The Braer - Dead Birds and Mammals

Under the WRCC plans all dead birds and mammals found during a large oil spill would be stored frozen until such time as a detailed examination of the sex and age structure of the mortalities could be determined for each species, as well as any other appropriate scientific investigations. All the corpses brought back by the beach teams were bagged, labelled and packed into a P. & 0. freezer container at the WRC. In February, this was shipped down to Edinburgh University's Department of Veterinary Pathology where the very messy examination was undertaken, At the time of writing the resulting bird data is being analysed but it will be a lengthy process and a detailed results will not be available for some time, although some preliminary data on the Shag corpses and an analysis of the gizzard contents of these and a few other species is presented below.

Dead birds, fish and mammals were tagged and frozen for later analysis.
Photo by RSPB

Apart from the fact that a lot of dead birds were never found, there are two problems associated with compiling the Braer casualty list. The first is that the storms during January were causing a high mortality of seabirds quite apart from the oil, and some of these storm victims may have become oiled after death by coming ashore on polluted beaches. Secondly, by the end of January small numbers of freshly oiled birds, which clearly had nothing to do with the Braer, were being found on beaches. Although the seas around Shetland are normally cleaner than in most parts of western Europe, oil pollution on a small scale does occur in winter and so a cut-off point for the Braer casualty list was fixed arbitrarily as the end of January 1993. The 'official' casualty list for January is presented below, which differs from some others as clean corpses and those unlikely to have been killed by the Braer have not been included.

Great Northern Diver		13
Black-throated Diver		1
Fulmar				31
Gannet				2
Cormorant			7
Shag				857
Grey Heron			3
Eider				70
King Eider			1
Long-tailed Duck			96
Red-breasted Merganser		1
Black-headed Gull			2
Common Gull			3
Herring Gull			16
Iceland Gull			7
Glaucous Gull			1
Great Black-backed Gull		45
Kittiwake				133
Guillemot				17
Razorbill				5
Black Guillemot			203
Little Auk				9
Puffin				15

			Total	1538

Examining the individual species in the list, some will have been greatly under-recorded in terms of the number of birds oiled and never found. As an example, in the first few days of the spill, several thousand oiled Kittiwakes and Herring and Great Black-backed Gulls were estimated to be in the Sumburgh/Quendale area, many feeding on the dead fish and marine invertebrates washed up in Quendale Bay. A large proportion of these were judged to be too heavily oiled to have survived, but relatively few corpses of these species were found. In addition, small flocks of shoreline feeding waders such as Turnstone and Purple Sandpiper were seen to have become oiled along Garths Ness and Scatness (and probably also further north, especially along the more polluted beaches on Burra), but no corpses of these species were found.

Shags dominated the mortality with Black Guillemot, Long-tailed Duck and Eider the other main diving seabirds involved, while Kittiwake was the main gull species involved. Thankfully, the very storms which caused the Braer (although causing the wreck of Kittiwakes) kept Fulmars and the pelagic Auks well out to sea, and the numbers of these species found oiled was very low. Fewer Great Northern Divers were found than had been feared and the leucistic bird that has wintered in Sand Sound for the past 12 years also survived, despite the heavy sheens which entered that area, The single Black-throated Diver picked up on Quendale beach was one of two known to have been in the area for several weeks, while the King Eider was a female found lightly oiled at Sand.

Examination of all the corpses at the Edinburgh University Veterinary Field Station showed that almost all were oiled, with the majority heavily so, in particular the Shag corpses. During examination, all the Shags that were mostly intact were aged from plumage characteristics and sexed from examination of the genitals. A proportion of Shags which could not be sexed in this way were tentatively sexed from measuring the gonys depth (the depth of the bill at the widest point) - a depth of more than 10.6mm taken as being a male. The results are shown below with the numbers of birds for which sex was inferred from their gonys depth shown in brackets.

			Male		Female		Unsexed		Total
Adult			95 (15)		240 (33)		30		365
Immature (total)		133 (18)		221 (39)		34		388
First Winter		20 (2)		43 (9)		2		65
Second Winter		20 (2)		28 (2)		0		48
Unaged			4 (2)		10 (8)		44		58

		Total	232		471		108		811

There is a clear excess of females over males which is significant within both immature and adult groups, although the bias is greater for adults than for immatures. The ratio of first to second winter birds was not significantly different from 1:1 for either sex.

As well as providing details on the age and sex structure of the casualty list the examination of the gizzard contents of many of the corpses gave information on the diet of the seabirds around the area of the spill. After birds had been aged, sexed and measured those that were no longer required but had a gizzard present were dissected to remove the gizzard and intestines. Prey remains from the gizzards were stored in 70% alcohol and subsequently identified to the lowest taxonomic group conveniently possible, with most attention given to the identification of fish otiliths *1.

Oiled Black Guillemot corpses awaiting examination at the University of Edinburgh.
Photo by S.McOrist

Most of the Shag gizzards were empty with those containing prey remains mostly fish otiliths, especially sandeel Ammodytes, with most of the other fish otiliths present rather small (3-9mm). A very high proportion of the Shags had parasitic worms in the gizzard. The proportion of Shags with prey in the gizzard declined rapidly with finding date, 375 containing sandeel otiliths on the 6-7th January, but only 7% containing sandeel otiliths after the 16th January.

Black Guillemots held crab remains more often than otiliths, with most of the latter being sandeels. Eiders contained mostly bivalves, gastropods and crabs. Long-tailed Ducks had mainly empty gizzards or small quantities of algal material that may or may not have been ingested while feeding on invertebrates, while their animal prey remains consisted of bivalves, gastropods, sandeels and polychaetes, thus showing only partial overlap with Eider diet.

Kittiwakes had mostly empty gizzards, but sandeels were present in 38% of those with prey. A further 10% contained plastic pellets, as did 50% of the Fulmars examined. Guillemots contained mostly fish otiliths, predominantly those of sandeels or gadids *2. The small numbers of gizzards of other species provided little clear indication of diet, but it was surprising that Great Northern Divers had only bivalve fragments and and fish otiliths on their gizzards

The high frequency of sandeels in diets of Shags, Tysties Kittiwakes and Guillemots in January provides clear evidence that this fish is available to seabirds even during winter, and appears to be the main food of Shags and Guillemots in this season in Shetland. The clear trend for stomachs of Shags to be empty if the bird was collected later after the spill may suggest that some of the later collected birds died of starvation rather than dying rapidly from the immediate effects of the oil. However, the ratio of sandeels to gadids found in the Shag gizzards did not change over time, suggesting that diets and rates of otilith loss in gizzards were not noticeably different over the month. Also the number of otiliths per gizzard with otiliths changed little, the reduction was predominantly in the proportion of Shags with any otiliths in the gizzard.

• 1. A fish otilith is a small bone found in the skull whose size increases with the age of the fish. Because this bone is relatively indigestible in birds stomachs, and because there is only one bone of this type in each fish, by counting and measuring each otilith the number and size of food prey can be identified.
• 2. Gadids are immature fish such as cod, etc. usually quite small, 3-10 cm long.

Of the 23 dead seals found on beaches during the incident, 11 were recovered with the remainder being tagged to prevent double recording. Four of the grey seals were post-mortemed by vets from the Scottish Agricultural College (SAC), three were determined to be pre-spill deaths and the fourth died from unknown causes, though it may have been oil-related. Many of the unidentified seals were badly decomposed and some of these are likely to have also been pre-spill deaths. It is impossible to put these figures into context, since there is very little previous information on the number of dead seals which normally come ashore in Shetland.

Number of dead mammals found on beaches

Grey Seal			12
Common Seal		1
Grey/Common Seal		10
Otter			2

	  	      Total  25

Six dead otters were found during the course of the incident, all of which were examined by an SAC vet. Of these, two were road traffic accidents (one of which was run over by a Norwegian film crew), one was oiled but was found at Brae, well outside the Braer affected area, two were lightly oiled and may have been oil related, and one was not oiled and presumably died from natural causes.

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