Population Viability Analysis of the Sula Sgeir Gannet Population

Scottish Natural Heritage Commissioned Report No. 897

Population viability analysis of the Sula Sgeir gannet population

COMMISSIONED REPORT

Commissioned Report No. 897 Population viability analysis of the Sula Sgeir gannet population

For further information on this report please contact:

Dr Andrew Douse Scottish Natural Heritage Great Glen House Leachkin Road INVERNESS IV3 8NW Telephone: 01463 725241 E-mail: [email protected]

This report should be quoted as:

Trinder, M. 2016. Population viability analysis of the Sula Sgeir gannet population. Scottish Natural Heritage Commissioned Report No. 897.

This report, or any part of it, should not be reproduced without the permission of Scottish Natural Heritage. This permission will not be withheld unreasonably. The views expressed by the author(s) of this report should not be taken as the views and policies of Scottish Natural Heritage.

Summary

Population viability analysis of the Sula Sgeir gannet population

Commissioned Report No. 897 Project No: 15222 Contractor: Mark Trinder, MacArthur Green Year of publication: 2016

Keywords Gannet; Sula Sgeir; Population Viability Analysis; harvesting.

Background The Sula Sgeir gannet Morus bassanus population is subject to an agreed annual harvest of full grown chicks (known locally as guga) each summer. SNH commissioned MacArthur Green to undertake a population viability analysis of the Sula Sgeir population to assist in the understanding of the population’s dynamics in relation to the current harvest level and potential future changes. This report provides details of the approach taken and the results obtained.

Main findings  The modelling presented suggests that the harvest of gannet chicks at Sula Sgeir has reduced the rate of population growth rate below the level that would be predicted in the absence of harvest.  It is likely that this has had an effect (to a smaller extent) on other populations that are linked through immigration and emigration.  Nevertheless, the population has continued to grow and it seems probable that this would continue to be the case at the current harvest level (of 2,000 chicks per year) and at increased levels of harvest up to 3,500.  Harvest levels above 3,500 are very likely to lead to long-term decline in the Sula Sgeir gannet population.

For further information on this project contact: Dr Andrew Douse, Scottish Natural Heritage, Great Glen House, Inverness, IV3 8NW. Tel: 01463 725241 or [email protected] For further information on the SNH Research & Technical Support Programme contact: Knowledge & Information Unit, Scottish Natural Heritage, Great Glen House, Inverness, IV3 8NW. Tel: 01463 725000 or [email protected]

i Table of Contents Page

1. INTRODUCTION 1 2. METHODS 1 2.1 Estimation of immigration and emigration from Scottish breeding populations 1 2.2 Simulation of the Sula Sgeir population with varying levels of annual harvest 2 3. RESULTS 3 3.1 Estimated demographic rates for the Scottish gannet population 3 3.2 Estimated rates of immigration and emigration from Scottish colonies 3 3.3 Simulations of the Sula Sgeir population under varying levels of harvest 4 4. DISCUSSION 6 5. CONCLUSIONS 7 6. REFERENCES 7 ANNEX 1: SULA SGEIR GANNET POPULATION PREDICTIONS GRAPHICAL OUTPUTS 8 ANNEX 2: SULA SGEIR GANNET POPULATION PREDICTIONS – TABULATED OUTPUTS 16

Acknowledgements

The work was commissioned by Andrew Douse, Policy & Advice Manager, Scottish Natural Heritage. Professor Bob Furness and an anonymous reviewer provided comments on the draft report.

iii

1. INTRODUCTION The Sula Sgeir gannet (Morus bassanus) population is subject to an agreed annual harvest of full grown chicks (known locally as guga) each summer. SNH commissioned MacArthur Green to undertake a population viability analysis of the Sula Sgeir population to assist in understanding the population’s dynamics in relation to the current harvest level and potential future changes. This report provides details of the approach taken and the results obtained. The modelling is based on that presented in WWT (2012).

2. METHODS The British gannet population is one of the best studied seabird populations, with all known breeding colonies counted at least once every decade since 1985 (Murray et al., 2015). Between 2004 and 2014 (the most recent pair of counts) the Scottish population increased at an average annual rate of 2.9%, although the rate has varied across Scottish colonies between 41.8% (Sule Skerry) and -0.15 (Sule Stack). Sula Sgeir increased between 2004 and 2014 at an average of 2.2%, despite an annual harvest of up to 2,000 chicks.

2.1 Estimation of immigration and emigration from Scottish breeding populations As a first step in developing a probabilistic model for predicting the impact of harvesting on the Sula Sgeir population, a preliminary deterministic model was developed to help understand the potential role of external inputs (i.e. immigrants) and outputs (i.e. harvested chicks) to the observed population growth. This modelling also considered the potential exchange between other Scottish breeding colonies. To do this the following steps were taken:

 A deterministic model was used to project the Scottish population between 2004 and 2014 using the demographic rates presented in WWT (2012) and the 2004 population estimate.  The population growth rate predicted by this model was 0.58% per year, which is lower than the observed annual growth rate of 2.9%. It was not clear what accounted for this difference (survival, reproduction or immigration from elsewhere). However, it was considered unlikely that there has been substantial immigration into the Scottish population from elsewhere so it was assumed that elevated demographic rates were responsible for the difference. Three alternative approaches for increasing the demographic rates were considered.

o Scenario 1: multiplying all demographic rates by the same amount; o Scenario 2: multiplying all survival rates by the same amount but retaining the original productivity value; o Scenario 3: retaining all survival rates at their original values and multiplying productivity.

 For each scenario a multiplication factor was found which increased the demographic rates such that the model prediction matched the observed trend. The resulting three sets of baseline demographic rates were used for the remaining population modelling.  Each Scottish colony was simulated from its 2004 size using each of the three sets of adjusted demographic rates. The difference between the observed and predicted 2014 population sizes was then converted into an annual level of immigration or emigration depending on whether the colony had grown above or below the national average. This approach assumes that demographic parameters are the same at all colonies. There is some evidence that breeding success is very similar across colonies (JNCC Seabird Monitoring Programme database), and the only study of survival rates found

no evidence for any differences in survival rates among colonies (Wanless et al., 2006). For the Sula Sgeir population the model also included the average annual harvest of chicks. Note that four Scottish populations were omitted from this analysis for the following reasons:  Noss and Foula have grown at (or near) the Scottish rate and therefore immigration and emigration are assumed to be balanced;  Rockall and Barra are very small colonies and are thus not considered to play a role in the wider Scottish population dynamics.

2.2 Simulation of the Sula Sgeir population with varying levels of annual harvest Having established estimates of the average level of immigration into the Sula Sgeir population, a stochastic model was developed. This model was based on that presented in WWT (2012) which used survival rates estimated from analysis of ring recovery data for individuals ringed at several colonies over a period of four decades (Wanless et al., 2006); note that the values for all colonies combined were used here. The productivity rate was derived from observations made at a wide range of colonies (data extracted from the JNCC Seabird Monitoring Programme database) over a similar period as the survival analysis. As noted above, using these demographic values the model predicts average annual growth at 0.58%. However, the Scottish population has grown at an annual rate of 2.9% over this period. Therefore the demographic rates were adjusted as described above for each scenario, prior to modelling. Environmental stochasticity was included using the standard deviations provided in WWT (2012). Demographic stochasticity was included by employing a binomial function on the survival rates. The model was density independent which is likely to be appropriate for this population at its current size, although as the colony expands will become less so. However, we have little information with which to guide the estimation of density dependent relationships at this site.

Immigration was set at the average rate estimated using the deterministic models, and harvesting was applied across a range of values from 0 to 4,000 at increments of 250. In all cases the model was run for a period of 25 years, with 5,000 simulations at each harvesting level.

The following graphical outputs from each of the three model scenarios are provided:

 population projection for the un-harvested and maximum harvest (4,000) simulations;  population growth rate and change in population growth rate (plotted separately), presented across the full range of harvesting rates, calculated at the following percentiles: 5%, 33%, 50% (median), 67%, 95% (change in population growth rate was calculated by subtracting the baseline growth rate values (i.e. no harvest) from the harvested growth rates);  ratio of median harvested population size to median un-harvested size (Counterfactual of Population Size; CPS) across the full range of harvesting rates, calculated at 5 year intervals up to 25 years;  probability that the population will decline below a range of percentages of the initial size (0, 1, 2, 5, 10, 15, 20, 25 and 50%), and change in this probability (plotted separately); and,  probability that the harvested population size in the final year will be less than a range of percentages of the un-harvested final population size (0, 1, 2, 5, 10, 15, 20, 25, 50%), and change in this probability (plotted separately).

The following tabulated outputs are provided:

 population growth rate (range of percentiles) at each harvest level;

 median population size at 5 year intervals at each harvest level;  probability of population decline at each harvest level; and,  probability that the harvested population size in the final year will be less than a range of percentages of the un-harvested final population size, and change in this probability.

It should be noted that the population growth rate has been calculated between year 5 and year 25 of each simulation. This avoids the rate being affected by initial conditions and allows the population structure (i.e. the ratio of each age class) to adjust from the initial value to one which reflects the modelled harvest level.

3. RESULTS 3.1 Estimated demographic rates for the Scottish gannet population The demographic rates presented in WWT (2012), the adjusted rates for Scenarios 1 to 3 and the magnitude of adjustment required in order for the rates to generate growth rates of 2.9% are provided in Table 1.

Table 1. Baseline and adjusted gannet demographic rates for modelling the Scottish population. The baseline survival rates are taken from Wanless et al. (2006).

Demographic rate Baseline (WWT Scenario Scenario Scenario 2012) 1 2 3 Adjustment factor NA 1.0217 1.0231 1.3880 Survival age 0-1 0.424 0.433 0.434 0.424 Survival age 1-2 0.829 0.847 0.848 0.829 Survival age 2-3 0.891 0.910 0.912 0.891 Survival age 3-4 0.895 0.914 0.916 0.895 Survival age 5+ 0.919 0.939 0.940 0.919 Age at first breeding 5 Reproduction 0.69 0.705 0.690 0.958 Population growth rate 1.0058 1.029 1.029 1.029

The magnitude of rate adjustment required under scenarios 1 and 2 was comparatively modest (2.1% to 2.3%) while scenario 3 required a large adjustment of 39%. Furthermore, since gannets only lay a single egg, achieving an average fledging success of 0.96 is very unlikely to be realistic. This is also much higher than any reported productivity estimate (the highest single year estimate reported by the JNCC was 0.87 at Bempton Cliffs in 1994 (Walsh et al., 1995). Scenario 3 was therefore considered to be unrealistic and was not used in subsequent analysis or for generating model predictions.

3.2 Estimated rates of immigration and emigration from Scottish colonies Using the rates in Table 1 for each Scottish population, simulating growth between 2004 and 2014 the estimated annual rates of immigration or emigration required in order for each population to match its observed trend are provided in Table 2 (note that in some cases the populations were counted in 2013 rather than 2014, which was accommodated in the analysis).

Table 2. Estimated rates of annual emigration (negative values) and immigration (positive values) from Scottish populations required in order for observed trends to match the overall Scottish trend.

Observed annual Immigration (+ve) / emigration (-ve) Population growth rate 2004- Scenario 1 Scenario 2 2014 (%) St. Kilda 0.12 -4554 -4530 Sule Stack - 0.01 -388 -386 Ailsa Craig 2.05 -678 -674 Scar Rocks - 0.001 -196 -195 Sula Sgeir 2.21 273 276 Flannan 7.47 436 435 Sule Skerry 47.38 450 449 Westray 48.92 164 163 Fair Isle 6.71 249 249 Hermaness 5.05 1090 1087 Troup Head 15.16 982 979 Bass Rock 4.59 2579 2573 Balance (immigration minus 407 426 emigration)

As can be seen from Table 2, Scottish gannet populations appear to have been more or less balanced when considered at the national level. Overall exchange (i.e. emigration or immigration) is around 6,000 individuals each year although on the basis of the results in Table 2 it appears that slightly more immigration is required (around 400 per year) into the Scottish population to account for the overall observed trend. However, the modelling was conducted with the simplifying assumption that all the populations have had the same demographic rates. It is very likely that the rates will in fact vary between the colonies which would account for the small imbalance.

The Sula Sgeir population, which is also subject to an annual harvest of chicks (an average of 1,917 over the period 2004 and 2014), is estimated to have required just over 270 breeding age recruits each year in order to have achieved the level of growth observed. Without this immigration the population growth rate would have been just less than 1%.

3.3 Simulations of the Sula Sgeir population under varying levels of harvest Using the estimated annual rates of immigration into the Sula Sgeir population and the demographic rates for scenarios 1 and 2 (Table 1), the effect of harvesting across a range of levels from 0 to 4,000 per year was investigated. Full results are presented in Annex 1 (graphs) and Annex 2 (tables). The population has been subject to both harvest (at a known rate of approx. 2,000 per year) and immigration (predicted from modelling to be around 270 per year). The model outputs at a harvest level of 2,000 generated a population growth of 2.2%, which matched the observed rate of growth in the population between 2004 and 2014.

A summary of the key results is provided in Table 3. At the current harvest level of 2,000, and assuming that immigration remains at the level estimated over the period 2004 to 2014, the probability of population decline within 25 years was estimated to be between 11.3% and 10.2% (for scenarios 1 and 2 respectively). At an increased harvest of 3,000 this probability range increases to 70% to 71% (again for scenarios 1 and 2), and at a harvest of 4,000 both scenarios predict a 99% probability of decline.

The current harvest regime appears to have reduced the population growth rate by 1.2% below the level which could be expected in the absence of harvest.

Table 3. Summary outputs for scenarios 1 and 2 across a range of harvest sizes from zero to 4,000: the probability of population decline below the initial size (2014 census) during 25 year simulation; the ratio of the predicted population size in year 25 at each harvest level compared with that with no harvest; the reduction in the population growth rate for each harvest level compared with zero harvest. Values at a harvest of 2,000 (current size) are highlighted.

Harvest Probability of Counterfactual of Percentage change in population decline population size in year population growth below initial (2014) 25 (ratio of harvested rate relative to zero size to zero harvest harvest population size) Scenario Scenario Scenario Scenario Scenario Scenario 1 2 1 2 1 2 0 0 0 1 1 0 0 500 0 0 0.932 0.928 -0.24 -0.26 1000 0.002 0.004 0.860 0.855 -0.52 -0.55 1500 0.017 0.021 0.789 0.784 -0.83 -0.86 2000 0.106 0.115 0.718 0.715 -1.17 -1.19 2500 0.364 0.361 0.648 0.643 -1.55 -1.60 3000 0.701 0.716 0.577 0.573 -2.00 -2.03 3500 0.934 0.939 0.506 0.502 -2.51 -2.56 4000 0.995 0.994 0.436 0.431 -3.12 -3.16

Using the current harvest of 2,000 as the baseline for comparisons (Table 4), the population growth rate was predicted to decline by 0.4%, 0.8%, 1.3% and 1.9% for additional harvests of 500, 1,000, 1,500 and 2,000 respectively (i.e. at total harvests of 2,500 to 4,000).

Table 4. Summary outputs for scenarios 1 and 2 across a range of harvest sizes from zero to 4,000: the ratio of the predicted population size in year 25 at each harvest level compared with the current harvest (2,000); the reduction in the population growth rate for each harvest level compared with current harvest (2,000).

Harvest Counterfactual of population size in Percentage change in population year 25 (relative to current harvest growth rate relative to current of 2,000) harvest (2,000) Scenario 1 Scenario 2 Scenario 1 Scenario 2 0 1.393 1.400 1.17 1.19 500 1.297 1.298 0.93 0.93 1000 1.198 1.197 0.65 0.64 1500 1.099 1.098 0.34 0.33 2000 1 1 0 0 2500 0.903 0.900 -0.38 -0.40 3000 0.804 0.802 -0.83 -0.84 3500 0.705 0.703 -1.34 -1.37 4000 0.608 0.603 -1.95 -1.97

While the overall growth rate of the Scottish population between 2004 and 2014 was 2.9%, and that is the value which would be expected at a harvest of zero, the model included immigration (at the levels in Table 2) at all modelled harvest levels, including zero. Hence, at zero harvest the Sula Sgeir population was predicted to grow at a slightly higher rate of 3.5%. It is of interest to note that the model predicted growth at 2.9% with a harvest of 1,000 chicks. On the basis of the analysis of population exchange, with immigration simulated as the annual addition of around 270 sub-adults (4 year olds), this indicates that each 4 year old

bird recruited to the population represents approximately 3-4 chicks in terms of their predicted contribution to population growth. This is due to the higher mortality of immature birds, with only around 30% of fledged birds reaching breeding age.

4. DISCUSSION The British gannet population has continued to grow in recent decades, in contrast to that for many other seabird populations. However, not all breeding colonies have grown at the same rate. While some of the differences are likely to be due to variations in survival and reproduction among colonies, there also appears to be good evidence that some more established colonies have provided recruits for other colonies at the expense of their own growth. The breeding population on St. Kilda appears to be an example of such a population, with relatively little increase in numbers between 2004 and 2013. This contrasts markedly with the growth of other colonies in the area (e.g. the Flannan Isles and Sule Stack) which have grown well above the overall Scottish rate. Consequently it seems very probable that the growth of these colonies has been supported through recruitment of birds from colonies such as St. Kilda.

The Sula Sgeir population grew at a rate of 2.2% over the last decade, which is below the Scottish rate of 2.9%. However, this colony also appears to have been supported through recruitment from other colonies, since without immigration the estimated growth would have been less than 1%. Given that the analysis presented here indicates exchange between Scottish colonies, the removal of individuals from one colony would seem very likely to have effects on other connected colonies. While it remains possible that the level of estimated immigration to Sula Sgeir may not be affected by the magnitude of harvest experienced, it does seem likely that the reduction in internal recruitment (i.e. by chicks hatched at Sula Sgeir) presents increased opportunities for external recruitment. Thus, the interchange between colonies indicates that harvesting from Sula Sgeir has in the past, and likely will in future, also have effects on other populations.

The demographic rates available for gannet are considered to be good quality (Horswill & Robinson, 2015). Nonetheless, when combined in a population model they are unable to replicate the recently observed rate of growth. In order to generate predictions that match those observed in recent years it was therefore necessary to modify the rates so that the modelled growth rate of the gannet population matched the actual, observed changes in population size. As there is no information available to indicate the most appropriate rates to modify, three alternative approaches were considered: (1) increase all rates by the same amount, (2) increase all survival rates by the same amount and (3) increase productivity. Of these, the third (increased productivity alone) required such a large adjustment that it was subsequently discounted from further consideration. In contrast the adjustments required under scenarios 1 and 2 were quite similar and hence the results obtained were also in closer agreement. This gives some confidence in the assumption that a combination of slightly higher survival and reproduction has accounted for recent trends, and also that it is not necessary to understand the precise balance of rate improvements in order to make robust predictions of future population trends. It is also important to highlight that, when considering population model outputs the most robust comparisons on which to base management decisions are those which compare scenarios with and without impact (e.g. the change in growth rate and the counterfactual of population size resulting from an additional factor), rather than individual predictions of expected population size. This is because the former are much less sensitive to model assumptions than the latter.

The estimated level of exchange between Scottish colonies indicates that changes at one site could lead to changes at others. For example, an increase in the harvest at Sula Sgeir might lead to an increase in recruitment to the population, thereby offsetting losses there.

This, in turn, would lead to a reduction in the number of recruits available for other colonies, although as the Scottish gannet population grows there would be an expected overall increase in potential recruits that could at least partially compensate. While it is also possible that disturbance caused by the harvest would act to limit recruitment, the fact that the harvest occurs late in the breeding season, for a comparatively short period of time (two weeks) and only within those areas that are accessible, suggests that such an effect would probably be comparatively small.

The population growth rate is predicted to decline below its current level with an increase in the harvest, and the converse is also expected to be the case. Nonetheless, the median population growth rate is predicted to remain positive up to a harvest of 4,000 chicks per year (the maximum modelled). It should be noted, however, that at harvest levels above 3,500 there is a risk of negative growth: at a harvest of 3,500 per year 4.7% of simulations declined in size, which increased to 27% at a harvest of 4,000.

5. CONCLUSIONS It appears from the modelling presented that the harvest of gannet chicks at Sula Sgeir has reduced the rate of population growth rate below the level that would be predicted in the absence of harvest. It is also likely that this has had an effect (to a smaller extent) on other populations that are linked through immigration and emigration. Nevertheless, the population has continued to grow and it seems probable that this would continue to be the case at the current harvest level (of 2,000 chicks per year) and at increased levels of harvest up to 3,500.

6. REFERENCES Horswill, C. & Robinson R. A. 2015. Review of seabird demographic rates and density dependence. JNCC Report No. 552. Joint Nature Conservation Committee, Peterborough.

Murray, S., Harris, M.P. & Wanless, S. 2015. The status of the gannet in Scotland in 2013- 14. Scottish Birds, 35, 3-18.

Walsh, P.M., Brindley, E. & Heubeck, M. 1995. Seabird numbers and breeding success in Britain and Ireland, 1994. Peterborough, JNCC (UK Nature Conservation, no, 18).

Wanless, S., Frederiksen, M., Harris, M.P. & Freeman, S.N. 2006. Survival of gannets Morus bassanus in Britain and Ireland, 1959-2002. Bird Study 53, 79-85.

WWT 2012. Gannet population viability analysis: demographic data, population model and outputs. SOSS Report 04 to The Crown Estate. WWT Consulting, Slimbridge.

ANNEX 1: SULA SGEIR GANNET POPULATION PREDICTIONS GRAPHICAL OUTPUTS Scenario 1 Gannet demographic rateset 1

100000 Median (no.impact) Median (Harvest: 4000) 95% c.i. (no.impact) 80000 95% c.i. (Harvest: 4000)

60000

40000 Population

20000

5 10152025

Year Figure 1. Gannet demographic rateset 1. Population projection for harvests of 0 and 4,000, showing sum of all age classes. Note that immigration of sub-adults (4th year birds) at a rate of 273 is included in all years.

1.04

1.03

1.02

1.01 95% 66% Median 33%

Population growth rate Population growth 1.00 5% 0.99

0 1000 2000 3000 4000 Guga harvest

Figure 2. Gannet demographic rateset 1. Population growth rate with increasing harvest from 0 to 4,000. Lines show the median (black) and percentage ranges around the median (red lines). Note the outer range contains 90% of the simulated output, with 5% of predictions both above and below these lines. Rate calculated between year 5 and 25 of each simulation.

0.00

-0.01

-0.02 95%

66% -0.03 Median 33%

Change in population growth rate Change in growth population -0.04 5%

0 1000 2000 3000 4000 Guga harvest Figure 3. Gannet demographic rateset 1. Change in population growth rate with increasing harvest from 0 to 4,000. Lines show the median (black) and percentage ranges around the median (red lines). Note the outer range contains 90% of the simulated output, with 5% of predictions both above and below these lines. Rate calculated between year 5 and 25 of each simulation.

1.0

0.9

0.8 5 yrs

0.7 10 yrs

0.6 15 yrs

0.5 20 yrs 25 yrs

Predicted medianPredicted population size 0.4 as proportion of un-impacted population un-impacted of as proportion 0 1000 2000 3000 4000

Guga harvest Figure 4. Gannet demographic rateset 1. Counterfactual of population size at 5-year intervals with increasing harvest from 0 to 4,000, relative to current harvest (2,000).

1.0 Reduction (%) 0 15 1 20 0.8 2 25 5 50 10 0.6

0.4

0.2

Probability of population declinepopulation of Probability 0.0

0 1000 2000 3000 4000 Guga harvest

Figure 5. Gannet demographic rateset 1. Probability that population will decline below percentages (0 to 50%) of the initial size between years 5 and 25 of the simulation with increasing harvest from 0 to 4,000.

1.0 Reduction (%) 0 15 1 20 0.8 2 25 5 50 10 0.6

0.4

0.2

Probability of population declinepopulation of Probability 0.0

0 1000 2000 3000 4000 Guga harvest

Figure 6. Gannet demographic rateset 1. Increase in the probability that population will decline below percentages (0 to 50%) of the initial size between years 5 and 25 of the simulation with increasing harvest from 0 to 4,000. Note that as the baseline probability of population decline (i.e. at 0 harvest) was 0 for all percentage reductions the increase in risk relative to this baseline was the same as the absolute probability (Figure 5).

1.0

0.8

0.6

0.4 Population sizes (%) 100 85

Probability of decline of Probability 0.2 99 80 98 75 95 50 relative to unimpacted scenario unimpacted to relative 0.0 90

0 1000 2000 3000 4000 Guga harvest

Figure 7. Gannet demographic rateset 1. Probability that population will be less than percentages (0 to 50%) of the median un-harvested population in the final year of the simulation with increasing harvest from 0 to 4,000.

1.0 Population sizes (%) 100 85 99 80 0.8 98 75 95 50 90 0.6

0.4

Probability of decline of Probability 0.2

relative to unimpacted scenario unimpacted relative to 0.0

0 1000 2000 3000 4000 Guga harvest

Figure 8. Gannet demographic rateset 1. Increase in the probability that population will be less than percentages (0 to 50%) of the median un-harvested population in the final year of the simulation with increasing harvest from 0 to 4,000.

Scenario 2 Gannet demographic rateset 2

100000 Median (no.impact) Median (Harvest: 4000) 95% c.i. (no.impact) 80000 95% c.i. (Harvest: 4000)

60000

40000 Population

20000

5 10152025

Year Figure 9. Gannet demographic rateset 2. Population projection for harvests of 0 and 4,000, showing sum of all age classes. Note that immigration of sub-adults (4th year birds) at a rate of 273 is included in all years.

1.04

1.03

1.02

1.01 95% 66% Median 33%

Population growth rate Population growth 1.00 5% 0.99

0 1000 2000 3000 4000 Guga harvest

Figure 10. Gannet demographic rateset 2. Population growth rate with increasing harvest from 0 to 4,000. Lines show the median (black) and percentage ranges around the median (red lines). Note the outer range contains 90% of the simulated output, with 5% of predictions both above and below these lines. Rate calculated between year 5 and 25 of each simulation.

0.00

-0.01

-0.02 95%

66% -0.03 Median 33%

5% Change in population growth rate Change in growth population -0.04

0 1000 2000 3000 4000 Guga harvest

Figure 11. Gannet demographic rateset 2. Change in population growth rate with increasing harvest from 0 to 4,000. Lines show the median (black) and percentage ranges around the median (red lines). Note the outer range contains 90% of the simulated output, with 5% of predictions both above and below these lines. Rate calculated between year 5 and 25 of each simulation.

1.4

1.2

1.0

5 yrs 0.8 10 yrs 15 yrs 20 yrs 0.6 25 yrs Ratio of harvested median population size population median harvested Ratio of

as proportion of population at current harvest current at population of proportion as 0 1000 2000 3000 4000

Guga harvest

Figure 12 Gannet demographic rateset 2. Counterfactual of population size at 5-year intervals with increasing harvest from 0 to 4,000, relative to current harvest (2,000).

1.0 Reduction (%) 0 15 1 20 0.8 2 25 5 50 10 0.6

0.4

0.2

Probability of population decline population of Probability 0.0

0 1000 2000 3000 4000 Guga harvest

Figure 13. Gannet demographic rateset 2. Probability that population will decline below percentages (0 to 50%) of the initial size between years 5 and 25 of the simulation with increasing harvest from 0 to 4,000.

1.0 Reduction (%) 0 15 1 20 0.8 2 25 5 50 10 0.6 population declinepopulation 0.4 ility of of ility 0.2

Probab 0.0

0 1000 2000 3000 4000 Guga harvest

Figure 14. Gannet demographic rateset 2. Increase in the probability that population will decline below percentages (0 to 50%) of the initial size between years 5 and 25 of the simulation with increasing harvest from 0 to 4,000. Note that as the baseline probability of population decline (i.e. at 0 harvest) was 0 for all percentage reductions the increase in risk relative to this baseline was the same as the absolute probability (Figure 13).

1.0

0.8

0.6

ility of decline of ility 0.4 Population sizes (%) 100 85

Probab 0.2 99 80 98 75 95 50 relativeto unimpacted scenario 0.0 90

0 1000 2000 3000 4000 Guga harvest

Figure 15. Gannet demographic rateset 2. Probability that population will be less than percentages (0 to 50%) of the median un-harvested population in the final year of the simulation with increasing harvest from 0 to 4,000.

1.0 Population sizes (%) 100 85 99 80 0.8 98 75 95 50 90 0.6

0.4

Probability of decline of Probability 0.2

relative to unimpacted scenario unimpacted to relative 0.0

0 1000 2000 3000 4000 Guga harvest

Figure 16. Gannet demographic rateset 2. Increase in the probability that population will be less than percentages (0 to 50%) of the median un-harvested population in the final year of the simulation with increasing harvest from 0 to 4,000.

15 ANNEX 2: SULA SGEIR GANNET POPULATION PREDICTIONS – TABULATED OUTPUTS Scenario 1 Gannet demographic rateset 1

Table A2.1 Population growth rate in relation to harvest level.

Harvest Median population growth rate Median Lower 95% Upper 95% c.i. c.i. 0 1.0346 1.0275 1.0416 250 1.0335 1.0265 1.0402 500 1.0322 1.0250 1.0394 750 1.0307 1.0236 1.0378 1000 1.0294 1.0222 1.0367 1250 1.0280 1.0210 1.0350 1500 1.0263 1.0190 1.0336 1750 1.0247 1.0174 1.0319 2000 1.0229 1.0154 1.0304 2250 1.0211 1.0136 1.0285 2500 1.0191 1.0113 1.0268 2750 1.0170 1.0091 1.0245 3000 1.0146 1.0069 1.0226 3250 1.0123 1.0040 1.0201 3500 1.0095 1.0008 1.0180 3750 1.0067 0.9977 1.0147 4000 1.0034 0.9940 1.0123

Table A2.2 Median population size at 5-year intervals.

Harvest Median population size in years 5 to 25 5 10 15 20 25 0 43372 51750 61489 72559 85520 250 42779 50720 59957 70448 82618 500 42245 49686 58278 68214 79663 750 41785 48770 56742 65849 76429 1000 41195 47734 55129 63740 73550 1250 40631 46689 53454 61337 70623 1500 40170 45697 51976 59160 67456 1750 39600 44603 50322 56873 64524 2000 39072 43613 48675 54566 61406 2250 38556 42614 47139 52422 58541 2500 37968 41516 45571 50087 55421 2750 37491 40632 44011 47948 52381 3000 36939 39534 42396 45617 49349 3250 36388 38544 40862 43502 46480 3500 35850 37515 39202 41116 43296 3750 35317 36467 37524 38862 40339 4000 34805 35437 35990 36545 37319

Table A2.3 Probability that population will decline below total initial size (0%) and percentages of the initial size (99% – 50%) between years 5 and 25 of simulations.

Harvest Probability of decline below percentages of the initial population size 0 99 98 95 90 85 80 75 50 0 0.000 0 0 0 0 0 0 0 0 250 0.000 0 0 0 0 0 0 0 0 500 0.000 0 0 0 0 0 0 0 0 750 0.001 0.001 0.000 0 0 0 0 0 0 1000 0.002 0.001 0.000 0 0 0 0 0 0 1250 0.007 0.004 0.002 0 0 0 0 0 0 1500 0.017 0.008 0.004 0.000 0 0 0 0 0 1750 0.044 0.024 0.013 0.000 0 0 0 0 0 2000 0.106 0.064 0.038 0.003 0 0 0 0 0 2250 0.207 0.131 0.082 0.012 0.000 0 0 0 0 2500 0.364 0.260 0.176 0.032 0.000 0 0 0 0 2750 0.525 0.404 0.295 0.077 0.001 0 0 0 0 3000 0.701 0.591 0.470 0.161 0.008 0 0 0 0 3250 0.841 0.754 0.655 0.312 0.027 0.001 0 0 0 3500 0.934 0.885 0.816 0.499 0.075 0.007 0.000 0 0 3750 0.976 0.953 0.914 0.700 0.205 0.028 0.003 0.001 0 4000 0.995 0.989 0.976 0.862 0.405 0.124 0.032 0.006 0

Scenario 2 Gannet demographic rateset 2

Table A2.4 Population growth rate in relation to harvest level.

Harvest Median population growth rate Lower Upper Median 95% c.i. 95% c.i. 0 1.0349 1.0280 1.0417 250 1.0337 1.0264 1.0405 500 1.0323 1.0252 1.0392 750 1.0310 1.0237 1.0379 1000 1.0294 1.0223 1.0366 1250 1.0278 1.0208 1.0350 1500 1.0263 1.0191 1.0336 1750 1.0248 1.0176 1.0320 2000 1.0230 1.0155 1.0305 2250 1.0211 1.0136 1.0284 2500 1.0189 1.0111 1.0266 2750 1.0168 1.0090 1.0243 3000 1.0146 1.0066 1.0226 3250 1.0120 1.0040 1.0200 3500 1.0093 1.0011 1.0176 3750 1.0064 0.9976 1.0151 4000 1.0033 0.9935 1.0118

Table A2.5 Median total population size at 5-year intervals.

Harvest Median population size in years 5 to 25 5 10 15 20 25 0 43022 51428 61224 72409 85358 250 42464 50429 59589 70137 82383 500 41971 49385 57983 67875 79191 750 41509 48413 56488 65663 76334 1000 40921 47390 54737 63240 73002 1250 40393 46329 53165 60919 69916 1500 39807 45275 51471 58631 66951 1750 39324 44341 50018 56656 64211 2000 38774 43249 48383 54261 60989 2250 38225 42257 46796 51983 57993 2500 37720 41190 45174 49664 54889 2750 37156 40182 43519 47367 51830 3000 36611 39207 41967 45182 48913 3250 36101 38183 40465 43004 45910 3500 35595 37135 38736 40572 42872 3750 35045 36151 37204 38477 39746 4000 34477 35063 35586 36144 36754

Table A2.6 Probability that population will decline below total initial size (0%) and percentages of the initial size (99% – 50%) between years 5 and 25 of simulations.

Harvest Probability of decline below percentages of the initial population size 0 99 98 95 90 85 80 75 50

0 0 0 0 0 0 0 0 0 0 250 0 0 0 0 0 0 0 0 0 500 0 0 0 0 0 0 0 0 0 750 0.001 0 0 0 0 0 0 0 0 1000 0.004 0.001 0 0 0 0 0 0 0 1250 0.007 0.003 0.001 0 0 0 0 0 0 1500 0.021 0.010 0.004 0.000 0 0 0 0 0 1750 0.048 0.022 0.012 0.001 0 0 0 0 0 2000 0.115 0.066 0.037 0.002 0 0 0 0 0 2250 0.219 0.141 0.089 0.010 0 0 0 0 0 2500 0.361 0.257 0.173 0.030 0.001 0 0 0 0 2750 0.545 0.419 0.302 0.076 0.002 0 0 0 0 3000 0.716 0.611 0.488 0.172 0.008 0.000 0 0 0 3250 0.847 0.766 0.655 0.311 0.030 0.001 0 0 0 3500 0.939 0.891 0.824 0.513 0.083 0.008 0.000 0 0 3750 0.979 0.955 0.923 0.712 0.212 0.032 0.004 0.000 0 4000 0.994 0.989 0.975 0.869 0.443 0.141 0.040 0.010 0

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