Experience over the past four decades has shown that most ecosystems have the capacity to recover fairly quickly from the effects of oil pollution - indeed, the Royal Commission on Environmental Pollution concluded as much in 1981. There are exceptions to the rule - sensitive habitats such as mangrove swamps and coral reefs, and the polar regions where oil volatilises and degrades slowly - but in general the grosser effects of oil are not much in evidence after a year or two.
Nevertheless, tanker disasters tend to follow a familiar pattern. Someone suggests that an environmental apocalypse is near at hand, and the media seize on the claim for the day's headlines. Then, after the black tide has receded and the dust settled, contrarian commentators are equally quick to seize on the reassurances of the official inquiry to brand "environmentalists" as irrational doomsters, and the media go into a period of soul-searching about how they could possibly have swallowed the scare story in the first place.
There has, though, been little sniping of this kind since the Sea Empress Environmental Evaluation Committee (SEEEC), appointed by the Government to assess the impacts of the 72,000 tonne oil spill off the Pembrokeshire coast in February 1996, reported on the second anniversary of the disaster.
The explanation may be found in the tone set by the Committee in releasing its report. Alerted by a newspaper story which suggested that the report would be a whitewash job, the Committee made a point of stressing at the launch at a conference in Cardiff on 10-13 February: "We were lucky: it could have been much worse."
Lady Luck takes a hand
As with the grounding of the oil tanker Braer off the Shetlands in 1993, several factors came together to prevent some very serious ecological consequences. One was the nature of the cargo carried by the Sea Empress - a light crude, 40% of which quickly evaporated.
A second was the change in wind direction soon after the tanker went aground. The wind shifted round to blow from the north, which it does for only 15% of the time in February. Most of the oil escaping from the vessel was blown away from the coast, making dispersant spraying possible, in turn reducing the cost and difficulty of cleaning polluted coasts. Little oil accumulated in offshore sediments for resuspension by storms.
Thirdly, the spill occurred in the biologically most favourable window. Fish life was at its lowest ebb during the year. And most of the nesting birds which make the area one of the most important for bird conservation in the UK had yet to return from overwintering elsewhere. The common scoter, a rare exception which overwinters in Carmarthen Bay and only one other site in Britain, was badly affected by the spill.
The report estimates that 3,700 to 5,300 tonnes of the tanker's cargo came ashore along 200 kilometres of coast. Most of this was removed by summer 1996, although some remains buried below sandy beaches and may re-emerge. Over-enthusiastic dispersant spraying and high-pressure hosing during the shoreline cleaning operation caused localised damage. In addition, up to 125 tonnes of heavy fuel oil released from the Sea Empress at a late stage in the incident was still coating the shores of Milford Haven last year.
Fish and commercial shellfish populations appear hardly to have been affected by the spill, although speakers at the Cardiff conference stressed that it would be difficult to identify any effects against the wide natural variation in populations. A residual uncertainty is whether juveniles were affected, but this may not be known until around 2001.
On rocky coasts, the most visible impact of the spill followed a familiar sequence. Large-scale limpet mortalities in some areas were followed by a flush of the green algae on which they normally graze, and then by a red algal flush. However, limpet populations are now recovering.
In rock pools and lower shores, red coralline algae were bleached by contact with oil. But only the surface cells were affected, and recovery is now under way. In contrast, lichens found in the splash zone along Milford Haven were killed or bleached by fuel oil, and recovery will take many years.
A visible consequence of the spill on inter-tidal sediments was the mass "strandings" of razorshells, cockles, bivalves, crabs, starfish and other animals between February and May 1996. Apparently irritated by contact with oil, the animals emerged to die on the surface. SEEEC, which felt that the dispersant spraying operation was generally justified by the benefits, concedes that the strandings may have been due to exposures to oil which would not have occurred without the use of dispersants. A more speculative possibility raised at the conference is that the toxicity of the dispersants themselves caused or contributed to these events.
Similar uncertainty surrounds the exact causes of the severe damage to populations of several species of amphipod. These small crustacea were virtually wiped out in much of Milford Haven and off Skomer island, and are showing very little sign of recovery. Their absence is of wider concern because they are important food items for flat fish and larger crustaceans.
A species which achieved fame during the Sea Empress affair is the starfish Asterina phylactica, found at only half-a-dozen sites in Britain. Discovered and studied for 30 years by local naturalist Robin Crump, its population of about 200 in West Angle Bay was reduced to only five individuals after its habitat was inundated with oil. Fears that the population would die out because the survivors would be unable to find each other to breed have been alleviated by the discovery that the species is capable of self-fertilisation, and numbers now exceed a dozen.
The local naturalists whose work over the years helped to provide extensive baseline data against which the effects of the spill could be measured are relieved that their worst fears have not been realised. But the loss of oil from the Sea Empress, though by far the largest in the area, was by no means the first. As one scientist put it: "Little by little, we are chopping away at the biodiversity."
Grey seals and other mammals appear not to have been affected by the spill. But some bird species were less fortunate. A total of 3,495 birds were found dead, and another 3,428 were recovered alive. The most hard hit were the common scoter and guillemot, accounting for over 6,000 of the total, and the razorbill. SEEEC believes that the real death toll may have been several times higher than these number indicate.
The evidence from past bird counts suggests that guillemot and razorbill numbers at some key sites were several percentage points lower than expected in 1996, although this must be set against a healthily rising trend in the nesting population. Scoter numbers visiting Carmarthen Bay have halved from the pre-Sea Empress level of 10,000, but scientists believe that the birds have moved on to other sites after finding local populations of benthic organisms depleted by the spill.
Cleaning oiled birds
The incident has raised questions about the merits of cleaning oiled sea birds. Although the survival rate of those returned to sea is not yet known, a study by the British Trust for Ornithology has found that barely 1% of guillemots survived for more than one year after cleaning following other spills.
But the Trust has cautioned that these findings, which attracted media attention, should not be used to draw hasty conclusions. In South Africa, jackass penguins and Cape gannet have shown much higher survival rates. Mute swans also appear to do better than guillemot. Further work on cleaning and release practices is needed to clarify the issues.
However, the Sea Empress affair did show that groups which take on cleaning of oiled birds are not well organised to do the job, relying heavily on untrained volunteers. Proposals have been put to the RSPCA to set up training schemes and establish records of trained individuals and seabird rescue centres prepared to work to professional standards.
SEEEC's report closes with a series of recommendations. It urges further research on the causes of the mass strandings of bivalves and the vulnerability of amphipods to oil pollution - including the role which dispersants may have played. Clarification of the procedures for oil spill clean-up and for imposing fisheries closure orders is also recommended.
SEEEC regards its two key recommendations as covering the arrangements for future post-spill evaluation exercises and their funding. It has urged the preparation of a national plan which would see the immediate establishment of an impact assessment group to monitor the fate and effects of spilled oil. The Committee itself was not set up until six weeks after the spill. SEEEC believes that the environment agencies are best fitted to take the lead in establishing such groups and reviewing the availability of baseline coastal environmental data.
Secondly, the Committee wants to see the "polluter pays" principle extended so that the costs of post-spill assessments are met by those responsible for the spill. It has urged the International Oil Pollution Compensation Fund (IOPCF) to consider extending its compensation rules to allow for this.
Meanwhile, as the ecosystems in and around Milford Haven continue their recovery, the intriguing question is whether the events of two years ago have left a deeper mark on the human psyche than on the environment. Nic Wheeler of the Pembrokeshire Coast National Park Authority told the Cardiff conference that the Sea Empress affair had resulted in a "deep feeling of shock, anger, outrage, even personal violation" among most people in the area.
The longer-term psychological reaction of local people had, he contended, played a part in the increased opposition to plans to import Orimulsion into Milford Haven - a proposal since withdrawn by National Power - in the rise of several local environmental groups, and in widespread disaffection with the official bodies involved in the emergency response. The compensation available from the IOPCF, he concluded, "does not begin to touch the total costs to society of such a spill."