The Braer - 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 (Suddaby 1991, 1992). The previous counts main findings were: 

*1: British wildfowl population figures are taken from Kirby et al (1993) and other species from Lack (1986).
*2: 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. 


	       	Mean	Mean   % difference
Section     Oil   	1991/2	1993	1991/2
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1	  0	 51	 51	  0%
2	 2/3	 1	 5	+370%
3	 3/4	 29	 13	 56%
4	1/2/3	 95	 69	-27%
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TOTAL           	176     	138    	 -22%

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. 


		 	Mean	Mean    % difference
Section	 Oil     1991   1991/2	1993	1991	1991/2
--------------------------------------------------------------------------------------------
1	  0       508      646	319	-37	-51%
2	 2/3                 351    427	 	+22%
3(i)	 3/4	      279	180	 	-35%
3(ii) 	  3       28	  	18		-36%	  
4              1/2/3   804	      808	329	-59	-59%
--------------------------------------------------------------------------------------------
TOTAL               1340     2084   1273           -50           -40%

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. 


			Mean	Mean	% difference
Section	 Oil	1991   1991/2	1993	1991   1991/2
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1	  0	 773	720	719	 -7%	  0%
2	 2/3	  	638	489	 	-23%
3	 3/4	 155	158	 72	-54%	-55%
4	1/2/3	 402	391	435	+8%	+11%
--------------------------------------------------------------------------------------------------------
TOTAL 		1330        1907         1715	-8%	-10%

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 (Table 1). 


                        Mean   Mean 	          %difference
Section  Oil    1991   1991/2  1993      1991   1991/2
----------------------------------------------------------------------------------------
1            0	      246	339    226	         -8%	     -33%
2           2/3	 94     75		     -20%
3           3/4     224	120    86	        -62%	     -28%
4         1/2/3    306	319   306	          0%	      -4%
----------------------------------------------------------------------------------------
TOTAL            776    872    693           -20%     -20%

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%.


Black Guillemot
Photo by RSPB


                 Mean                 Mean % difference
Section  Oil 1991 1991/2 1993 1991   1991/2
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1           0      361   220   267   -26      +21
2(i)       2/3            23     41 +78
2(ii)      2/3    121    39     88    -27      +125
3(i)       3/4    55     18     15     -73      -18
3(ii)       3      21                      12       -40
4       1/2/3   404   473    342   -15       -28
-------------------------------------------------------------------------
TOTAL           962     773    765     -25      -3

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. 



 			2/1 	23/1	26.1	5/2 	22/2	9/3	 Dead
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Great Northern Diver 	QB	8	4	2	3	3	8	    5
                     	WS		3	1	2	7	6

Shag		QB	420	0	0	4	4	21	  517
		WS		114	130	142	300	170

Eider		QB	142	12	9	17	32	13	   49
		WS		20	14	26	14	55

Long-tailed Duck	QB	290	25	18	9	20	7	   78
		WS		64	45	57	35	61

Black Guillemot	QB	35	0	0	1	2	6	   83
		WS		1	7	10	12	24

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TOTAL		QB	895	41	29	34	61	55	  732
		WS		202	197	237	368	316

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