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Oil spill impact assessment for seabirds: The role of refugia and growth centres
Biological Conservation 40 (1987) 1-9
Oil Spill Impact Assessment for Seabirds: The Role of Refugia and Growth Centres D. K. Cairns Newfoundland Institute for Cold Ocean Science and Psychology Department, Memorial University of Newfoundland, St John's, Newfoundland A1B 3X9, Canada
& R. D. Elliot Canadian Wildlife Service, Box 9158, Station B, St John's, Newfoundland A1A 2X9, Canada (Received 26 April 1986, accepted 28 August 1986) A BSTRA CT Assessments of oil spill risk to seabirds ought to consider the possibility that large spills adjacent to colonies of pursuit-diving seabirds could kill all birds at the colony. The rate of population recovery following destruction of a colony depends on the size and location of neighbouring colonies which may act as refugia, or as growth centres that accelerate recovery by attracting recruits. Population recovery following destruction of a major common murre Uria aalge colony in Newfoundland would likely be slow because of the absence of suitable refugia and growth centres near large colonies. Full assessment of recovery potential of colony populations requires further information on inter-colony recruitment, movement of pre-breeders and their potential to repopulate a colony, dynamics of colony foundation, and the role of food competition in determining colony size.
INTRODUCTION Because seabirds are very sensitive to oil spills, environmental managers have sought to assess the vulnerability of seabird communities to oil pollution and to estimate their recovery potential following spills (Ford et aL, 1982; Samuels & Lanfear, 1982). These studies have assumed that an oil 1
Biol. Conserv. 0006-3207/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
2
D.K. Cairns, R. D. Elliot
spill will eliminate some fraction of a seabird population, but have not considered situations in which the spill kills all of a colony's breeding adults. A major oil spill that persisted for several days adjacent to a high latitude seabird colony could kill all pursuit-diving birds associated with it because such species spend much time on the water (King & Sanger, 1979), and because even small amounts of oil in contact with seabird plumage can be lethal in cold conditions (Levy, 1980). This paper develops an approach to evaluate the recovery potential of seabirds following the destruction of a colony by oil pollution. We illustrate our approach, which assesses the ability of other colonies to contribute to population recovery, with the example of common murres Uria aalge breeding in eastern Newfoundland. Concerns about the effects of oil on murres and other seabirds are particularly relevant in Newfoundland because of planned oil production in the region.
ASSESSING RECOVERY POTENTIAL We assume that the parameter of interest is the time required for regional population recovery following the destruction of a colony by an oil spill. Recovery could come about through re-establishment and growth of the destroyed colony, by growth of other colonies in the region, or by a combination of these. Since recovery requires both a source of recruits and available breeding habitat, the location, size, and physical characteristics of neighbouring colonies are critical in the assessment of recovery potential.
Refugia When an oil spill destroys one seabird colony in a region but spares or only partially destroys others, these other colonies are refugia. The effectiveness with which birds from a refugium could re-stock the former colony depends on: (1) The likelihood that the refugium will escape the spill at the destroyed colony, which depends on distance between the colonies and strength and direction of prevailing winds and currents. (2) The likelihood that birds from the refugium will recruit to the decimated colony. Most adult seabirds show high fidelity to their breeding colonies once they begin breeding, so most recruits will be first-time breeders which are generally less philopatric. Recruitment rates probably depend on distance between the colonies as well as size of the source colony (e.g., Atlantic puffins Fratercula arctica; Harris, 1983).
Oil spills and seabirds
3
(3) The likelihood that a pre-breeding cohort, hatched at the affected colony, is absent from the area at the time of the oil spill and therefore escapes its consequences. The importance of nearby refugia would be reduced if such a cohort were available and capable of re-stocking the affected colony in the absence of established breeders. Such a cohort would have the effect of a refugium centred on the affected colony. Growth centres
Because newly founded colonies are small and do not have experienced breeders, they may lack advantages of anti-predator protection and social facilitation of food finding that accrue to birds breeding in established colonies (Birkhead, 1985). Thus birds nesting at devastated sites may suffer low breeding success Which may cause recolonisation to fail (e.g., Campbell et al., 1975). This suggests that regional population recovery may be more rapid if colonies are present close to the affected site where recruits might breed with higher success. Recruits may be attracted to established colonies because the presence of breeders indicates that the site is suitable for nesting (Lack, 1967). For example, 13 Atlantic puffins artificially reared at recently recolonised Eastern Egg Rock, Maine, bred in 1985 at the established puffin colony at Matinicus Rock, Maine, despite the presence of good nesting habitat and abundant food at the site where they were reared (Kress, 1985). Colonies which contribute to regional population recovery by increasing in size following the destruction of a neighbouring colony can be called growth centres. Suitability of a colony as a growth centre depends on: (1) The likelihood that the growth centre will escape the oil spill, which depends on distance, winds, and currents. (2) The extent to which potential foraging ranges of the decimated colony and the growth centre overlap. If seabird populations are limited by summer food supply (Ashmole, 1963; Furness & Birkhead, 1984), densitydependent population growth should occur at the growth centre if breeders are able to exploit feeding habitat formerly used by the decimated colony. (3) The availability of nesting habitat for population expansion.
A CASE STUDY: RECOVERY POTENTIAL OF COMMON MURRE COLONIES IN N E W F O U N D L A N D We surveyed small Newfoundland common murre colonies to supplement published censuses of large colonies (Fig. 1), and located two previously
4
D.K. Cairns, R. D. Elliot
unreported breeding sites (The Drook and Cape Pine). Murre colonies reported by Brown et al. (1975) at the Guppy Islets and Grand Columbier are not occupied (personal observations; Desbrosse et al., 1984). Table 1 lists present populations for each colony and maximum populations if all available nesting habitat is used. Maximum populations in fiat colonies were estimated by extrapolating breeding densities given by Birkhead & Nettleship (1980) over the available area. Population caPacities of cliff
® Funk I. ~
e
South~CCabot I.
Common Murre colonies: <1,000 pairs o ~1,000 I~ <10,000 poirs • :~I0,000 poirs ®
wind frequency
O I
50 l
tO0 I
150 I
km
Fig. 1. Locations of common murre colonies in Newfoundland. Wide black arrows indicate prevailing surface currents (Petrie & Anderson, 1983). Wind rose shows mean percent frequency of wind directions in July at St. John's (47°37', 52°45'W; Atmospheric Environment Service, 1982). colonies could not be objectively measured and those we present are based on our subjective judgement of the amount of ledge habitat available. We assessed the likely value of neighbouring colonies as refugia and growth centres following disaster at Newfoundland murre colonies using the criteria given above (Table 1). Since one-year-old murres do not frequent areas around Newfoundland colonies such as Witless Bay (Piatt et al., 1984; see also Johnson, 1940), this cohort should survive an oil spill at the natal colony. Thus recolonising recruits could come from both nearby refugia and the affected colony itself. If an oil spill destroyed the large colony at Funk Island, prevailing winds would tend to move the slicks past South Cabot Island (Fig. 1), which
77 000 a
12b 18 ~
27 b
Witless Bay
The Drook
Cape Pine
Western Head
a From Nettleship (1980). b From present study.
10000 °
1 500 °
Baccalieu I.
Cape St Mary's
2 600 b
400 000 °
Breeding population (pairs)
South Cabot I.
Funk I.
Colony
20000
1000
500
100
730 000
10000
30 000
3 200 000
Maximum breeding capacity (pairs) South Cabot Baccalieu Funk Baccalieu Witless Bay South Cabot Funk Island The Drook Cape Pine Baccalieu Western Head Cape Pine Witless Bay Western Head The Drook Cape St Mary's Witless Bay Cape Pine The Drook Cape St Mary's Witless Bay Western Head Cape Pine The Drook Witless Bay
Neighbouring colonies
62 184 62 128 104 128 184 83 103 104 115 21 83 12 21 58 103 12 32 47 115 47 58 74 153
Distance to neighbouring colony (km) fair poor good poor fair poor poor poor poor poor poor poor fair poor poor poor fair fair poor poor fair poor poor poor poor
as a refugium
fair poor poor poor poor poor poor poor poor poor poor poor poor fair poor poor poor fair poor poor poor poor poor poor poor
as a growth centre
Suitability of neighbouring colony
TABLE 1 Present and Potential Populations of Newfoundland Common Murre Colonies, and their Suitability as Refugia and Growth Centres
6
D.K. Cairns, R. D. Elliot
would lessen the chance that birds there would be affected by the spill. However, the role of South Cabot as a refugium and a growth centre would be limited because of its small population and the limited habitat available for colony expansion. The nearest neighbours to the large murre colony at Witless Bay are Baccalieu and the three small colonies on the southeast Avalon Peninsula (Fig. 1). These are all too far from Witless Bay to serve as effective growth centres (Table 1) and are poor refugia because they are small and distant. If the colony at Cape St Mary's were destroyed by a n oil spill, recolonisation might come from the small colonies on the southeast Avalon, which are upwind and upcurrent from Cape St Mary's. However, the tiny size of these colonies would limit their role as refugia, and most recolonisation would probably be from Witless Bay. This analysis suggests that regional population recovery would occur slowly if a major Newfoundland murre colony were destroyed by an oil spill. Some small colonies have large neighbours which could serve them well as refugia or growth centres (Table 1), but the three large colonies where 99% of Newfoundland common murres breed do not have wellplaced refugia or growth centres. If we assume a breeding productivity of 0.7 chicks per nest and that 35% of fledglings reach breeding age (Birkhead & Hudson 1977), then a pre-breeding cohort that escaped an oil spill would represent about 12% of the original colony population at the time the cohort reached breeding age. Although the philopatry of pre-breeding cohorts absent from the colony at the time of the oil spill is uncertain, these birds probably offer the best chance for regional population recovery following a devastating oil spill.
CONCLUSIONS A N D I N F O R M A T I O N NEEDS Accurate assessments of seabird oil spill sensitivity depends on information which is frequently sparse or lacking (Wiens et aL, 1984). As our case study demonstrates, more data are required on the following factors for complete assessment of recovery potential following the destruction of all breeding seabirds at a colony.
Inter-colony recruitment patterns Harris (1983) estimated that 23% of young Atlantic puffins immigrate to non-natal colonies, and suggested that such behaviour is widespread among auks. The frequency of non-fidelity to natal site is important in estimating rates at which devastated colonies would be restocked.
Oil spills and seabirds
7
Movement of pre-breeding cohorts The absence of immature birds from their natal colonies during one or more breeding seasons may have a major effect on the availability of recruits to restock a decimated colony.
The dynamics of colony foundation and growth Although models of population recovery following oil spills (Ford et al., 1982; Samuels & Lanfear, 1982) assume no inter-colony movement, many seabird colonies have grown faster than intrinsic rates of increase (e.g., Tuck, 1961; Nelson, 1978; Sowl, 1979; Harris, 1984). Accurate prediction of recovery rates requires that the roles of immigration and internally generated growth be clarified through studies of the development of newly founded colonies.
The potential of pre-breeders to repopulate a colony The ability of naive birds to nest successfully in the absence of experienced breeders is poorly known, although the initiations of new colonies (e.g., Nelson, 1978) indicate that they may be successful.
Size and nest-site availability of small colonies Although small colonies may be important in recovery of regional populations following oil spills, many seabird censuses cover only large ones.
Food competition and foraging range In several seabird species, colony size tends to be inversely correlated with the size of neighbouring colonies (Furness & Birkhead, 1984). This suggests that destruction of a colony might be followed by density-dependent growth of other colonies which share potential foraging ranges. However, the link between seabird populations and food supply in the breeding season has not been demonstrated directly (Birkhead & Furness, 1985), and further information on these factors is required before the role of growth centres in population recovery can be fully assessed. Because of many methodological difficulties, these information needs are unlikely to be met in the near future. Nevertheless inter-specific and interregional comparisons, based on qualitative assessments of recovery potential, may be useful in planning petroleum-related activities. For
8
D.K. Cairns, R. D. Elliot
example, c o m m o n murres, which breed at only eight sites in Newfoundland, are likely at greater risk from an oil spill that destroys a colony than are Atlantic puffins, which breed in at least 19 colonies (Cairns et al. unpublished data). Similarly, Newfoundland murres are at a higher risk from a colony-destroying spill than those breeding in Labrador and the Gulf of St Lawrence, where populations are dispersed in numerous medium and small colonies (Brown et al., 1975). These comparisons suggest that high priority should be placed on the avoidance of petroleum activity that involves risk of spills near Newfoundland murre colonies.
ACKNOWLEDGEMENTS We thank J. Piatt for providing information on seabird colonies and E. Murray, N. Wakely, J. Wells and E. A. Young for helping with censuses. T. R. Birkhead, G. Finney, M. P. Harris, W. A. Montevecchi and J. Piatt improved the manuscript with their comments. The Newfoundland and Labrador Wildlife Division authorised our observations on F u n k Island, Witless Bay, and Cape St Mary's, which are Ecological Reserves under the provincial Wilderness and Ecological Reserves Act. Field work was supported by the Canadian Wildlife Service and grants from the Natural Sciences and Engineering Research Council of Canada to W. A. Montevecchi. This is Contribution no. 130 of the Newfoundland Institute for Cold Ocean Science.
REFERENCES Ashmole, N. P. (1963). The regulation of numbers of tropical oceanic birds. Ibis, 103b, 458-73. Atmospheric Environment Service (1982). Canadian climate normals, 5. Wind 1951-1980. Ottawa, Environment Canada. Birkhead, T. R. (1985). Coloniality and social behaviour in the Atlantic Alcidae. In The Atlantic Alcidae, ed. by D. N. Nettleship and T. R. Birkhead, 355-82. London, Academic Press. Birkhead, T. R. & Furness, R. W. (1985). Regulation of seabird populations. Symp. Brit. Ecol. Soc., 147-67. Birkhead, T. R. & Hudson, P. J. (1977). Population parameters for the common guillemot Uria aalge. Ornis. Scand., 8, 145-54. Birkhead, T. R. & Nettleship, D. N. (1980). Census methods for murres, Uria species: A unified approach. Can. Wildl. Serv. Occ. Pap., No. 43. Brown, R. G. B., Nettleship, D. N., Germain, P., Tull, C. E. & Davis, T. (1975). Atlas of eastern Canadian seabirds. Ottawa, Canadian Wildlife Service.
Oil spills and seabirds
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Campbell, R. W., Ward, J. G. & Shepard, M. G. (1975). A new common murre colony in British Columbia. Can. Field-natur., 89, 244-8. Desbrosse, A., Borotra, M. & Etcheberry, R. (1984). Les oiseaux marins de Saint Pierre et Miguelon. Saint-Pierre, France, Service d6partemental de ragriculture. Ford, R. G., Wiens, J. A., Heinemann, D. & Hunt, G. L. (1982). Modelling the sensitivity of colonially breeding marine birds to oil spills: Guillemot and kittiwake populations on the Pribilof Islands, Bering Sea. J. appl. Ecol., 19, 1-31. Furness, R. W. & Birkhead, T. R. (1984). Seabird colony distributions suggest competition for food supplies during the breeding season. Nature, Lond., 311, 655-6. Harris, M. P. (1983). Biology and survival of the immature puffin Fratercula arctica. Ibis, 125, 56-73. Harris, M. P. (1984). The puffin. Calton, T. and A. D. Poyser. Johnson, R. A. (1940). Present range, migration and abundance of the Atlantic murre in North America. Bird-Banding, 11, 1-17. King, J. G. & Sanger, G. A. (1979). Oil vulnerability index for marine oriented birds. In Conservation of marine birds of northern North America, ed. by J. C. Bartonek and D. N. Nettleship. U.S. Dep. Interior, Fish Wildl. Serv. Wildl. Res. Rep., No. 11,227-39. Kress, S. W. (1985). Island updates. Egg Rock Update, 1985, p. 3. Lack, D. (1967). Interrelationships in breeding adaptations as shown by marine birds. Proc. int. Orn. Congr., 14th, 3-42. Levy, E. M. (1980). Oil pollution and seabirds: Atlantic Canada 1976-77 and some implications for northern environments. Mar. Pollut. Bull., 11, 51-6. Nelson, J. B. (1978). The gannet. Berkhamsted, T. and A. D. Poyser. Nettleship, D. N. (1980). A guide to the major seabird colonies of eastern Canada. Dartmouth, Nova Scotia, Canadian Wildlife Service (unpublished manuscript). Petrie, B. & Anderson, C. (1983). Circulation of the Newfoundland continental shelf. Atmosphere-Ocean, 21, 207-26. Piatt, J. F., Nettleship, D. N. & Threlfall, W. (1984). Net-mortality of common murres and Atlantic puffins in Newfoundland, 1951-81. In Marine birds: Their feeding ecology and commercial fisheries relationships, ed. by D. N. Nettleship, G. A. Sanger and P. F. Springer, Can. Wildl. Serv. Spec. Publs, 196-206. Samuels, W. B. & Lanfear, K. J. (1982). Simulations of seabird damage and recovery from oilspills in the northern Gulf of Alaska. J. environ. Mgmt, 15, 169-82. Sowl, L. W. (1979). The historical status of nesting seabirds of the northern and western Gulf of Alaska. In Conservation of marine birds of northern North America, ed. by J. C. Bartonek and D. N. Nettleship, U.S. Dep. Interior, Fish Wildl. Serv. Wildl. Res. Rep., No. 11, 47-71. Tuck, L. M. (1961). The murres. Ottawa, Can. Wildl. Serv. Monogr., No. 1. Wiens, J. A., Ford, R. G. & Heinemann, D. (1984). Information needs and priorities for assessing the sensitivity of marine birds to oil spills. Biol. Conserv., 28, 21-49.
Oil Spill Impact Assessment for Seabirds: The Role of Refugia and Growth Centres D. K. Cairns Newfoundland Institute for Cold Ocean Science and Psychology Department, Memorial University of Newfoundland, St John's, Newfoundland A1B 3X9, Canada
& R. D. Elliot Canadian Wildlife Service, Box 9158, Station B, St John's, Newfoundland A1A 2X9, Canada (Received 26 April 1986, accepted 28 August 1986) A BSTRA CT Assessments of oil spill risk to seabirds ought to consider the possibility that large spills adjacent to colonies of pursuit-diving seabirds could kill all birds at the colony. The rate of population recovery following destruction of a colony depends on the size and location of neighbouring colonies which may act as refugia, or as growth centres that accelerate recovery by attracting recruits. Population recovery following destruction of a major common murre Uria aalge colony in Newfoundland would likely be slow because of the absence of suitable refugia and growth centres near large colonies. Full assessment of recovery potential of colony populations requires further information on inter-colony recruitment, movement of pre-breeders and their potential to repopulate a colony, dynamics of colony foundation, and the role of food competition in determining colony size.
INTRODUCTION Because seabirds are very sensitive to oil spills, environmental managers have sought to assess the vulnerability of seabird communities to oil pollution and to estimate their recovery potential following spills (Ford et aL, 1982; Samuels & Lanfear, 1982). These studies have assumed that an oil 1
Biol. Conserv. 0006-3207/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
2
D.K. Cairns, R. D. Elliot
spill will eliminate some fraction of a seabird population, but have not considered situations in which the spill kills all of a colony's breeding adults. A major oil spill that persisted for several days adjacent to a high latitude seabird colony could kill all pursuit-diving birds associated with it because such species spend much time on the water (King & Sanger, 1979), and because even small amounts of oil in contact with seabird plumage can be lethal in cold conditions (Levy, 1980). This paper develops an approach to evaluate the recovery potential of seabirds following the destruction of a colony by oil pollution. We illustrate our approach, which assesses the ability of other colonies to contribute to population recovery, with the example of common murres Uria aalge breeding in eastern Newfoundland. Concerns about the effects of oil on murres and other seabirds are particularly relevant in Newfoundland because of planned oil production in the region.
ASSESSING RECOVERY POTENTIAL We assume that the parameter of interest is the time required for regional population recovery following the destruction of a colony by an oil spill. Recovery could come about through re-establishment and growth of the destroyed colony, by growth of other colonies in the region, or by a combination of these. Since recovery requires both a source of recruits and available breeding habitat, the location, size, and physical characteristics of neighbouring colonies are critical in the assessment of recovery potential.
Refugia When an oil spill destroys one seabird colony in a region but spares or only partially destroys others, these other colonies are refugia. The effectiveness with which birds from a refugium could re-stock the former colony depends on: (1) The likelihood that the refugium will escape the spill at the destroyed colony, which depends on distance between the colonies and strength and direction of prevailing winds and currents. (2) The likelihood that birds from the refugium will recruit to the decimated colony. Most adult seabirds show high fidelity to their breeding colonies once they begin breeding, so most recruits will be first-time breeders which are generally less philopatric. Recruitment rates probably depend on distance between the colonies as well as size of the source colony (e.g., Atlantic puffins Fratercula arctica; Harris, 1983).
Oil spills and seabirds
3
(3) The likelihood that a pre-breeding cohort, hatched at the affected colony, is absent from the area at the time of the oil spill and therefore escapes its consequences. The importance of nearby refugia would be reduced if such a cohort were available and capable of re-stocking the affected colony in the absence of established breeders. Such a cohort would have the effect of a refugium centred on the affected colony. Growth centres
Because newly founded colonies are small and do not have experienced breeders, they may lack advantages of anti-predator protection and social facilitation of food finding that accrue to birds breeding in established colonies (Birkhead, 1985). Thus birds nesting at devastated sites may suffer low breeding success Which may cause recolonisation to fail (e.g., Campbell et al., 1975). This suggests that regional population recovery may be more rapid if colonies are present close to the affected site where recruits might breed with higher success. Recruits may be attracted to established colonies because the presence of breeders indicates that the site is suitable for nesting (Lack, 1967). For example, 13 Atlantic puffins artificially reared at recently recolonised Eastern Egg Rock, Maine, bred in 1985 at the established puffin colony at Matinicus Rock, Maine, despite the presence of good nesting habitat and abundant food at the site where they were reared (Kress, 1985). Colonies which contribute to regional population recovery by increasing in size following the destruction of a neighbouring colony can be called growth centres. Suitability of a colony as a growth centre depends on: (1) The likelihood that the growth centre will escape the oil spill, which depends on distance, winds, and currents. (2) The extent to which potential foraging ranges of the decimated colony and the growth centre overlap. If seabird populations are limited by summer food supply (Ashmole, 1963; Furness & Birkhead, 1984), densitydependent population growth should occur at the growth centre if breeders are able to exploit feeding habitat formerly used by the decimated colony. (3) The availability of nesting habitat for population expansion.
A CASE STUDY: RECOVERY POTENTIAL OF COMMON MURRE COLONIES IN N E W F O U N D L A N D We surveyed small Newfoundland common murre colonies to supplement published censuses of large colonies (Fig. 1), and located two previously
4
D.K. Cairns, R. D. Elliot
unreported breeding sites (The Drook and Cape Pine). Murre colonies reported by Brown et al. (1975) at the Guppy Islets and Grand Columbier are not occupied (personal observations; Desbrosse et al., 1984). Table 1 lists present populations for each colony and maximum populations if all available nesting habitat is used. Maximum populations in fiat colonies were estimated by extrapolating breeding densities given by Birkhead & Nettleship (1980) over the available area. Population caPacities of cliff
® Funk I. ~
e
South~CCabot I.
Common Murre colonies: <1,000 pairs o ~1,000 I~ <10,000 poirs • :~I0,000 poirs ®
wind frequency
O I
50 l
tO0 I
150 I
km
Fig. 1. Locations of common murre colonies in Newfoundland. Wide black arrows indicate prevailing surface currents (Petrie & Anderson, 1983). Wind rose shows mean percent frequency of wind directions in July at St. John's (47°37', 52°45'W; Atmospheric Environment Service, 1982). colonies could not be objectively measured and those we present are based on our subjective judgement of the amount of ledge habitat available. We assessed the likely value of neighbouring colonies as refugia and growth centres following disaster at Newfoundland murre colonies using the criteria given above (Table 1). Since one-year-old murres do not frequent areas around Newfoundland colonies such as Witless Bay (Piatt et al., 1984; see also Johnson, 1940), this cohort should survive an oil spill at the natal colony. Thus recolonising recruits could come from both nearby refugia and the affected colony itself. If an oil spill destroyed the large colony at Funk Island, prevailing winds would tend to move the slicks past South Cabot Island (Fig. 1), which
77 000 a
12b 18 ~
27 b
Witless Bay
The Drook
Cape Pine
Western Head
a From Nettleship (1980). b From present study.
10000 °
1 500 °
Baccalieu I.
Cape St Mary's
2 600 b
400 000 °
Breeding population (pairs)
South Cabot I.
Funk I.
Colony
20000
1000
500
100
730 000
10000
30 000
3 200 000
Maximum breeding capacity (pairs) South Cabot Baccalieu Funk Baccalieu Witless Bay South Cabot Funk Island The Drook Cape Pine Baccalieu Western Head Cape Pine Witless Bay Western Head The Drook Cape St Mary's Witless Bay Cape Pine The Drook Cape St Mary's Witless Bay Western Head Cape Pine The Drook Witless Bay
Neighbouring colonies
62 184 62 128 104 128 184 83 103 104 115 21 83 12 21 58 103 12 32 47 115 47 58 74 153
Distance to neighbouring colony (km) fair poor good poor fair poor poor poor poor poor poor poor fair poor poor poor fair fair poor poor fair poor poor poor poor
as a refugium
fair poor poor poor poor poor poor poor poor poor poor poor poor fair poor poor poor fair poor poor poor poor poor poor poor
as a growth centre
Suitability of neighbouring colony
TABLE 1 Present and Potential Populations of Newfoundland Common Murre Colonies, and their Suitability as Refugia and Growth Centres
6
D.K. Cairns, R. D. Elliot
would lessen the chance that birds there would be affected by the spill. However, the role of South Cabot as a refugium and a growth centre would be limited because of its small population and the limited habitat available for colony expansion. The nearest neighbours to the large murre colony at Witless Bay are Baccalieu and the three small colonies on the southeast Avalon Peninsula (Fig. 1). These are all too far from Witless Bay to serve as effective growth centres (Table 1) and are poor refugia because they are small and distant. If the colony at Cape St Mary's were destroyed by a n oil spill, recolonisation might come from the small colonies on the southeast Avalon, which are upwind and upcurrent from Cape St Mary's. However, the tiny size of these colonies would limit their role as refugia, and most recolonisation would probably be from Witless Bay. This analysis suggests that regional population recovery would occur slowly if a major Newfoundland murre colony were destroyed by an oil spill. Some small colonies have large neighbours which could serve them well as refugia or growth centres (Table 1), but the three large colonies where 99% of Newfoundland common murres breed do not have wellplaced refugia or growth centres. If we assume a breeding productivity of 0.7 chicks per nest and that 35% of fledglings reach breeding age (Birkhead & Hudson 1977), then a pre-breeding cohort that escaped an oil spill would represent about 12% of the original colony population at the time the cohort reached breeding age. Although the philopatry of pre-breeding cohorts absent from the colony at the time of the oil spill is uncertain, these birds probably offer the best chance for regional population recovery following a devastating oil spill.
CONCLUSIONS A N D I N F O R M A T I O N NEEDS Accurate assessments of seabird oil spill sensitivity depends on information which is frequently sparse or lacking (Wiens et aL, 1984). As our case study demonstrates, more data are required on the following factors for complete assessment of recovery potential following the destruction of all breeding seabirds at a colony.
Inter-colony recruitment patterns Harris (1983) estimated that 23% of young Atlantic puffins immigrate to non-natal colonies, and suggested that such behaviour is widespread among auks. The frequency of non-fidelity to natal site is important in estimating rates at which devastated colonies would be restocked.
Oil spills and seabirds
7
Movement of pre-breeding cohorts The absence of immature birds from their natal colonies during one or more breeding seasons may have a major effect on the availability of recruits to restock a decimated colony.
The dynamics of colony foundation and growth Although models of population recovery following oil spills (Ford et al., 1982; Samuels & Lanfear, 1982) assume no inter-colony movement, many seabird colonies have grown faster than intrinsic rates of increase (e.g., Tuck, 1961; Nelson, 1978; Sowl, 1979; Harris, 1984). Accurate prediction of recovery rates requires that the roles of immigration and internally generated growth be clarified through studies of the development of newly founded colonies.
The potential of pre-breeders to repopulate a colony The ability of naive birds to nest successfully in the absence of experienced breeders is poorly known, although the initiations of new colonies (e.g., Nelson, 1978) indicate that they may be successful.
Size and nest-site availability of small colonies Although small colonies may be important in recovery of regional populations following oil spills, many seabird censuses cover only large ones.
Food competition and foraging range In several seabird species, colony size tends to be inversely correlated with the size of neighbouring colonies (Furness & Birkhead, 1984). This suggests that destruction of a colony might be followed by density-dependent growth of other colonies which share potential foraging ranges. However, the link between seabird populations and food supply in the breeding season has not been demonstrated directly (Birkhead & Furness, 1985), and further information on these factors is required before the role of growth centres in population recovery can be fully assessed. Because of many methodological difficulties, these information needs are unlikely to be met in the near future. Nevertheless inter-specific and interregional comparisons, based on qualitative assessments of recovery potential, may be useful in planning petroleum-related activities. For
8
D.K. Cairns, R. D. Elliot
example, c o m m o n murres, which breed at only eight sites in Newfoundland, are likely at greater risk from an oil spill that destroys a colony than are Atlantic puffins, which breed in at least 19 colonies (Cairns et al. unpublished data). Similarly, Newfoundland murres are at a higher risk from a colony-destroying spill than those breeding in Labrador and the Gulf of St Lawrence, where populations are dispersed in numerous medium and small colonies (Brown et al., 1975). These comparisons suggest that high priority should be placed on the avoidance of petroleum activity that involves risk of spills near Newfoundland murre colonies.
ACKNOWLEDGEMENTS We thank J. Piatt for providing information on seabird colonies and E. Murray, N. Wakely, J. Wells and E. A. Young for helping with censuses. T. R. Birkhead, G. Finney, M. P. Harris, W. A. Montevecchi and J. Piatt improved the manuscript with their comments. The Newfoundland and Labrador Wildlife Division authorised our observations on F u n k Island, Witless Bay, and Cape St Mary's, which are Ecological Reserves under the provincial Wilderness and Ecological Reserves Act. Field work was supported by the Canadian Wildlife Service and grants from the Natural Sciences and Engineering Research Council of Canada to W. A. Montevecchi. This is Contribution no. 130 of the Newfoundland Institute for Cold Ocean Science.
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