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Abstract from BTO Research Report No 309:

Burton, N.H.K., Holloway, S.J., & Clark, N.A.. (2003).

The Effects of Waterbirds of Dredging at the Cardiff Bay Barrage Report for April to Dec 02.

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 using data collected between April and December 2002. Results are compared to those reported in March 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, in February and March 2002 and, during the period of study, between 8 and 31 August 2002. Dredging 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. Data are presented separately for April to May 2002 and August to December 2002. Additionally, data are presented for August 2002 alone, when counts were undertaken during a period when dredging operations were being undertaken. Comparative data are presented for August 2001 and August to December 2001.

4. Nine waterbird species were recorded using the mudflats affected by dredging between April and December 2002. These included five of the 10 species of wildfowl and wader that had been recorded on the equivalent mudflats prior to barrage construction – Shelduck, Mallard, Oystercatcher, Curlew and Redshank – three species of gull – Black-headed Gull, Lesser Black-backed Gull and Herring Gull – and Cormorant. These species, and also Mute Swan Cygnus olor and Great Black-backed Gull L. marinus, had been recorded on these mudflats during the previous year’s fieldwork

5. By far the most numerous species on the mudflats affected by dredging were Black-headed Gull and Lesser Black-backed Gull. Aside from these, only Mallard and Herring Gull were recorded in numbers of greater than 10 on any one of the mudflats. Black-headed and Lesser Black-backed Gulls utilised all three mudflat areas, whilst Mallard particularly favoured the area within the outer harbour.

6. Although the overall numbers of wildfowl and waders using the mudflats affected by dredging are very low, the average low tide densities of four of the species found on these mudflats were higher than those on comparative areas of mudflat nearby. Densities of both Black-headed Gull and Lesser Black-backed Gull, the most numerous species on the mudflats by the barrage, were greater on the comparative mudflats, however.

7. Black-headed Gulls and Herring Gulls occurred in higher numbers on the mudflats by the barrage in 2002 than in 2001. Mallard numbers, in contrast, were lower on the mudflat area within the outer harbour in 2002.

8. There is no evidence for the dredging in August 2002 (like that in February/March 2002) having an effect on the numbers of birds using the mudflats. Indeed, numbers of the three most numerous species – Mallard, Black-headed Gull and Lesser Black-backed Gull – were higher on average than over August to December as a whole. Numbers of Mallard in August 2002 were also greater than those found the previous August.

9. This report will be updated again in March 2003 following the completion of counts over the winter of 2002/03 and a further period of dredging scheduled for February. This will allow a more complete assessment of the impacts of dredging to be made.

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