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M. I. Evans and G. 0. Keijl Sand grouse 15
Plate 2. Oiled saltmarsh at Abu Ali (Saudi Arabia), Apri11991. (Arnoud B. van den Berg)
ered by oil at one time or another due to continual re-flotation and re-depositionof oil by the tides. Only two small areas had escaped oiling by then: the innermostreaches of a small tidal inlet at Safaniyah, and a small bay in the Ras Tanajibembayment (count site 6, Figure 2, p. 59). However, the latter site was totally oiledon 9-10 May by refloated oil.
The exposed beaches that had been oiled in early February, e.g. at al-Khafji, ap-peared clean by March, but this was merely because oil had soaked deep into thesediment and storms had washed and floated off the remainder (southwards), afterwhich the beach had been covered with a fresh layer of unpolluted sediment fromthe shallow subtidal zone.
By April-May the beached oil was most concentrated on the highest part of theintertidal flat, beneath the storm berm; upper to middle reaches of the flats hadless free oil standing on them, but had everywhere been soaked and stained by oilto variable depths (usually at least several centimetres), giving the substrate apatchy, black and dark brown appearance. The lowest areas of the flats were stainedvarying shades of paler brown, indicating less prolonged exposure to beached oil.
No slicks reached as far south as Tarut Bay (although patches of sheen did) butthe area has a long history of oil pollution due to the high concentration of oilindustry facilities around the Tarut Bay /Dhahran area (e.g. Spooner 1970). Forinstance a small oil slick was noted on 2 May off Ras Tannurah Coastguard sta-tion; two days later it had come ashore as a 50 cm wide band along 3 km of Rahimahbeach. Patches of sheen 200 m long were also noted twice in the Bay in April.
RESULTS
Numbers of waders in the impact zone after the spill
Before it ended on 28 February, the Gulf War prevented any investigation of theimpact zone. During the first half of March about one third of the Saudi Arabian
88
M. I. Evans and G. 0. Keijl Sand grouse 15
already being close to carrying capacity with respect to feeding waders. It isincreasingly clear that many if not most individual waders use the same set ofmigration and wintering areas from year to year: they appear to depend on theirknowledge of their traditional feeding grounds in order to migrate and breed suc-cessfully. It is thought likely that in attempting to establish themselves in newareas, the increased feeding density of waders would cause increased depletion offood stocks and increased behavioural interference between feeding waders, res-ulting in lower feeding success for some birds (most likely the interlopers, plus theyoungest individuals in the population), leading to unsuccessful migration or breed-ing that year, or to death.
Thus the actual fate of the displaced population is impossible to state with cer-tainty , but the loss to wader species of such a large area of feeding habitat willhave almost certainly led to a net reduction in their flyway population sizes. Thedrastic reductions recorded in Saudi Arabian Gulf wintering wader populationsduring November-December 1991, e.g. of Lesser Sand Plover and Dunlin, are evi-dence in support of this pessimistic view.
Plate 3. Oiled Lesser Sand Plovers Charadrius mongolus and a Dunlin Calidris alpina, Jubail(Saudi Arabia), April 1991. (Arnoud B. van den Berg)
Density of waders feeding in the oiled intertidal zone
It appears from the (non-oiled) control sites that natural seasonal/ annual variationin the density of feeding birds is very unlikely to account for such a drastic reduc-tion at the oiled site: it is almost certainly the oiling itself that has caused thismassive drop in wader feeding density. The magnitude of this reduction (c. 98%)is closely similar to the scale of depopulation of the oiled coast (97% ) indicated bythe earlier comparison of pre-oil spill and post-oil spill counts, suggesting someindependent corroboration.
Assessments of oiling frequency in wader populations
Overall frequency of oiling
The existence of a minority (c. 25% or less) of unoiled birds can be explained bythe continuous arrival of fresh, inexperienced migrants from further south during
98
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