Abstract from BTO Research Report No 285:

Burton, N.H.K. & Clark, N.A.

The effects on waterbirds of dredging at the Cardiff Bay barrage.
Report for 2001/2002
ISBN: 1-904870-56-2

Executive summary

1. This report investigates the potential impact of maintenance dredging on the birds utilising mudflats within and adjoining the outer harbour of the Cardiff Bay barrage and updates results reported in January 2002. Dredging is required to maintain a channel from the outer harbour to the sea and to prevent sediment build up within this harbour. Dredging took place in August 2000, May 2001 and, during the period of study, between 15 February and 12 March 2002, and will be undertaken twice a year henceforth. Within the outer harbour, mudflats reform naturally after dredging.

2. Cardiff Bay was formed by the combined estuaries of the Rivers Taff and Ely and is situated at the mouth of the larger Severn Estuary. The bay was impounded by a barrage constructed at its mouth in November 1999. The mudflats that now adjoin the Cardiff Bay barrage historically formed part of the intertidal mudflats of the bay

3. The ornithological significance of the mudflats that adjoin the barrage is assessed by comparing counts made between August 2001 and March 2002 with historic data collected prior to the construction of the barrage and with concurrent count data from two adjacent areas of mudflat.

4. Only five of the 10 species of wildfowl and wader recorded between 1990/91 and 1993/94 on the mudflats that were subsequently dissected by the barrage were seen on the equivalent mudflats between August 2001 and March 2002. These were Shelduck, Mallard, Oystercatcher, Curlew and Redshank. In addition, the mudflats were utilised by Cormorants, Black-headed, Lesser Black-backed and Herring Gulls, a single Mute Swan and a single Great Black-backed Gull.

5. Densities of Shelduck, Mallard, Oystercatcher, Curlew and Redshank were also between 38 and 99% lower on these mudflats than in the four years prior to construction of the barrage in 1994. Thus, it is clear that much of the study area’s value has been lost due to the building of the barrage and the displacement of intertidal waterbirds from the bay.

6. Although the overall numbers of wildfowl and waders using the mudflats affected by dredging are now very low, average low tide densities of Curlew were not dissimilar to those on comparative areas of mudflat, whilst Mallard and Redshank only occurred on the mudflats by the barrage. Densities of Cormorants and gulls on these comparative mudflats were considerably higher, however.

7. In view of the similarity in the densities of most waders and wildfowl using the mudflats by the barrage and those surveyed for comparison, it is reasonable to conclude that the dredging undertaken prior to the start of counts has had no long-term impact on these species.

8. Following the dredging undertaken between 15 February and 12 March 2002, there was a decrease in the numbers of Mallard, Black-headed Gull and Lesser Black-backed Gull using the mudflats by the barrage. However, the fall in these species’ numbers would have been partly or wholly due to movements of birds away from the area to breeding grounds elsewhere.

9. This report will be updated again in March 2003 following the completion of counts through 2002 and over the winter of 2002/03 and a further assessment made of the impacts of dredging.

Back to Research Reports 248-286