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The Effects on Breeding Seabirds Oil spills can affect breeding bird populations in a number of ways but the most obvious are: # Direct casualties i.e. birds being killed or disabled by being in direct contact with the oil # Populations can be affected by sub-lethal doses of oil, received either by direct ingestion or by eating oil contaminated food, This can affect the individuals ability to breed successfully by affecting the birds natural responses. This apparently happened following the Exxon Valdez oil spill, when it was reported that the timing of the breeding season was affected in the case of colonies of Common Murres (Guillemots). However it can also lead to nesting failure or reduced brood sizes. # Populations can be affected by a reduction in the food supply, e.g. fish, numbers being reduced as a result of mortality as part of the oil spill. This fear was raised when a large kill of various fish species was discovered in the Quendale Bay and Garths Ness area during the early days of the Braer spill. Again food shortages could lead to nesting failure or reduced brood sizes. The two main groups of birds affected by the spill were locally resident species (e.g. Shags, Tysties, Eiders, etc.) and winter visitors from the north (e.g. Long-tailed Ducks, Great Northern Divers). There were also two main groups of birds little affected by the spill. Firstly those species that are largely summer visitors to Shetland (e.g. Guillemots, Razorbills, Puffins, skuas, terns and Red-throated Divers); the bulk of the populations of these species would have been out of Shetland waters at the time of the spill. Secondly, there are Shetland resident species (e.g. Cormorants, Gannets and Fulmars) which, as few casualties were reported for these species, can be assumed to have either moved to other parts of Shetland, probably as a reaction to the weather to gain a lee shore from the prevailing south westerly gales, or perhaps offshore in the case of Fulmars and Gannets. In any review of Shetland's seabird populations the food shortage of the 1980's should be remembered. Between 1984 and 1989, sandeels (one of the basic food fish for many of the seabirds in Shetland) became less available, with the result that there was a significant reduction in the breeding success especially amongst the smaller surface feeding seabirds. Glasgow University Applied Ornithology Unit undertook a 3 year study of the seabirds at Sumburgh (1990 to 1992) in response to this reduction in breeding success and were funded for a further year to look at seabird breeding success at Sumburgh in the wake of the Braer. These studies concentrated on monitoring nests and radio tracking breeding adults to ascertain chick feeding regimes. Their findings indicate that the 1993 season was successful for seabirds monitored at Sumburgh Head compared with previous years. Red-throated Diver The 1993 breeding season was an early season, although the overall number of successful pairs (that is, pairs which managed to rear any chicks), was lower within the study areas than in 1992. Divers may raise broods of either one or two chicks and the number that manage to rear broods of two is one measure of the relative success of the season. The ratio of one chick broods to two chick broods fell from 1:0.74 in 1992 to 1:0.65 in 1993. The poor breeding season was not evenly reflected throughout the county however, both Hermaness and Foula held good numbers of successful pairs with most other areas less successful, especially the West Mainland and Yell. Cormorant Shetland's Cormorant colonies have been slowly declining since at least the mid 1970's. However, there were 264 pairs in the colonies in 1993, compared with 225 pairs in 1992 (all colonies surveyed). The mean brood size across all colonies was 3.1 chicks/nest, which is slightly above average and the same as in 1992. Black Guillemot After the Esso Bernicia oil spill at the Sullom Voe Terminal in December 1978, SOTEAG commissioned Oxford University to devise a method to monitor changes in breeding numbers of Black Guillemot. This work resulted in a bi-annual programme of surveys, alternately in Yell Sound where the numbers were severely reduced by the Esso Bernicia spill, and at 12 further sites around Shetland not directly affected by the spill. Thereafter, a complete Shetland census of pre-breeding adults was carried out in the springs of 1982-84. The plan for the spring of 1993 was to carry out a census of the south Mainland coast of Shetland (within which there are four control sites for Yell Sound), and survey the other monitoring sites including Yell Sound and Fair Isle - twice. Despite poor weather in late March and early April, the vast majority of this work was carried out. Although the detailed results require careful interpretation, counts up to 1992 at monitoring sites suggested a general increase in numbers since the early 1980's, especially on the west coast. For much of the coastline however, the only comparisons that can be made are with the 1982-84 counts. Preliminary results of the spring 1993 counts have indicated that the numbers of Black Guillemots on the south-east Mainland coast were similar to the most recent counts, made in 1991/92. In the area of Quendale Bay and the Fitfull Head cliffs immediately to the north of the Braer, numbers were generally lower than in 1983 but there are anomalies. Along the cliffs a kilometre each side of the wreck, 58 birds were counted in 1983 compared with 96 in 1993, Further north along the coastline, numbers at West Burra were reduced by 35% since 1992 and there may also have been a decrease among the Scalloway Islands, although further north still there was little change since the most recent counts in regularly monitored sections, and considerable increases since the 1982-84 counts on other coasts. Surveys on Fair Isle found a 25% decrease since the last count of the whole island in 1989. The conclusion is therefore that the Braer appears to have had little or no effect on breeding numbers of Black Guillemots along the south-east Mainland coast, although along the south-west coast (allowing for probable decreases between 1982/83 and 1992), decreases between 1992 and 1993 in some areas (e.g. the Scalloway Islands) has probably been greater than the 35% observed at the Burra monitoring sites, but probably less elsewhere (e.g. counts of 77 at St. Ninian's Isle in 1983, and counts of 75, 72 and 86 in 1993). Shag Breeding success studies began in March 1993 at Sumburgh Head and quickly confirmed a decrease in numbers in this area. By the 21st April, birds had reoccupied only 60% of the nest sites used in the 1992 study plots, and were building nests or had laid eggs in 47% of these. After experience gained during fieldwork for the Seabird Colony Register (SCR) the overall monitoring strategy for Shags has been to count and map nests and adults along entire stretches of coastline from an inflatable boat, with land counts made only at colonies where a high proportion of nests could be seen from the clifftop. In June 1993 these counts were repeated for as much coastline as possible for which post-SCR counts existed. At Sumburgh Head a count from the land revealed 151 nests compared with 304 in 1992 and 508 in 1988. No 1992 counts were available for the coast of south-west Mainland (where smaller numbers breed), and while 1993 counts indicated considerable reductions (e.g. 9 nests at St. Ninian's Isle compared with 38 in 1989), it is less clear exactly when the decreases occurred. Further north along the west coast and along most of the east coast, the 1993 counts indicated relatively small decreases in breeding numbers that broadly correspond with the recent trend in a slow reduction of breeding numbers. A survey of Fair Isle found 946 nests, only a 9% decrease on the 1043 recorded in 1990. There are three main Shag ringing sites at Sumburgh Head and the number of pairs at these sites had dropped, with the largest drop on the West Steps site. In the main ringing area there was only one nest (with eggs), and it was the first time that chicks have not been ringed in this area since it was first visited in 1978. The reduction in numbers was not even throughout the colony with some areas worse hit than others. A small colony north of Compass Head was reduced from c. 12 pairs to 3. Compared to the average total of nests in the ringing sites during the last four years, the 1993 total was only 61 %. However, numbers of shags in these colonies have been falling for the last few years and the 1993 total is 75.5% of the number of nests in 1992. The average brood size of chicks at the age of ringing is given below for the Sumburgh colony since 1982. |
Oil 0 2/3 3/4 1/2/3 Total |
Mean 1991/92 51 1 29 95 176 |
Mean 1993 51 5 13 69 138 |
% difference 0% +370% -56% -27% -22% |
Section 1 2 3 4 |
Section 1 2 3(i) 3(ii) 4 |
Oil 0 2/3 3/4 3 1/2/3 Total |
Mean 1991/92 646 351 279 . 808 2084 |
Mean 1993 319 427 180 18 329 1273 |
1991 508 . . 28 804 1340 |
% difference 1991 -37% . . -36% -59% -50% |
% difference 1991/92 -51% +22% -35% . -59% -40% |
Section 1 2 3 4 |
Oil 0 2/3 3/4 1/2 Total |
1991 773 . 155 402 1330 |
Mean 1991/92 720 638 158 391 1907 |
Mean 1993 719 489 72 435 1715 |
% difference 1991 -7% . -54% +8% -8% |
% difference 1991/92 0 -23% -55% +11% -10% |
Section 1 2 3 4 |
Oil 0 2/3 3/4 1/2/3 Total |
1991 246 . 224 306 776 |
Mean 1991/92 339 94 120 319 872 |
Mean 1993 226 75 86 306 693 |
% difference 1991 -8% . -62% 0 -20% |
% difference 1991/92 -33% -2-% -28% -4% -20% |
Section 1 2(i) 2(ii) 3(i) 3(ii) 4 |
Oil 0 2/3 2/3 3/4 3 1/2/3 Total |
1991 361 . 121 55 21 404 962 |
Mean 1991/92 220 23 39 18 . 473 773 |
Mean 1993 267 41 88 15 12 342 765 |
% difference 1991 -26% . -27% -73% -40% -15% -25% |
% difference 1991/92 +21% +78% +125% -18% . -28% -3% |
Great Norther Diver . . Shag . . Eider . . Long-tailed Duck . . Black Guillemot . . Total |
QB WS . QB WS . QB WS . QB WS . QB WS . QB WS |
02/01 8 . . 420 . . 142 . . 290 . . 35 . . 895 . |
23/01 4 3 . 0 114 . 12 20 . 25 64 . 0 1 . 41 202 |
26/01 2 1 . 0 130 . 9 14 . 18 45. . 0 7 . 29 197 |
05/02 3 2 . 4 142 . 17 26 . 9 57 . 1 10 . 34 237 |
22/02 3 7 . 4 300 . 32 14 . 20 35 . 2 12 . 61 368 |
09/03 8 6 . 21 170 . 13 55 . 7 61 . 6 24 . 55 316 |
Dead 5 . . 517 . . 49 . . 78 . . 83 . . 732 . |
Year Brood Size |
82 2.3 |
83 2.0 |
84 2.2 |
85 2.0 |
86 2.3 |
87 2.1 |
88 2.4 |
89 2.1 |
90 2.1 |
91 2.2 |
92 2.1 |
93 2.0 |
Shag - The species most affected by the spill - both in breeding & wintering numbers. |
Great Northern Diver - wintering numbers declined around the south Mainland following the spill. |
Eider - Numbers declined by about 10% around the south Mainland following the spill. |
Black Guillemot (Tystie) - declined by approximately 80% in the vicinity of the spill. |
Long-tailed Duck - declined by around 20% in the Quendale Bay area. |
Kittiwake - breeding numbers have been declining for several years & it is unclear whether the spill had any significant population effects. |
braer.net |
M.V. Braer grounding and oil spill Garth's Ness, Shetland, 5 Jan 1993 |
Origins of Ringed Seabirds found dead. A total of 39 ringed birds were recovered as a result of the Braer oil spill. Thirty-four were Shags, fifteen from the nearby Sumburgh colony, and while this was the worst hit colony, birds from colonies throughout Shetland, (and as far away as Orkney), suffered mortalities. The two main groups of birds affected by the oil spill were locally resident species (e.g. Shags, Tysties, Eiders, etc.) and winter visitors from the north (e.g. Long-tailed Ducks, Great Northern Divers). Of the local resident species Shags have been a target species for ringing effort and this was reflected in the numbers of ringed birds found. None of the winter visitors found dead were ringed e.g. Long-tailed Ducks, Great Northern Divers, etc., probably because very few individuals of these species have been ringed in their natal areas. Shag There were 15 birds recovered that had been ringed at the colony at Sumburgh, 11 from Fair Isle, 6 from Foula and singles from Hermaness in Unst and Swona in Orkney. This was 3.5% of the Shags collected. All of the colonies in Shetland where any significant numbers of Shags have been ringed are represented. It is not surprising that the bulk of the recoveries come from the Sumburgh colony as this is by far the closest to the site of the wreck. However because a chick was ringed in a colony it does not necessarily mean that the bird will return to that colony to breed, but it is likely that a large proportion of Shags will do so. Four birds ringed as breeding adults (2 from Sumburgh and singles from Foula and Fair Isle), were recovered. It was previously assumed that a large proportion of breeding Shags were sedentary, remaining in the vicinity of their colony throughout the year. If this had been the case then the bulk of the dead birds would have been from the colony at Sumburgh which would consequently have been wiped out by the spill. In fact, the recoveries show that there were birds present from over a wide area and that breeding adults from other colonies were in the area at the time of the spill. It is possible that birds from remote colonies such as Fair Isle and Foula are inclined to move to the Mainland during winter as the outer isles will be particularly exposed to severe weather conditions. Eider A single ringed bird was recovered during the spill. This had been ringed as a breeding female on Fair Isle in 1987. Presumably Eiders move to the more sheltered areas of Mainland during the winter to avoid the exposed coastlines of the outer isles. If a significant number of the casualties were established breeders rather than juveniles, recovery of the population could be long term. Black Guillemot Two ringed birds were recovered during the incident, one ringed and recovered within the Scalloway Islands, the other a bird ringed as a chick on Isbister Holm off Whalsay in 1982 and recovered at Quendale. This was quite a significant movement for a Black Guillemot as most are very sedentary and represents the furthest movement of any Tystie in the records of the Shetland Ringing Group. Kittiwake A single bird was recovered, having been originally ringed in the Farne Islands in 1964. This is the oldest Kittiwake recorded in the British ringing scheme. Fairly large numbers of Kittiwakes have been ringed in some of the Shetland colonies over the years and the fact that none of these locally ringed birds were found amongst the 136 birds collected in the incident would suggest that many of the individuals involved were not from Shetland colonies. The Shetland Ringing Group has no previous records of birds from Shetland colonies being recovered in the islands in winter. The only recovery of a Kittiwake in Shetland in winter was of a bird from a colony in West Greenland. Birds from Shetland colonies have been recorded in the southern North Sea and France during the winter. |
The Effects on Wintering and Breeding Seabirds following the spill The effects on Wintering Seabirds From the 23rd January to the 9th March 1993 the RSPB coordinated winter seabird surveys with assistance from SNH and SOTEAG, to assess the immediate impact of the oil spill on the seabirds present in the affected areas. Fortunately these counts could be compared with previous recent surveys of the same areas carried out by SNH from February to March 1991, and from November 1991 to March 1992. The previous counts main findings were: # approximately 4% of the British wintering population of Great Northern Diver (50% of the Shetland wintering population) winter in the south Mainland *1 with the main concentrations between Wadbister Voe to South Nesting Bay and from Skelda Voe to Whiteness Voe. # approximately 2% of the British wintering population of Eider (25% of the Shetland population) winter in the south Mainland with the main concentrations between Gulberwick and Laxfirth, Mousa Sound and from Skelda Voe to Kettla Ness. This species is particularly vulnerable as the Shetland population has undergone considerable decline in recent years. # approximately 4% of the British wintering population of Long-tailed Duck (44% of the Shetland wintering population) winter in the south Mainland with the main concentrations between Gulberwick and Laxfirth, Quendale Bay, and from Skelda Voe to Kettla Ness. # approximately 1% of the British wintering population of Shag and Black Guillemot winter in the south mainland. The Shetland population of Shag is steadily declining, while that of Black Guillemot is considered to be increasing. *1: from Skelda Ness south around to South Nesting Bay, hereafter regarded as south Mainland. Counts only took place in favourable weather conditions, with good visibility, winds below force 4 and calm sea conditions. The coastline of south Mainland (Skelda Ness to South Nesting Bay) was divided into four main sections each of which was then divided into areas which could be counted by one observer between the hours of 0900 to 1600 (Map 1). All but one area were land based counts, the exception being the Scalloway islands, which were counted by a combination of a fixed zodiac route and land based counts from three of the islands. Section totals were derived by adding together the area totals. If an area had multiple counts then a mean count was taken, and each winter count was compared by percentage differences. Sections counted were restricted mainly to the 'oil affected' areas, i.e. 2,3 and 4, with one count from a 'non-oil affected' area, i.e. section 1. Map1. The southern half of Shetland showing the four count areas for wintering seabirds, and the areas counted within each section. |
The most important species, in both local and national terms, recorded in previous surveys were Great Northern Diver, Shag, Eider, Long-tailed Duck and Black Guillemot. Totals and percentage differences between this and previous surveys of these five species in each section are given in each species account. Counts in 1993 of Quendale Bay and the West Voe of Sumburgh are given in Table 1. The overall reduction in numbers from previous surveys in the south Mainland was by approximately 20% (c.1,200 birds). Section 3, especially around Quendale Bay, showed the largest apparent reduction in numbers from previous survey data, approximately 60%. North of this, on the west side of south Mainland (section 4) numbers were reduced by approximately 25% and on the east side (section 1) by approximately 18%. The only slight increase in numbers was in section 2, approximately 1 %. The following tables in the species accounts show totals and percentage differences between species in each section surveyed. Oil in each area on the 9th January 1993 (worst extent) was categorised as, no oil =0, streaks of sheen = 1, light sheen =2, heavy sheen =3, and heavy oil =4. Great Northern Diver Numbers had declined by 38 birds (22%) from the 1991/92 count, which represents approximately 13% of the Shetland wintering population (1% of the British wintering population). Of the three main wintering areas, numbers in section 1 remained stable whereas sections 3 and 4 showed an apparent reduction in numbers of 56% and 27% respectively. |
Shag Numbers had declined by approximately 500 birds (1991 count) and 800 birds (1991/92 count) (50% and 40%). The only increase in numbers was noted in section 2, with 22%. The other sections (1, 3 and 4) showed an apparent reduction in numbers by 100 and 500 birds, in section 1 by 37% and 50%, section 3 by 35% and section 4 by 59%. Within section 3, the largest apparent reduction in numbers was in Quendale Bay (Table 1) and within section 4, the largest apparent reduction in numbers was in Whiteness Voe, off West Burra and in Clift Sound. |
Eider Numbers had declined by approximately 100 birds (1991 count) and 200 birds (1991/92 count) (8% and 10%) which represents approximately 2% of the Shetland population. Numbers in sections 1 and 4 remained relatively stable with apparent reductions in numbers of approximately 23% and 55% in sections 2 and 3 respectively. |
Long-tailed Duck Numbers had declined by approximately 150 birds (20%) which represents approximately 7% of the Shetland wintering population. Numbers in section 4 were similar to the 1991 and 1991/92 counts whereas the other sections showed an apparent reduction in numbers by 30-150 birds. Quendale Bay suffered the largest reduction in numbers. |
Black Guillemot The largest apparent reductions in numbers were within section 3, of up to 73% and section 4, of 15%-28%. Numbers in Quendale Bay declined by 83% after the Braer had grounded (Table 1) and within section 4 numbers declined in Whiteness Voe, off West Burra and in Clift Sound by 60%, although in the northern half of the Scalloway islands numbers increased by approximately 30%. |
Seabirds, especially Black Guillemot, are difficult to census during the winter due mainly to variable light conditions and sea states, which greatly affects the visibility of the birds to the observer. As there were few counts carried out in each area within the sections over the past three winters, there may be inaccuracies in the data presented here. Persistent strong winds can cause movement of birds from exposed areas to more sheltered coastlines and during each winter the weather was similar with strong to gale force winds predominantly from a westerly direction. The total number of birds counted in each section were very similar on the 1991 and 1991/92 surveys, suggesting that movements of birds in response to weather are mainly local within each section. Therefore the differences in the numbers found between 1993 and the previous two surveys in sections 3 and 4 are considered to be mainly a result of birds dying of oil contamination. Section 1 was surveyed as a control area and showed no increase in total numbers to compensate for the reduced numbers in sections 3 and 4. However in section 2 there was an increase, mainly of Shags, suggesting a movement of birds possibly from section 3, although numbers in section 2 did not decrease during February and March when the storms abated and numbers increased around Sumburgh Head and the West Voe of Sumburgh. The reduction in Shag numbers in section 1 may be attributable to birds moving into section 2 but some variation in wintering numbers is perhaps to be expected. Quendale Bay suffered the heaviest impact of the oil and the largest reduction in seabird numbers with at least 732 birds found dead. It is interesting to note that by the end of February small numbers of Black Guillemot (2-6 birds) and Shag (4-21 birds) were seen back in Quendale Bay although to the east of Scatness in the West Voe of Sumburgh the situation was different with c.300 Shags and c.20 Black Guillemots present (Table 1). This may have been due to little food being available in Quendale Bay. The reduction in Shag numbers in section 4 is mainly attributable to birds dying of oil contamination and exhaustion as reflected by numbers found dead or oiled on beaches in this section. This is also the case for Black Guillemots although there may have been movement from the Trondra and Burra area to the northern half of the Scalloway islands (less affected by oil) where there was an increase of approximately 50 birds overall from previous winter counts. The counts of the Trondra and Burra area on the 27th January 1993 and 9th February 1993 were of 56 and 50, respectively, increasing to 131 by the 3rd March 1993 suggesting a movement of birds back into the area. Eider numbers only decreased markedly within section 3. The reduced numbers in section 2 may have been due to birds not being recorded as they could have moved to the north and east coast of Mousa where they would not have been seen from the Mainland. The similar numbers of Eider recorded within sections 1 and 4 in the three winters is reflected in the small numbers found dead or oiled on beaches in these sections. This was also the case for Long-tailed Duck. The reduced numbers in section 1 may (as in Shag) have been affected by small local oil spills around the Lerwick Harbour area during December 1992 although there was no direct evidence for this. The number of Long-tailed Duck wintering in Shetland can fluctuate between years and prior to the Braer grounding there was evidence that the 1992/93 wintering numbers were relatively high (M.Heubeck pers comm.), as reflected by the count in Quendale Bay on the 2nd January (Table 1) and therefore the oil spill mortality may have been greater than inferred from the species counts. Table 1. Totals of Great Northem Diver, Shag, Eider, Long-tailed Duck and Black Guillemot, before and after the Braer grounded, in Quendale Bay (QB) and West Voe of Sumburgh (WS) from January to March 1993 with totals of these species found dead between Quendale Bay and West Voe of Sumburgh (mostly in Quendale Bay) up to the end of January 1993. |
Although there was an obvious reduction in breeding numbers, the average brood size was only slightly lower than the last few years, and the chick growth curve (wing lengths x weights) of the measured shag chicks in 1993 was better than that for 1991. Guillemot and Razorbill The colonies at Sumburgh and Compass Heads had a very successful season, although the season was over a week earlier than usual. Visits to these colonies were made in late June and early July and by this time many of the chicks were large and in the process of fledging. Undoubtedly many of the chicks had fledged by the time of the visits. There were large numbers of display fish discarded in the Guillemot colonies, noticeably more than in previous years, largely sandeels of a wide range of sizes, and the chick growth curves for both species (wing lengths x weights) of the measured chicks in 1993 was better than that for 1992. Puffin Puffin chick survival was investigated at Sumburgh Head and Hermaness, Unst. From the beginning of July. 26 chicks at Sumburgh Head and 27 chicks at Hermaness were examined at approximately seven day intervals until the last chick had left its burrow. The percentage of the sample of chicks which survived to fledging (chick survival) at Sumburgh Head was 100%, a continued increase since 1990 when monitoring began, chick survival in 1990 being 56%. In contrast, at Hermaness, chick survival decreased from 87% in 1992 to 78%, although this was still an increase on 1990 and 1991 when 56% and 67% respectively, of chicks survived to fledging. The reasons for this decrease in 1993 are not clear but may have included increased predation by Great Skuas and rats and a shortage of sandeels. All monitored chicks at Sumburgh Head fledged before the 2 August and at Hermaness before the 10 August. Arctic Tern Numbers (counted in the first week of June) at 53 colonies declined slightly from 4305 adults in 1992 to 4242 in 1993 (-1.5%). The mean clutch size per pair increased from 2.14 in 1992 to 2.42 with a c.60% hatching success and productivity, (expressed as young fledged per Apparently Incubating Adult (AIA)) decreased from 0.65 in 1992 to 0.42, Large numbers of chicks of all ages were found either in a weak state or dead from the end of June onwards, reflecting both the adverse weather conditions and the acute food shortage from the first week of July. As in 1992, Mousa suffered heavy predation from otter/s and several other colonies by predation from Great Skuas, notably at Noss Arctic Skua Numbers at eleven monitored colonies decreased from 194 Apparently Occupied Territories (AOTS) in 1992 to 171 AOTs (-12%), all but two (which showed no change) decreased by 4% - 23%. The mean clutch size (1.85) and hatching success (78%) were slightly lower than in 1992. Productivity (expressed as young fledged per AOT) was 1.08, very similar to 1992, with a slight decreases on Yell, Fetlar and Noss due to predation from Great Skuas and Great Black-backed Gulls. Great Skua Numbers at eight monitored colonies increased slightly from 258 AOTs in 1992 to 267 AOTs (+3%). The numbers at Noss Hill (36 AOTs in 1993) continued to increase, with a 20% increase from 1992. The mean clutch size increased slightly from 1.79 in 1992 to 1.87, the slight increase as a result of the success on Noss and Noss Hill. Hatching success (76%) was similar to 1992. Productivity increased from 0.68 young fledged per AOT in 1992 to 0.82, the increase being noted on Noss, Mousa and Mainland where very little cannibalism was noted, although post-fledging cannibalism by adults was noted at Hermaness, Lambhoga and Noss. Kittiwake The Shetland Kittiwake population has declined markedly in recent years, but at greatly varying rates at different colonies or regions. In 1993, counts of nests and adults were made at all colonies except those on Foula, Noss, north-west Mainland and Unst. Apart from increases at some of the smaller colonies on Fetlar, Yell and Out Skerries, the recent decreases had continued, in some (but not all) areas at greater rates than recorded recently. At colonies in the southern half of Shetland (from Watsness in the west to Mousa in the east), totals of 3693 nests were recorded in 1993, 4917 in 1991 and 8905 in 1981. Because of the considerable recent changes at colonies it is difficult to know how much the reductions in 1993 were attributable to the Braer or simply reflected a continuing downward trend. Eider Surveys of flocks of moulting Eiders suggest that after a considerable decline from an estimated 16,500+ birds in 1977, the Shetland population (assumed to be largely resident) had stabilised by 1991/92 at c.7,200, again based on surveys of moulting flocks which located 6544 birds. In January 1993 most of the 122 oiled Eiders were found in the Sumburgh area with later counts of moulting birds about 450 lower than in 1992. The proportion of the total population moulting in each area varies from year to year but it is interesting to note that further north on the oil-affected coast of south-west Mainland (where few oiled Eiders were found), moulting numbers were actually higher in 1993 (636) than in 1992 (529). It is likely therefore that the impact of the Braer on Shetland's Eider population was relatively small and that had the oil spill not occurred, numbers may have increased slightly between 1992 and 1993. In general, the 1993 breeding season was successful for most species that may have been affected by the Braer. The exceptions however, were Shags whose breeding numbers were significantly reduced at Sumburgh and to a lesser degree at other south Mainland colonies, and Black Guillemots with numbers along the south-west Mainland coast reduced by 30-40%, although few appear to have been affected along the south-east Mainland coast. There was little evidence that other species suffered from food shortages or a reduction in breeding numbers as a result of the Braer. Brood sizes and chick growth were good in species such as Guillemot, Razorbill, Tystie, Cormorant and Shag and although Arctic Terns and to some extent Kittiwakes had some mortality amongst chicks late in the season, this occurred in many colonies away from the area contaminated by the Braer spill and was presumably due to difficulties in obtaining sandeels, as happened throughout much of the 1980's. Red-throated Divers had a poor year but most of these breed and feed outside the area likely to be directly affected by the Braer. |
Handbook on Oil Impact Assessment - Seabirds As part of an EU project (2006-2007) a Handbook for Impact Assessment is being developed. A scientific workshop has been held in La Coruņa, Spain in September 2006, where scientists from all across Europe discussed the contents of the Handbook and identified the main issues with regards to the integration of Impact Assessment into oiled wildlife response and the wider oil spill response. The main conclusions from the workshop were: * Where possible, the impact of an oil spill incident on seabirds should be prevented where possible, by using ideally pre-spill prepared information on the seasonal distribution of vulnerable species and consulting scientists in the planning of response operations * Impact Assessment should be an integrated part of oil spill response planning * There is a need for an internationally developed manual, protocol or set of guidelines that could be used to design an adequate, standardized system of data collection and analysis as an integrated part of oil spill response so that a reliable assessment of impact on wildlife populations can be made. Handbook on Oil Impact Assessment 4.0 SPILL RESPONSE 4.1 Assessing the damage Introduction The four basic questions to be answered by an impact assessment are (i) what has been killed, (ii) how many, (iii) where did they come from, (iv) can any effect be detected at the population level? Demonstrating any effect on seabird populations of even a major oil spill has not been easy, and few case studies have been able to do so convincingly. Firstly, the different species affected must be identified and the mortality quantified. This normally involves searches of the polluted coastline for oiled victims (beached bird surveys), and these surveys are most effective if they are planned and directed, in co-operation with the teams responsible for cleaning up the oil. Not all birds that become oiled will be found on even the most thorough beached bird surveys, and this proportion will vary depending on factors such as the distance from shore of the point source of the spill, the nature of the coastline, the type and volume of oil, and prevailing winds. A drift experiment, where real or simulated seabird carcasses are marked and dropped in the vicinity of slicks, to be later recovered on beached bird surveys, can help estimate the proportion of the total mortality that was actually found. This is best planned for in advance, and conducted during the early stages of an incident. For many seabird species the only data on population sizes and trends comes from counts at the breeding colonies, but most seabirds are migratory, or at least spend much of the year well away from their breeding sites, whereas most oil spills occur during winter months. A small proportion of seabirds have been ringed (banded) at their colonies, which may indicate breeding origins, but for most species this percentage is either extremely small or heavily biased towards particular well-studied locations. Fortunately, widespread seabird species normally vary in size over their breeding range and careful measurements (biometrics) can help indicate likely breeding origins, and where best to look for population impacts. Seabirds also have a period of immaturity lasting several years before attaining breeding age, with the different age classes having different natural survival rates, and often different winter distributions. It is therefore important to established the age structure of the mortality for each species, since their populations are most likely to be impacted if the victims are mostly breeding adults (with normally high winter survival), rather than birds in their first winter, many of which would have died naturally before the following summer. Finally, it is important to establish the sex ratio of the mortality since male and female seabirds can have different wintering distributions, and an incident could kill adult birds of either sex disproportionately. These key elements of a wildlife response for impact assessment are generic to any incident: organised beached bird surveys, a drift experiment where feasible, and the recording and examination of the dead birds collected. This needs close co-operation with those involved in the rehabilitation response for live birds, and with the authorities in charge of the overall response. Being prepared should also involve collating data on the numbers and distribution of the wildlife resources at risk in your area. This will not only help guide the overall response, but may itself be part of the impact assessment for species such as arctic waterfowl whose breeding origins, numbers and densities are poorly known, and for which counts in winters following the spill may be the best means of detecting any population effect. This manual provides guidance for wildlife responders during an oil spill, and introduces standard protocols and methodology that aid the impact assessment and help provide comparisons between different incidents. Click the link to access the digital handbook http://www.oiledwildlife.eu/drupal -4.7.3/?q=node/243 |