Abstract from BTO Research Report No. 465:

Burton, N.H.K., Maclean, I.M.D. (2006)

The Effects on Waterbirds of Dredging at the Cardiff Bay Barrage, Report for 2006/07. ISBN 1-904870-98-8

Executive Summary

1. This study reports on the effects of maintenance dredging on the birds utilising mudflats within and adjoining the outer harbour of the Cardiff Bay barrage using data collected in January and March 2007. Results are compared to those reported between 2002 and 2006. Dredging is required to maintain a channel from the outer harbour to the sea and to prevent sediment build up within this harbour. Initial dredging took place during the construction of the barrage and since August 2000 has usually taken place twice annually (in February and August). During the period of study, mudflats were dredged between 2 and 22 February 2007. 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 for the months of January 2007, before the latest dredging commenced, and March 2007, after dredging operations had been completed.
4. Twelve waterbird species were recorded using the mudflats affected by dredging in January and March 2007. 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 – together with Cormorant and six species of gull – Black-headed Gull, Common Gull, Lesser Black-backed Gull, Yellow-legged Gull, Herring Gull and Great Black-backed Gull. With the exception of Yellow-legged Gull, all these species had been recorded on these mudflats during previous periods of post-barrage fieldwork.
5. By far the most numerous species on the mudflats affected by dredging were Black-headed Gull, Lesser Black-backed Gull and Herring Gull. Aside from these, only Cormorant and Mallard were recorded in numbers of greater than 10 on any one of these mudflats. Mallard and Black-headed, Lesser Black-backed and Herring Gulls utilised all three mudflats; mudflat B5, within the outer harbour, was also used by a single Shelduck and Great Black-backed Gull. These results are very similar to those reported previously.
6. Although the overall numbers of wildfowl and waders using the mudflats affected by dredging are very low, two species found on these mudflats – Mallard and Redshank – were not recorded on comparative areas of mudflat nearby. Low tide densities of Shelduck, Curlew, Black-headed Gull, Lesser Black-backed Gull and Herring Gull were greater on the comparative mudflats, however.
7. There is some evidence that the dredging in February 2007 may have had an effect in the short term, as numbers of Mallard, Black-headed Gull and Lesser Black-backed Gull fell on the mudflats by the barrage between January and March. However, it is important to note that other factors may have also affected numbers of these species over this period. Numbers of Black-headed and Lesser Black-backed Gulls using the mudflats by the barrage were probably particular high in January 2007 because of strong westerly winds which forced birds to seek shelter underneath Penarth headland. The observed declines of gulls may have also occurred because by March some birds were beginning to move back to their breeding areas. It should also be noted that there were slight increases in the numbers of Cormorants and Herring Gulls following the operations.
8. Evidence that dredging has had a long-term effect on the numbers of birds using the mudflats by the barrage is more limited. Over the six years of monitoring, numbers of Black-headed Gull showed a slight, but insignificant decline on the mudflats affected by dredging. This might suggest that the regular dredging that has occurred since the barrage was completed has had a limited effect on the long-term development of invertebrate food supplies – if so, this would be an inevitable consequence of complying with the statutory requirements of the Cardiff Bay Barrage Act 1993. However, it should also be noted that there was a much larger, significant decrease in Black-headed Gull numbers on adjacent mudflats, suggesting that changes were part of a wider trend. Numbers of Mallard, Cormorant and Herring Gull have actually increased on the mudflats affected by dredging and this would suggest that waterbird food supplies have not been detrimentally affected by the operations in the long-term.
9. Densities of wader and wildfowl species on the mudflats affected by dredging are lower than those found prior to construction of the barrage and this may in part be due to disturbance. Overall, the numbers of birds that might be affected by dredging are very small in relation to the substantial populations found locally.
10. Further monitoring is recommended in order to allow future assessment of the effects of dredging over both the short and long term.

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