of dredging operations:
1. A dedicated, qualified and experienced Marine Mammal Observer will conduct a 30 minute watch for marine mammals within 500m prior to start up. If a seal or cetacean (or otter) is sighted within 500m of the TSHD, start-up must be delayed until the animal(s) is observed to move outside the mitigation zone or the 30 minutes has passed without the animal being sighted within the mitigation zone. 2. Dredging activities shall only commence in daylight hours where effective visual monitoring, as performed and determined by the MMO, has been achieved. Where effective visual monitoring, as determined by the MMO, is not possible the sound-producing activities shall be postponed until effective visual monitoring is possible. 3. Once normal dredging operations commence, there is no requirement to halt or discontinue the activity at night-time, nor if weather or visibility conditions deteriorate nor if marine mammals occur within a 500m radial distance of the sound source, i.e., within the MZ.
8.1 | Residual Impacts
With implementation of the above mitigation measures, it is very unlikely that there will be negative residual impacts from the proposed dredging works on marine mammals in the area. It is also very unlikely that any animals will be injured or killed as a result of the proposed works. Seals using the area are likely to be tolerant of vessel noise and any animals which might be displaced from the vicinity of the excavator or barge can be expected to
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quickly re-establish use of the area following cessation of the works. Ongoing static acoustic monitoring of harbour porpoises will ensure no effects on the conservation objectives of the Rockabill to Dalkey Island SAC.
9 | SUMMARY
Dublin Bay and approaches to the port are important for marine mammals including the regular occurrence of grey and harbour seals and harbour porpoise. Recent dredging associated with the ABR project and ongoing maintenance dredging has provided an excellent understanding of the marine mammal community exposed to dredging and dumping and the likely effects. We recommend the NPWS Guidelines to minimise the acoustic impacts of dredging be implemented to enable the dredging campaign to be carried out which will result in no significant impacts to marine mammals or on the conservation objectives of the Rockabill to Dalkey Island SAC.
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