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Hearing Document 1 – Burbo Bank Extension Offshore Wind Farm Order page 1
BURBO BANK EXTENSION OFFSHORE WIND FARM
DONG Energy Burbo Extension (UK) Ltd.
Appendix 6 – AGREED Statement of Common Ground with Natural Resources Wales and Natural England on red-throated diver
for the proposed Burbo Bank Extension Offshore Wind Farm Order
Planning Inspectorate Reference: EN010026
Version: Final
13 March 2014 (submitted for Deadline VI (13 March 2014)
AGREED Supplementary Statement of Common Ground with Natural England and Natural Resources Wales regarding red throated divers
Appendix 6
2
Burbo Bank Extension Offshore Wind Farm
DONG Energy Burbo Extension (UK) Ltd.
Prepared by: NIRAS Consulting and Allen Risby (Environmental Manager, DONG Energy)
Checked by: Allen Risby (Environmental Manager, DONG Energy) and Stuart Livesey (Consents Project Manager, DONG Energy)
Accepted by: Stuart Livesey (Consents Project Manager, DONG Energy)
Approved by: Ferdinando Giammichele (Project Development Manager, DONG Energy)
Revision history
Version Date Author History
Version 1 22nd
January 2014 NIRAS Consulting Draft produced for comment
Version 2 3rd February 2014
Allen Risby (Environmental Manager, DONG Energy)
Updated with comments from Natural England
Version 3 4th February 2014
Allen Risby (Environmental Manager, DONG Energy)
Updated with comments by DONG Energy
Version 4 28th February 2014
Allen Risby (Environmental Manager, DONG Energy)
Updated for comment by Natural England and Natural Resources Wales
Version 4a 6th March 2014
Allen Risby (Environmental Manager, DONG Energy)
updated by DONG Energy to include reference to studies by Kaiser et al (2005) and Topping & Petersen (2011)
Version 5 10th March 2014
Allen Risby (Environmental Manager, DONG Energy)
Updated with comments from Natural England and Natural Resources Wales
V5 Review 11th March 2014
Stuart Livesey (Consents Project Manager, DONG Energy)
Review and acceptance
Final 12th March 2014
Ferdinando Giammichele (Project Development Manager, DONG Energy)
Final review and sign off
Burbo Bank Extension Offshore Wind Farm
c/o DONG Energy Burbo Extension (UK) Ltd.
33 Grosvenor Place
London
United Kingdom
SW1X 7HY
Telephone +44 020 7811 5235
Website: www.burbobankextension.co.uk
Email: burbobankextension@dongenergy.co.uk
Name & Registered Office:
DONG Energy Burbo Extension (UK) Ltd.
Watson, Farley & Williams LLP
15 Appold Street
London
United Kingdom
EC2A 2HB
Company No. 07307131
Produced by DONG Energy Wind Power (UK) Ltd., March 2014
© Copyright DONG Energy Wind Power (UK) Ltd., 2014. No part of this publication may be reproduced by any means without prior permission from DONG Energy Wind
Power (UK) Ltd. Text is correct at the time of writing. Every effort has been made to ensure the accuracy of the material published. However, DONG Energy Wind Power (UK)
Ltd. is not liable for any inaccuracy.
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Burbo Bank Extension offshore wind farm Supplementary SOCG between DONG Energy and Natural England and Natural Resources Wales with regard to impacts on wintering red-throated diver (Liverpool Bay / Bae Lerpwl SPA)
Version 1, 22nd January 2014 DRAFT
Version 2 with comments from Natural England
Version 3 with updated comments by DONG Energy, 4th February 2014
Version 4, 28th February 2014, updated for comment by Natural England and Natural Resources
Wales
Version 4a, 6th March 2014, updated by DONG Energy to include reference to studies by Kaiser
et al (2005) and Topping & Petersen (2011)
Version 5, 10th March 2014, with comments from Natural England and Natural Resources
Wales
Page 2
Contents 1. Introduction .................................................................................................................................... 3
2. Red-throated diver .......................................................................................................................... 3
Conservation objectives ...................................................................................................................... 3
Favourable condition .......................................................................................................................... 4
Data analysis ....................................................................................................................................... 5
Interaction ........................................................................................................................................... 6
Magnitude of displacement (N) .......................................................................................................... 6
Proportion of birds lost to the SPA (P) ................................................................................................ 8
Conclusions ....................................................................................................................................... 16
References ............................................................................................................................................ 18
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1. Introduction
DONG Energy Burbo Extension (UK) Ltd. (“the Applicant”) has made an application to the Planning
Inspectorate (PINS) for a Development Consent Order (DCO) for the proposed Burbo Bank Extension
offshore wind farm (the Project) located partly within Liverpool Bay / Bae Lerpwl SPA1.
The Applicant has provided an assessment of, amongst other things, the Project's likely effect on the
population of red-throated diver (Gavia stellata), a qualifying feature of the Liverpool Bay / Bae
Lerpwl SPA, as a consequence of permanent displacement from the wind farm site during
construction and operation of the wind farm2. That assessment is documented in the Environmental
Statement3 and Habitats Regulations Assessment Report4 which accompanied the application
documents submitted to the Planning Inspectorate on March 22nd 2013.
Further analysis has been undertaken by the Applicant in response to representations from Natural
England and discussions held with Natural England and Natural Resources Wales. This analysis is
documented in NIRAS (2013), DONG (2013a), and DONG (2014a). This latter Paper ("Red-throated
diver displacement: Clarification of density dependent effects") is used as the basis for this
document to record areas of agreement between DONG Energy and Natural England and Natural
Resources Wales.
A subsequent Paper (DONG, 2014b), submitted by the Applicant for Deadline V, updates this analysis
to calculate displacement effects out to 3 km from the wind farm site. A further update to that Paper
(DONG, 2014c and DONG, 2014d) identifies further evidence to support the Applicant's case that
density dependent effects will not result in an adverse effect on the red-throated diver population of
Liverpool Bay SPA.
2. Red-throated diver
Conservation objectives Red-throated divers are a qualifying feature of the Liverpool Bay / Bae Lerpwl SPA (“Liverpool Bay
SPA”). This SPA was classified in 2010. The SPA conservation objectives for this species are:
• Subject to natural change, maintain or enhance the red-throated diver population
and its supporting habitats in favourable condition.
• The interest feature red-throated diver will be considered to be in favourable
condition only when both of the following two conditions are met:
1 Special Protection Area. 86% of the wind farm area lies within the Liverpool Bay / Bae Lerpwl SPA.
2 It is acknowledge by all parties that red-throated diver as a species are particularly sensitive to displacement
(Garthe and Hüppop (2004), Furness et al.(2013)) 3 Planning Inspectorate document reference 5.1
4 Planning Inspectorate document reference 4.3
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o The size of the red-throated diver population is at, or shows only non-
significant fluctuation around the mean population at the time of
designation of the SPA, to account for natural change;
o The extent of supporting habitat within the site is maintained.
The Applicant’s position Natural England / Natural Resources Wales
position
The key potential impact on the red-throated
diver interest feature of the Liverpool Bay SPA
is permanent displacement from the proposed
wind farm area. This could lead to an adverse
effect on the SPA if it led to a significant
reduction in the cited red-throated diver
population of the Liverpool bay SPA due to
emigration or mortality as a result of
permanent displacement from the areas
occupied by offshore wind farms.
The Applicant notes that the Liverpool Bay SPA
covers an area of approximately 1703 square
kilometres. The offshore wind farms
considered in the assessment cover
approximately 7.81% of this area (excluding
any displacement buffer area).
We agree that displacement is the key impact.
Studies of operational offshore wind farms
(OWFs) indicate a consistent avoidance of
constructed OWFs, with little or no evidence of
habituation over time. Such displacement
could cause birds to leave the SPA, or increase
competition for food as birds are obliged to
occupy a smaller area. Mortality from such
‘density-dependent’ competition is a resulting
possibility, although it is not possible to
quantify this directly for red-throated divers.
Including the 3 km buffers of the relevant
OWFs increases the area from which birds are
displaced to approximately 20% of the SPA
habitat covered (though not all divers within
these areas are expected to be displaced). We
also note not all available remaining habitat
within the SPA is likely to be suitable for this
species.
Favourable condition
Although not regarded as threatened within the EU, the conservation status of this species is
regarded as unfavourable because of declines in the European breeding population between 1970-
1990. The population is now considered stable though depleted (Natural England & CCW 2010).
The five year peak mean population at the time of designation was 922 individuals during winter
(2001/02 – 2006/07; insufficient data was recorded for period 2003/2004, JNCC 2010) although
more recent estimates indicate a population of 1,188 (Bradbury et al., 2011).
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The Applicant’s position Natural England / Natural Resources Wales
position
The conservation status of red-throated diver
in Europe is considered to be unfavourable
due to historic declines in the breeding
population.
The species is amber listed in the UK, according
to Birds of Conservation Concern (Eaton et al.
2009). This reflects its ‘depleted’ status as
defined by Species of European Concern
(SPEC)(BirdLife International 2004), within
which it is a SPEC 3 species (unfavourable, not
concentrated in Europe).
The red-throated diver population of Liverpool
Bay SPA is currently in Favourable Condition,
the most recent population estimate indicating
the population exceeds that at the time of
designation.
The SPA was classified in 2010 and is assumed
to be in Favourable Condition. Formally
condition monitoring for the site has not yet
occurred. The most recent survey in 2011 was
designed to test aerial survey methods and
was not part of the monitoring program, which
would normally be expected to capture
variation across time (within and between
winters). However, the value of 1,188 divers
generated, albeit using a new method, was
within the range of variability from past
surveys (e.g. 1,599 in 2001/02; 1,210 in
2002/03).
Data analysis Subsequent to the application it was agreed with Natural England that further analysis of the
potential displacement effects of the proposed wind farm would be undertaken. Natural England
advised (in a meeting on 3/7/13) that the most appropriate data to inform this additional analysis
were an additional data set available from JNCC. These comprised aerial survey data carried out
during winter (October – March) between the winter of 2000/01 and 2009/10, using a line transect
method.
The Applicant’s position Natural England / Natural Resources Wales
position
The Applicant has used the most appropriate
data available for the analyses of displacement
impacts on the red-throated diver population
of Liverpool Bay SPA.
We agree that the data used are as advised
and represent the best available evidence to
our knowledge.
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Interaction Displacement could lead to an impact on Liverpool Bay SPA if it were to cause a loss of birds from
the qualifying population. The birds lost to the population (either through mortality or emigration
from the SPA) comprises that proportion of birds displaced from the wind farm that cannot
redistribute to other parts of the SPA. If the ‘interaction’ of the wind farm with the SPA population is
taken to be the proportion of the SPA population that is lost then this can be expressed as:
Interaction = N * P
Where:
N = Number of birds displaced from the wind farm (expressed as a proportion of the SPA
population as defined in the conservation objectives)
P = proportion birds unable to redistribute within the SPA (this is equivalent to 1 –
proportion successfully redistributing)
The Applicant’s position Natural England / Natural Resources Wales
position
Calculation of the interaction term (N * P) is
the most appropriate method available for
approximating the likely impact of
displacement on the red-throated diver
population of Liverpool Bay SPA.
We agree this is a useful way to conceptualise
the effect, noting that N is likely to be
evidence-based whereas P may not be.
Magnitude of displacement (N) The value N is estimated using assumptions about the density of divers present and the proportion
of birds within the wind farm and adjacent buffer areas that will be displaced.
Table 1 summarises the values of N for Burbo Bank and other wind farms within Liverpool Bay SPA
(assuming a displacement effect out to 2 km).
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Table 1: Cumulative displacement analysis for red-throated diver at four offshore wind farm sites
Offshore Wind farm Year operational Divers displaced % of SPA population
Burbo Bank
2007 11 1.19
Burbo Bank Extension
- 30 3.255
Gwynt y Môr
Expected 2014
(construction started
2012)
35 3.80
Rhyl Flats
2009 24 2.60
Total
100 10.85
The Applicant’s position Natural England / Natural Resources Wales
position
Table 1 includes those other projects which
could contribute to an in-combination effect
on the red-throated diver population of
Liverpool Bay SPA.
The Applicant understands that the JNCC data
used for the in-combination displacement
analysis were collected between 2000/01 and
2009/10 (See note from JNCC, "Supporting
information for interpretation of density maps"
December 2011). Consequently it is reasonable
to assume that any effect from the Burbo Bank
owf is included in the baseline data.
The Applicant now understands following
advice from Natural England (13/02/2014),
that the JNCC Note referred to here is in error
and that the analysis used data up to February
2007 only. Consequently the Applicant does
not now seek to exclude the Burbo Bank
We agreed at the Issue Specific Hearing that it
was appropriate to exclude North Hoyle OWF,
as the timing of operation coincided with the
beginning of SPA data collection, but not with
the exclusion of Burbo Bank OWF, as operation
did not occur until after the end of the data
collection period. Any putative construction
effects are restricted to the latter end of the
data collection period Assuming construction
began in 2005), and thus the bird distribution
data will be weighted towards pre-construction
years. Therefore it is equivocal that such
effects will have had time to manifest
themselves in the baseline data.
We can confirm that JNCC have indicated their
data for Liverpool Bay were collected up to and
including February 2007 and agree that Burbo
Bank OWF should not be excluded from in
5 Note this figure is lower than the 3.8% potentially displaced by the Project alone as it includes birds displaced
from the zone of overlap with Burbo Bank and its respective buffers.
Page 8
offshore wind farm from the in-combination
analysis. (The Burbo Bank wind farm was
constructed in 2006 and operational in 2007).
combination tests.
The magnitude of displacement for each wind
farm has been calculated on the basis of the
Kentish Flats Extension (KFE) approach (Model
3) which assumes displacement up to 2km
from the wind farm.
The Applicant has now undertaken
displacement analysis assuming effects out to
3 km. These results are provided in DONG
(2014b).
Natural England / Natural Resources Wales
position
The Applicant has followed the KFE approach,
although Natural England has consistently
advised that it is appropriate to consider
displacement out to 3 km, according to
monitoring data from Kentish Flats OWF.
The likely magnitude (N) of the in combination
effect (including Burbo Bank owf) is estimated
as 10.85% of the SPA population assuming
displacement effects out to 2 km and 11.88%
assuming displacement effects out to 3 km.
Natural England / Natural Resources Wales
position
We agree that these are the magnitude of
effects as calculated in the most recent
clarification note “Red-throated diver
displacement: clarification of density
dependent effects v4a”.
Proportion of birds lost to the SPA (P) It is agreed with Natural England that the proportion of birds lost to the population (P) is < 1 as a
value of 1 would indicate that all birds displaced are lost to the population. It was argued in the
Application for the Burbo Bank Extension that the value of P would be low, because the density of
divers in Liverpool Bay is correspondingly low, when compared to other areas known to support
significant numbers of this species. Consequently, there should be opportunity for many displaced
birds to redistribute.
In addition suitable values for P have also been inferred from density-dependent mortality observed
in oystercatchers (Durell et al., 2000, being the most relevant studies that can be found where this
effect has been quantified empirically, NIRAS 2013). Those studies indicate a density-dependent
mortality rate of between 2.5% - 5%, with 2.5% being the average value indicated by the data and
5% being the most extreme interpretation of the data, including extreme value data points. A
density-dependent mortality rate of 2.5% indicates that for every increase in the density of divers (in
this case assumed to arise when displaced birds redistribute to areas that are already occupied by
some divers) of 1% there will be an increase (of the existing background adult mortality rate) of
2.5%.
These density-dependent mortality rates can be also be expressed (in this case assuming
displacement effects out to 3 km) as a value of P:
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Density-dependent mortality of 2% is approximately equivalent to P of 0.31 (or 31%)
Density-dependent mortality of 2.5% is approximately equivalent to P of 0.39 (or 39%)
Density-dependent mortality of 3% is approximately equivalent to a P of 0.46 (or 46%)
Density-dependent mortality of 5% is approximately equivalent to P of 0.77 (or 77%)
A suitable value for P is not known, but is agreed to be less than 1. It is argued that the value for P
should be relatively low because it is considered that there is considerable scope for the
redistribution of divers within other parts of the SPA and other studies (eg. Topping & Petersen
(2011), Kaiser et al. 2005) suggest a low mortality effect for red-throated divers or equivalent
species.
The evidence to support this includes:
The conservation status of this species in Europe is regarded as unfavourable because of
declines in the European breeding population between 1970-1990. Whilst the population is
now considered to be stable it is also considered to be depleted (Natural England & CCW
2010). It is likely, therefore, that the wintering population of red-throated diver has been
higher in the past and that areas, such as Liverpool Bay, would have supported higher
numbers and densities than those currently observed.
The average density of divers within the SPA, in the absence of offshore wind farms, is
calculated to be 0.549 individuals / km2. This is relatively low compared to other areas that
support this species e.g. Thames Estuary, 4 individuals / km2 (Webb et al. 2009).
Once displacement from the proposed Burbo Bank Extension and other relevant offshore
wind farms is taken into account, this density increases to 0.616 individuals / km2 (2 km
displacement) and 0.621 individuals / km2 (3 km displacement) This is a relatively small
change (~12%). Shown graphically (Figure 1) it can be seen that the resulting densities are
well within the range of those observed in Liverpool Bay.
Kaiser et al. (2005) modelled the likely effect of displacement arising from expected wind
farm development on common scoter in Liverpool Bay. Common scoter are considered to
share the same sensitivity to disturbance and habitat specialism (although not the same prey
species) as red-throated diver (Furness et al., 2013) and therefore may be considered a
suitable proxy for an analysis of the consequences of displacement in Liverpool Bay SPA. The
aim of the model was to predict the consequences of displacement expressed as changes in
the overall mortality rate of common scoter in Liverpool Bay. The model incorporated
information about the distribution and variability in prey availability and correlated these
with observed patterns of common scoter distribution. Information on the energetics
associated with foraging by common scoter was also included to explore the effects of
displacing them from the wind farm area. The distribution of the prey of common scoter
(bivalves) are patchily distributed leading to distinct aggregations of common scoter within
Liverpool Bay. The model identifies that the displacement of common scoter from wind
farms at Rhyl Flats, Burbo Bank, North Hoyle and Gwynt y Mor is not predicted to have any
significant adverse effects on common scoter mortality. Only when the proposed (at that
time, but not subsequently delivered) wind farm at Shell Flats is included is a significant
effect on mortality predicted (increasing from a median value across Liverpool Bay of 7.3%
to a median value of 11.7%, an increase of 60% in the overall winter mortality rate)
reflecting the value of the Shell Flats area to over wintering common scoter. It is assumed
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that the consequences of displacement for common scoter are greater than they are for red-
throated diver which are much more widely dispersed within Liverpool Bay and which do not
show the same degree of aggregations, presumably because their prey is similarly dispersed
and disaggregated.
Topping & Petersen (2011) developed an Agent-based model to assess the population-level
consequences of displacement of red-throated divers by offshore wind farms in the Baltic
Sea and Danish waters. They modelled various scenarios, but the scenario which assumed
most wind farm development (all existing and proposed offshore wind farms in the Baltic
Sea and Danish waters) predicted a 1.7% reduction in red-throated diver numbers in the
flyway. Although the authors make many assumptions which consequently does not allow a
direct comparison with red-throated diver in Liverpool Bay SPA, the results point to a very
small population impact arising from a relatively large extent of development (15,000km2 of
wind farms).
Figure 1. Distribution of estimated density of red-throated diver in Liverpool Bay SPA6
6 Figure 1 presents the frequency distribution of estimated density of red throated diver for all 1 km x 1 km
grid squares within Liverpool Bay SPA. This dataset had been derived from JNCC data on the number and distribution of red-throated divers in Liverpool Bay SPA as determined using visual aerial survey methods which have been corrected for distance related detection errors. Bird observations were smoothed by JNCC using kernel density estimation (KDE) and were combined to create a mean modelled density surface of red-throated diver in Liverpool Bay SPA (NIRAS 2013).
Arrows and red columns identify the mean estimated density of birds in the SPA, outside of wind farms and buffers: (1) before displacement, (2) after displacement (2km buffer) (3) after displacement (3km buffer)
Page 11
The Applicant’s position Natural England / Natural Resources Wales
position
It is agreed that P<1 We agree that it is very unlikely that all of the
birds displaced will die, and thus agree that P
<1.
There are few empirical studies to inform likely
density-dependent mortality relationships. The
studies on oystercatchers are the most
relevant information that is available.
However, the Applicant notes that the
assumption of a 5% density dependent
mortality effect for red-throated diver in
Liverpool Bay SPA is highly pre-cautionary due
to the overall low density of divers in this SPA
and evidence for recent diver populations in
the SPA in excess of the citation value. The
Applicant argues that the calculated increase in
overall density is not sufficient to increase
diver mortality (or loss) to an extent which
would adversely affect the integrity of the SPA.
The Applicant additionally makes reference to
the study modelling predicted common scoter
mortality in Liverpool Bay (Kaiser et al. (2005))
and the modelling of red-throated divers off
the Danish coast (Topping and Petersen,
(2011)) to support its contention that a 5%
density-dependent effect is highly
precautionary.
Natural England / Natural Resources Wales
position
There are few studies to inform likely density-
dependent mortality relationships. The
oystercatcher is one of the few species in
which the density-dependence of
overwintering mortality has been quantified.
Oystercatchers’ foraging ecology makes them
particularly prone to strong density-dependent
competition for access to their food resources.
While clearly oystercatchers and red-throated
divers cannot be considered similar in terms of
their ecology, application of the oystercatcher
density-dependent mortality function to help
inform assessment of the significance of
predicted impacts of red-throated diver
displacement (and resultant increase of density
and hence mortality) provides a reasonably
precautionary basis on which to make that
assessment given the apparent severity of the
density-dependence inferred from this other
species.
Using the increase in density resulting from
displacement is probably a more useful
measure than referring to absolute densities,
as we do not know whether the current
Liverpool Bay SPA red-throated diver density is
limited (or limiting further increases).
Whilst the Kaiser et al. (2005) modelling
predicted a median increase of 60% in over
wintering mortality of common scoter, these
data can also be interpreted as a worst case of
121% additional mortality when expressed as
an increase from baseline mortality.
Page 12
We consider that there are several reasons to
have a low confidence in the application of the
modelling by Topping & Petersen (2011) to the
Liverpool Bay SPA situation, largely based on
the assumptions made and which the Applicant
partially acknowledges. For instance: the study
assumes a 500 m displacement buffer (not a
variable 3 km buffer as we would advise); the
density-dependent relationship is not
obviously defined (i.e. we can’t tell what P is);
the spatial scales are not comparable (i.e. we
don’t know if the % of habitat lost is
comparable with Liverpool Bay); the starting
population is ‘arbitrary’, not measured; and
changes are not expressed in terms of
increases to baseline mortality and thus we
cannot estimate P. There are several other
assumptions and caveats which the authors
themselves highlight.
The data on oystercatchers indicates that
density-dependent mortality is likely to be no
greater than 5% and probably lower (between
2-3%)
The Applicant agrees that in this case the
relationship between displacement and
mortality (loss) will be weaker and understands
that a "P" value between 0.30 and 0.45 is more
appropriate for this analysis and was the more
usual value reported in the Durell study (Durell
et al., 2000).
Natural England / Natural Resources Wales
position
The data on mussel feeding adult
oystercatchers indicates that the density-
dependence of over-winter mortality is such
that a doubling of density results in up to a
five-fold increase in baseline mortality but that
the relationship may be less strong than this
(i.e. between a 2 – 3 fold increase).
We do not agree that the lower P values are
‘more appropriate’ but recognise the situation
has some uncertainty.
Density-dependent mortality of 5% is
approximately equivalent to P = 0.75 for a
displacement effect out to 2 km, and P = 0.77
Natural England / Natural Resources Wales
position
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for displacement out to 3 km. Application of this strong density-dependent
mortality function to the Liverpool Bay SPA
red-throated diver data, and in particular to
the number of birds predicted to be displaced
and the density of birds in the remaining areas
to which displaced birds might relocate,
indicated a 12% increase in density might occur
in those areas and the predicted additional
mortality of a number of birds equivalent to
75% of the number predicted to be displaced
(P=0.75)out to 2 km, and 77% (P=0.77) out to 3
km.
There is evidence to suggest that it is unlikely
that P > 0.75 but likely that P < 0.75
The Applicant interprets the evidence from the
study on the Exe estuary to support the
contention that it is unlikely that there is a
strong density-dependent function relationship
in the Liverpool Bay SPA red-throated diver
population; and, on the contrary, the low
densities and variable population numbers
within the SPA suggest there is likely to be a
weak relationship between displacement and
loss to the SPA at this site. Data from
oystercatchers indicates that a P in the range
0.30 – 0.45 is more likely.
Natural England / Natural Resources Wales
position
Given that the value of P=0.75 is derived on
the basis of what is likely to be a strong
density-dependent function to the Liverpool
Bay SPA data , it is relatively unlikely that P
would be any greater than 0.75 in the case of
red-throated diver. This appears to be a
suitably evidence-based but precautionary
assumption in this instance. Note though that
application of the same density-dependent
mortality function to other instances of diver
displacement may result in P>0.75 if the
number of displaced birds is greater than that
considered here and in particular if the %
increase in density that results in the remaining
areas due to displacement of birds into them is
also greater than that calculated to occur in
this instance.
Figure 2 indicates, graphically, how the magnitude of displacement (N) relates to interaction
depending on the assumed value of P. The consented mortality (interaction) rates consented in
recent applications in the Thames Estuary are indicated (Kentish Flats Extension in 2013 at 9.3% and
Gunfleet Sands in 2008 at 6.5%). The purple rectangle indicates the value that would need to be
assumed for P if the interaction value for Burbo Bank Extension was to be within the range of
previous consented rates.
It can be seen that value of P would have to be very high (~0.75), which equates to the most
pessimistic interpretations of the density-dependent mortality data. The evidence for oystercatchers
Page 14
indicates that a density-dependent mortality rate between 2% to 3% is more likely, equivalent to
values of P that lie between 0.3 – 0.45, implying an interaction rate that is considerably lower than
that already consented elsewhere.
Figure 2: Interaction rates
The Applicant’s position Natural England / Natural Resources Wales
position
It is agreed that for a value P ≈ 0.75 the
interaction figure for Burbo Bank Extension
would be approximately 8.14% (10.85%
multiplied by 0.75).
The analysis of displacement out to 3 km, using
the suggested 20% displacement rate between
2km and 3km, identifies a total of 110 birds
displaced (including from Burbo Bank owf).
This is equivalent to 11.88% of the SPA citation
population (922 birds). The "P" value, based on
a worse case density dependent mortality, is
0.77 giving an interaction value of 9.15%.
Mathematically, we agree, noting the value of
N is based on a 2 km buffer which we advise
may underestimate the effect of displacement.
Page 15
It is agreed that these values would be in a
similar range to those already consented in the
Thames Estuary (between 6.5% at Gunfleet
Sands to 9.3% at Kentish Flats Extension).
Natural England / Natural Resources Wales
position
Mathematically, we agree, noting that Natural
England did not agree to all of the elements of
the assessment for KFE (notably the exclusion
of certain wind farms from assessment and the
appropriate displacement buffers), nor did we
advise that adverse effect on site integrity
could be excluded beyond reasonable scientific
doubt.
Slightly lower (but still credible) values of P
would result in interaction figures that are less
than those already consented in the Thames
Estuary.
Using the 2 km displacement scenario and
values of P = 0.3 and P = 0.45 the interaction
values are:
10.85% x 0.3 = 3.26%
10.85% x 0.45 = 4.88%
Using the 3 km displacement scenario and
values of P = 0.31 and P = 0.46 the interaction
values are:
11.88% x 0.31 = 3.68%
11.88% x 0.46 = 5.47%
Natural England / Natural Resources Wales
position
Mathematically, we agree.
It is likely, in any case, that the value of P <
0.75 because:
the density of divers in Liverpool Bay is
low, when compared to other areas known
to support significant numbers of this
species
the change in density of divers in receptor
areas arising from the displacement of
divers from the wind farm area is relatively
low
A value of P = 0.75 based on the
oystercatcher study (Durell et al., 2000)
Natural England / Natural Resources Wales
position
We advise that there is insufficient
understanding of the system to conclude that P
< 0.75 beyond reasonable scientific doubt.
Although diver density is lower than in the
Outer Thames Estuary, we do not know
whether the system in Liverpool Bay is at
carrying capacity. Similarly, the latest
assessment of density changes within Liverpool
Bay SPA following displacement (i.e. from an
average of 0.549 to 0.616 divers per km2)
refers to a scenario where all remaining
Page 16
uses the extreme value of mortality from
deaths observed in the 1990/91 winter.
habitat, regardless of quality, is available to
red-throated divers. We do not know whether
this is the reality. Finally, P = 0.75 is based on
empirical data from oystercatchers and
represents a scientifically derived value.
We do not know whether red-throated divers
respond to the same environmental variables
as oystercatchers (for example, sea state may
be more important than air temperature), nor
whether future winters will be more like that
of 1990/91 than any other winter.
Conclusions
The Applicant’s position Natural England / Natural Resources Wales
position
The Applicant has made use of the best
available evidence to assess the likely effects of
the proposed Burbo Bank Extension on
Liverpool Bay SPA.
It is the Applicant’s position that the density-
dependent mortality relationship for red-
throated diver at this SPA is unlikely to show
the worst case strong relationship suggested in
the study of mussel-feeding oystercatcher on
the Exe estuary (Durell et al., 2000). That study
showed that an approximate doubling of the
density of oystercatchers feeding on mussel
beds in the Exe estuary resulted in an
approximate tripling of the winter mortality
rate for those birds, a density dependent
mortality effect not shown for oystercatcher
feeding in other (more extensive) habitats on
the estuary. The 1:5 density dependent
mortality relationship (ie. 1% increase in
density leading to a 5% increase in mortality
rate) only occurs if the extreme mortality
values from the severe weather winter of
1990/91 are used. The Applicant notes that the
physically constrained mussel-bed food
resource and the high bird densities (up to 30
We agree the JNCC data used for in
combination are the best available data. We
acknowledge the use of oystercatcher density-
dependent data although we cannot test its
applicability to red-throated divers, especially
as we do not understand the extent to which
fish prey, and access to fish prey, are spatially
or temporally constrained ; however,
oystercatchers are expected show a
comparatively strong density-dependent
relationship. We continue to disagree over
interpretations of evidence on the appropriate
displacement buffers, though the Applicant has
now presented the outcomes from both
potential scenarios.
Page 17
birds per hectare) in that study are not
replicated for fish-eating red-throated diver in
Liverpool Bay SPA.
There is no indication that the proposed
development alone will adversely affect the
favourable condition of the SPA through a
reduction in the size of the red-throated diver
population.
The predicted level of displacement from the
Project alone (3.25% of the SPA population
assuming effects out to 2km and 3.56%
assuming effects out to 3 km) is substantially
less than that consented at other wind farms,
and this is the case regardless of the assumed
value of P.
Natural England / Natural Resources Wales
position
We have agreed the project alone is unlikely to
result in an adverse effect on site integrity.
There is no indication that the proposed
development in combination will adversely
affect the favourable condition of the SPA
through a reduction in the size of the red-
throated diver population.
Considering the extent of the habitat available
at this SPA and the relatively low densities of
red-throated diver observed at the site the
Applicant’s position is that P = 0.75 is too
precautionary and that the evidence points to
a value that is significantly lower than this. The
implication otherwise is that 3 out of 4 birds
displaced (that is, potentially unable to make
use of the wind farm area and displacement
buffer) subsequently die (or are lost to the
SPA) as a result and this is not a credible
scenario given the extent of available
alternative habitat and the relatively low
densities within those area. Notwithstanding
this point, however, even if a value of P=0.75 is
adopted (at either 2km or 3km displacement
scenarios) the resulting impact is, in every
case, within the scale of effect for this species
that has already been consented elsewhere.
Natural England / Natural Resources Wales
position
We advise that it is not possible to rule out an
adverse effect on site integrity beyond
reasonable scientific doubt. This is because of
different interpretation of the evidence
informing appropriate displacement buffers,
and because of the uncertainties surrounding
the mortality (P) resulting from displacement.
We note that even at P = 0.75, 1 in 12 divers
are predicted to suffer mortality.
Page 18
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