Barry Stewart

Introduction

The Common Scoters of Carmarthen Bay have perhaps never received so much attention since the dramatic impact of the Sea Empress oil spill that struck during February 1996. This event is now known to have directly affected just less than 4, 500 Common Scoters that were wintering off the Pendine to Amroth stretch of coast (3, 326 died and a further 1, 100 were rehabilitated and released) and may have indirectly affected a further 10, 000 birds that were using the eastern half of the bay at the time of the incident.

A consequence of the oil spill has been the establishment of a monitoring programme, set up by the Countryside Council for Wales (CCW) and conducted by the Wildfowl & Wetlands Trust, that will hopefully be maintained at least into the new millennium. This report summarises the findings of land-based and aerial surveys that were carried out during 1996 and is a much shortened version of a report submitted by the Wildfowl & Wetlands Trust to CCW (Stewart et al., 1997)

Methods

Land-based counts from all shores of Carmarthen Bay were conducted at two-week intervals. These were occasionally supplemented with co-ordinated aerial surveys to determine if birds were occurring further offshore and out of range of land-based observers (approximately 5km for scoters). These synchronised counts were also used to assess the effectiveness of each survey method so as to improve the interpretation of future surveys. Figure 1 shows the land-based observation points from which counts were made and also indicates the visual limit of land-based observations.

Aerial surveys were planned to take place at monthly intervals, but constraints, particularly with the weather, prevented regular flights from taking place and only four flights were made in total.

Results

When oil entered Carmarthen Bay on 24 February, approximately 8, 000 Common Scoter were present, approximately half in the north west of the bay between Amroth and Pendine, the remainder off Rhossili and Whiteford in the east. The 4, 500 Common Scoters directly affected by the oil were those wintering along the Amroth to Pendine stretch of coast. The remaining birds, off Rhossili and Whiteford, were joined by another 6, 000 birds in early April when total numbers in the bay swelled to around 10, 000 birds (Figure 2). However, some 3, 500 of these birds moved to the previously heavily oiled Amroth to Pendine stretch of coast after the amount of surface oil had decreased. By the end of April, most birds had left the bay on their northward breeding migration. Common Scoters then reappeared in the bay in June, but numbers only built to around 1, 000 birds during July, August and September as birds gathered to moult, mostly off Cefn Sidan.

Winter concentrations formed from late September onwards with birds again frequenting the

Rhossili area as well as their usual feeding areas from Pendine to Amroth. Over the whole study, the Rhossili area and Pendine to Amroth stretches of the bay held most birds, constituting some 41% and 21% of all birds counted, respectively.

Discussion

The large numbers of birds located in Rhossili Bay during the oil spill were unprecedented; this area typically holds no more than 1, 500. A possible explanation for this occurrence might have been that bivalves such as the Egg-shell Razor Pharus legumen, or other normally difficult to obtain prey, could have been narcotised by low levels of oil in the water column, which then caused them to evacuate from the sediment and become available to scoter. An analysis of the diet of scoters picked up during the spill showed the Egg-shell Razor to be the dominant food item (Hughes eta!, 1997). Additionally, birds were seen off Rhossili on more than one occasion manipulating and swallowing whole Egg-shell Razors at the surface.

The reappearance of birds off the Pendine to Amroth stretch in April was unexpected as it was assumed at the time that most invertebrates in that area would have been killed off by the oil. However, the fact that birds appeared to feed and display normally on the whole suggested that some of the benthos must have remained relatively unaffected. It is also worth noting that some birds seen during this period did show signs of oiling and it is quite possible that many of the apparently 'unaffected' birds could have suffered some sub-lethal effects which may have impaired their potential for migration and/or breeding.

Observations also showed that birds occurred thither offshore when compared to previous years and only very small numbers were noted in intertidal areas. It is suggested that is likely to have resulted from the impact being greatest on intertidal fauna which was hit by bulk oil with each tide.

Possible causes for the lower numbers of Common Scoter recorded in the latter half of the year include direct and indirect mortality from the Sea Empress oil spill and/or a high turnover of birds due to low food availability, particularly in intertidal areas of the bay. However, a lack of baseline data on Common Scoter numbers and benthic invertebrate communities in Carmarthen Bay prior to the Sea Empress oil spill, limited data on the effects of oiling on benthic invertebrates, and the naturally high level of variation in scoter numbers in the bay, preclude firm conclusions being drawn. Nevertheless, it appears likely that over one year after the oil spill, some areas of Carmarthen Bay, particularly intertidal areas, remained unsuitable, or at least unprofitable, for foraging Common Scoters.

References:

Hughes, B., Stewart, B., Brown, M.J. & Ream, RD. (1997) Studies of Common Scoters killed by the Sea Empress oil spill, Report to Countryside Council for Wales.

Stewart, B., Hughes, B., Bullock, I. & Haycock, B. (1997) Common Scoter Monitoring in Carmarthen Bay following the Sea Empress Oil Spill, Report to Countryside Council for Wales.