Abstract from BTO Research Report No 298:

Burton, N.H.K., Rehfisch, M.M. & Clark, N.A. (2006)

The Effect of the Cardiff Bay Barrage on Waterbird Populations 13.
Distribution and Movement Studies August 2001 – May 2002
ISBN: 1-904870-58-9

Executive summary

1. This report presents the results of the thirteenth year of intensive monitoring of the waterbirds of Cardiff Bay and adjacent areas. The report concentrates upon results from the winter of 2001/02, the third following the closure of the Cardiff Bay barrage. The programme of monitoring followed that used over the previous 12 years, thus allowing direct comparisons to be made between results from each year.

2. The Cardiff Bay barrage was closed on the morning of 4 November 1999, impounding the Bay with seawater. Thereafter, the Bay was drained overnight approximately once every week until September 2001, whereafter it was impounded permanently with freshwater.

3. The numbers of birds using Cardiff Bay in the three winters following barrage-closure have been greatly reduced. A very few individuals of the four key species - Shelduck, Dunlin, Curlew and Redshank – have continued to use the Bay, though primarily as a high tide roost site. A total of 28 species of waterbird, and an annual mean of 21.0, have been recorded at the site since barrage-closure in comparison to an annual mean of 26.5, and a total of 50, in the 10 years before.

4. The decline in waterbird species’ diversity in Cardiff Bay since barrage-closure has been due, primarily, to a loss of waders. However, at the same time, there has been a slight increase in the numbers of ‘other’ waterbird species such as grebes and rails. Among waders and wildfowl, only the numbers of Mute Swan and two diving duck species – Pochard and Tufted Duck – have increased. There has thus been a change from a diverse waterbird community dominated by large numbers of estuarine specialists, to a less diverse community comprising relatively small numbers of freshwater species.

5. Although numbers of both Shelduck and Curlew increased at Orchard Ledges in the two winters following barrage-closure, they fell back to previous levels in 2001/02. In neither case could the increase account for the loss of birds displaced from the Bay. At Rhymney, densities of Shelduck have decreased since barrage-closure, whilst those of Curlew have remained the same. Whilst it seems probable, therefore, that some of the Shelduck and Curlew from the Bay initially settled at Orchard Ledges, it is likely that either many have been forced to disperse to more distant areas or that there has been increased mortality in their populations or reduced recruitment of young birds. Until the full programme of work has been completed, however, these conclusions should be treated with caution.

6. Numbers of both Dunlin and Redshank had declined on the main study sites in the 10 years prior to barrage-closure. Dunlin numbers fell further at Rhymney in the winter of 1999/2000, though no further over the following two winters. Redshank numbers, in contrast, have risen at Rhymney since barrage-closure. The observed increase in the number of Redshank at this site over the three winters subsequent to barrage-closure could account for the loss of birds from the Bay.

7. Numbers of Dunlin and Curlew in the winters of 2000/01 and 2001/02 at low tide at Peterstone and St. Brides were lower than in any previous winter, whilst those of Shelduck and Redshank were unchanged.

8. Information from colour-ringing confirmed that the increase in Redshank numbers at Rhymney in the three winters post-closure was largely due to an influx of birds from Cardiff Bay. In the winter following closure, Redshank originally colour-ringed in the Bay were recorded as far as the River Usk in Newport and the River Axe in Somerset. In contrast, only single colour-ringed birds were seen in Somerset in 2000/01 and 2001/02 and none any further east than Peterstone. In all three years, the majority of colour-ringed birds were seen on the Rhymney Estuary and on the area of mudflats by Cardiff Heliport. Radio-tracking had shown that the latter area was formerly used only at night, probably due to disturbance.

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