Abstract from BTO Research Report No 434:

Burton, H.H.K. & Holloway, S.J. (2006)

The effects on waterbirds of dredging at the Cardiff Bay Barrage Report for 2005/06
ISBN: 1-904870-88-0

Executive summary

1. This study reports the impact 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 2006. Results are compared to those reported between 2002 and 2005. 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 4 and 20 February 2006. 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 2006, before dredging commenced, and March 2006, after dredging operations had been completed.

4. Eleven waterbird species were recorded using the mudflats affected by dredging in January and March 2006. These included four of the 10 species of wildfowl and wader that had been recorded on the equivalent mudflats prior to barrage construction – Shelduck, Mallard, Curlew and Redshank – together with Cormorant, Mute Swan and five species of gull – Black-headed Gull, Common Gull, Lesser Black-backed Gull, Herring Gull and Great Black-backed Gull. These species, and also Oystercatcher 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 and Lesser Black-backed Gull. Aside from these, only Cormorant, Herring Gull and Mallard were recorded in numbers of greater than 10 on any one of these mudflats. Blackheaded, Lesser Black-backed and Herring Gulls utilised all three mudflat areas, whilst Mallard particularly favoured the area within the outer harbour. 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 – Mute Swan and Redshank – were not recorded on comparative areas of mudflat nearby. Densities of Black-headed, Lesser Blackbacked and Herring Gulls were greater on the comparative mudflats, however.

7. There is some evidence that the dredging in February 2006 may have had an effect in the short term, as numbers of both Mallard and Black-headed Gull fell on the mudflats by the barrage. However, there were slight increases in the numbers of Cormorants and Herring Gulls following the operations and Mute Swan and Shelduck were only recorded on these mudflats in March.

8. Over the longer-term (i.e. the five years of monitoring), numbers of Black-headed Gull have declined on the mudflats affected by dredging, while remaining stable on adjacent mudflats. Herring Gull numbers have also remained stable on the mudflats affected by dredging, while they have increased on the adjacent mudflats. This might suggest that the regular dredging that has occurred since the barrage was completed has prevented invertebrate food supplies from developing – if so, this would be an inevitable consequence of complying with the statutory requirements of the Cardiff Bay Barrage Act 1993. However, it is notable that numbers of Mallard have actually increased on these mudflats over the same period. It is perhaps more likely, therefore, that gull numbers have been reduced due to disturbance from people on the barrage – Mallard being more tolerant of human activity.

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