The Braer - The Effects on Breeding Seabirds


Oil spills can affect breeding bird populations in a number of ways but the most obvious are:

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 (Heubeck 1989). 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.


Year       1982   83   84   85   86   87   88   89   90   91   92   93
Brood size 2.3    2.0  2.2 2.0  2.3  2.1  2.4  2.1  2.1  2.2  2.1  2.0

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 1 993 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.

 


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