Provisional Regional PBR for Scottish seals in 2010
Objective: This document estimates PBR values for Scottish grey and harbour seal
“populations”. The coast of Scotland is divided into ten broad areas, and sets of possible values
are calculated for each one. The setting of the recovery factor parameter determines the PBR
value. Traditionally this has been done by a subjective assessment of the state of the population.
More recently SMRU (Boyd et al., unpublished) has suggested a more objective approach. We
have used that approach, but modified it to incorporate a more precautionary approach to
populations undergoing rapid and as yet unexplained declines.
Areas used in the calculations. Figure 1 (a & b) and Table 1 shows the boundaries used to
divide the Scottish seal populations for this study together with the summer haulout distributions
for both grey and harbour seals.
Table 1 Definitions of the suggested management areas
East coast
Moray Firth
Shetland
Orkney + North coast
Outer Hebrides
West Highland
South-West Scotland
(Clyde + Dumfries &Galloway)
Border to Fraseburgh
Fraseburgh to Duncansby Head
Duncansby Head to Cape Wrath
Cape Wrath to Mull of Kintyre
Mull of Kintyre to the Border
Particularly for grey seals, there will probably be substantial movement of animals between
regions. The suggested division is a pragmatic compromise taking some consideration of: current
biological knowledge; distances between major haulouts; environmental conditions; the spatial
structure of existing data; practical constraints on future data collection and management
requirements. Survey data are available at finer resolution and PBR estimates could be allocated
within subdivisions regions based on the local distribution of seals
Origins of the PBR approach: Potential Biological Removal is a widely used way of
calculating whether current levels of anthropogenic mortality are consistent with reaching or
exceeding a specific target population, chosen to be the Optimum Sustainable Population. The
method has been supported by simulations demonstrating its performance under certain
assumptions (Wade 1998).
The PBR calculation:
PBR = Nmin.(Rmax/2).FR
where:
PBR is a number of animals considered safely removable from the population.
Nmin is a minimum population estimate (usually the 20th percentile of a distribution)
Rmax is the population growth rate at low densities (by default set 0.12 for
pinnipeds), this is halved to give an estimate of the growth rate at higher
populations. The estimate should be conservative for most populations at their OSP.
Sea Mammal Research Unit
1
16/02/2016
FR is a recovery factor, usually in the range 0.1 to 1. Low recovery factors give
some protection from stochastic effects and overestimation of the other parameters.
They also increase the expected equilibrium population size under the PBR.
Data used in these calculations
Nmin values used in these calculations are from the most recent summer surveys of each area, for
both species:
 Harbour seals: The surveys took place during the harbour seal moult, when the majority
of this species will be hauled out, so the counts are used directly as values for Nmin.
(An alternative approach would be to assume that the proportion hauled out was 2/3, a
value supported by telemetry data, with a CV of 0.15 to calculate Nmin. That approach
would increase the PBR by up to 20%.).
 Grey seals: Provisional analyses of telemetry data suggests that around 1/3 of grey seals
were hauled out during the surveys, though the precision of this estimate is unknown.
These counts were doubled to give values used here as minimum population estimates,
equivalent to the Nmin values for harbour seals.
Rmax = 0.12, the default value for pinnipeds, since we have no information on this parameter for
Scottish seals.
FR needs to be chosen from the range [0.1, 1]. Estimated PBR values for the entire range of FR
values are presented. A recommended FR value is indicated for each species in each region,
together with a justification for the recommended value.
Sea Mammal Research Unit
2
16/02/2016
Results
PBR values for grey and harbour seals for each suggested management region. Recommended FR values are highlighted in grey cells.
PBR values for harbour seals
Region
East coast
Moray Firth
Shetland
Orkney + North coast
Outer Hebrides
West Highland
South-West Scotland
Year of data
collection
2008
2009
2009
2008
2008
2007/2009
2007
Count
(Nmin)
487
776
2989
2986
1815
10530
834
0.1
3
5
18
18
11
63
5
0.2
6
9
36
36
22
126
10
0.3
9
14
54
54
33
190
15
0.4
12
19
72
72
44
253
20
0.5
15
23
90
90
54
316
25
0.6
18
28
108
107
65
379
30
0.7
20
33
126
125
76
442
35
0.8
23
37
143
143
87
505
40
0.9
26
42
161
161
98
569
45
1
29
47
179
179
109
632
50
0.5
138
76
81
479
204
149
22
0.6
166
91
98
575
245
178
27
0.7
194
106
114
671
285
208
31
0.8
221
121
130
767
326
238
36
0.9
249
136
146
863
367
268
40
PBR values for grey seals
Year of data
Region
East coast
Moray Firth
Shetland
Orkney + North coast
Outer Hebrides
West Highland
South-West Scotland
collection
2008
2009
2009
2008
2008
2007/2009
2007
Sea Mammal Research Unit
Count
2307
1263
1355
7988
3396
2478
374
(Nmin)
4614
2526
2710
15976
6792
4956
748
0.1
28
15
16
96
41
30
4
0.2
55
30
33
192
82
59
9
0.3
83
45
49
288
122
89
13
3 16/02/2016
0.4
111
61
65
383
163
119
18
1
277
152
163
959
408
297
45
Rationale for the suggested recovery factors:
The original PBR methodology leaves the setting of the recovery factor as a subjective
choice for managers. Boyd et al. (unpublished) attempted to systemise this by evaluating
the factors that may be affecting the population, categorising these impacts according to
whether they primarily influence the effective population size or its growth rate. A “level of
certainty” (“low”, “intermediate” or “high”) is then assigned to each variable and a lookup
table used to map these on to the range [0.1,1] of permissible Recovery Factors. The factors
affecting the value for each species in each area are given below (only the worst effect on
each variable is considered in setting the Recovery Factor, so only these are listed):
Harbour seals
1) Shetland, Orkney+ North Coast and Eastern Scotland (FR= 0.1)
FR set to minimum because populations are experiencing prolonged declines.
2) Outer Hebrides (FR = 0.5)
Population is undergoing a protracted but gradual decline, it is only partly closed
being close to the relatively much larger population in the West Highland region
and the Rmax parameter is derived from other seal populations.
4) Western Scotland (FR = 0.7)
The population is largely closed, likely to have limited interchange with much
smaller adjacent populations. The population is apparently stable and the intrinsic
population growth rate is taken from other similar populations giving a FR of 0.7.
4) South West Scotland (FR = 0.7)
The population is apparently stable, is closed to the south and the adjacent
population to the north is apparently stable. The intrinsic population growth rate is
taken from other similar populations giving a FR of 0.7.
5) Moray Firth
The recent data for the Moray Firth indicates a continued gradual decline following
the rapid declines around 2000. The population may be regarded as relatively
closed with respect to the neighbouring Firth of Tay and Orkney populations. This
would suggest a FR = 0.5, similar to the Outer Hebrides. However, the
neighbouring Orkney and Tay populations have undergone unexplained, rapid and
catastrophic declines of 70% and 85% respectively. It can be argued that the FR for
the Moray Firth should be set to the same value as that for the adjacent Orkney and
East Scotland populations. This would suggest a FR = 0.1.
The detailed trend data and removals data for the Moray Firth may allow us to
select an appropriate value within this range. The population is apparently stable or
declining slowly. It is likely that removing the same number of seals as in previous
years (actual take rather than quota) will have the same effect and the population
will continue to behave as it has done recently. Recent takes in the Moray Firth
have averaged 14 seals over the past 5 years (with a PBR set in the low 20s) during
which time the population has remained stable or declined slowly. This would be
achieved by setting FR = 0.3.
Grey seals
All regions (FR = 1.0)
During the summer the populations are highly mobile and the management areas
can not be considered closed (“intermediate” N). However, all the populations
have shown prolonged growth and some have now stabilised. This should reduce
the importance of the openness of sub populations and we recommend a FR of 1.
Sea Mammal Research Unit
4
16/02/2016
References
Wade, P.R. (1998) Calculating limits to the allowable human-caused mortality of cetaceans and
pinnipeds. Marine Mammal Science 14(1):1:37
Boyd, I.L., Thompson, D. & Lonergan, M (unpublished) Potential Biological Removal as a
method for setting the impact limits for UK marine mammal populations. Draft briefing paper to
2009 SCOS meeting.
Sea Mammal Research Unit
5
16/02/2016
Figure 1 Suggested management areas for grey and harbour seals, for the purposes of calculating PBRs
in Scotland.
a)
management areas with grey seal haulout sites.
Sea Mammal Research Unit
6
16/02/2016
b) management areas with harbour seal haulout sites.
Sea Mammal Research Unit
7
16/02/2016