Background

Over the last decade Northumbrian Water Ltd (NWL) has implemented a series of major improvements to the treatment and discharge of sewage at sites along the coast between Berwick-upon-Tweed and Saltburn (from Northumberland to Cleveland), so as to comply with the EC’s Urban Waste Water Treatment Directive.

The impact of these directives on coastal waterbirds has raised concern as waste water discharges from outfalls may provide considerable supplies of food for bird species, either as directly edible matter or by artificially enhancing concentrations of invertebrate food through nutrient enrichment.

This report reviews the latest results of an investigation, commissioned by NWL and begun in 1996/97 by the University of Durham, of the impacts of improvements to sewage discharges on waterbirds wintering on a 36 km stretch of the Northumbrian coast between the Coquet Estuary and St. Mary’s Island (Figure 1.1). The area comprises extensive areas of rocky shore which are included in the Northumbria Coast Special Protection Area (SPA), which is designated for its importance for wintering Purple Sandpipers Calidris maritima and Turnstones Arenaria interpres. Between these rocky areas are the sandy (bathing) beaches of Druridge Bay, Cambois and South Blyth. The majority of improvements to sewage discharges in the area were completed by the end of the winter of 2000/01.

The report outlines work carried out by the British Trust for Ornithology (BTO) in the winter of 2004/05 and investigates how numbers of waterbirds wintering in the study area have changed since monitoring was begun by the University of Durham. Additional analyses investigate whether changes in Turnstone numbers following the improvements to sewage treatment might be explained by changes in this species’ survival rates or movements. All waterbird species in the study area were included in counts, though analyses here are restricted to Purple Sandpiper and Turnstone. Analyses were carried out at two scales: firstly, for the whole coast from the Coquet Estuary to St. Mary’s Island and secondly, for the Amble- Hauxley and Newbiggin areas alone. Two of the three largest outfalls in the study area discharge in these areas; both discharges received improved treatment from late in the winter of 2000/01. 

Changes in Waterbird Numbers Following Improvements to Sewage Treatment

Across the study area as a whole, counts suggested that the changes to sewage treatment affected both of the species for which the SPA is designated in winter – Purple Sandpiper and Turnstone. These species showed declines following the winter of 2000/01, having previously risen in number. At a regional level, Wetland Bird Survey (WeBS) counts indicated that Purple Sandpipers were declining at least up to 2003/04, but that Turnstone numbers had stabilised after a decline between the mid-1980s and mid-1990s. However, for neither species were numbers in the study area related to regional numbers. At the more local scale, after a period of high stability in numbers, there was a clear decline (of >35%) in the numbers of Turnstone at Amble-Hauxley over the four years following the improvement to the Amble discharge. Purple Sandpiper numbers also fell here in 2003/04, having risen prior to the improvement to the discharge. However, no significant declines were apparent in the numbers of either species, over the study area as a whole or at Amble-Hauxley, between the winters of 2003/04 and 2004/05. 

Changes in Turnstone Survival Rates Following Improvements to Sewage Treatment

Analysis of data from resightings of colour-ringed Turnstone suggested that adult survival fell in 2002, from 0.769 (95% confidence limits = 0.723-0.810) to 0.615 (0.471-0.742), a rate similar to that of first-winter birds. However, the difference between these rates was not significant (P = 0.0555) and thus a constant annual survival rate (for adults and first-winter birds) of 0.747 (0.707-0.783) was estimated for the period 1997/98 to 2004/05. Nevertheless, it seems likely that the drop in Turnstone numbers seen at Amble-Hauxley following the improvements to the Amble discharge was in part a consequence of a fall in adult survival. No evidence was found to suggest that the decline in Turnstone numbers at Amble-Hauxley was the result of individuals increasingly using other sites in the area. 

Conclusions

Count data suggested that the changes to sewage treatment may have impacted numbers of both Purple Sandpiper and Turnstone, notably at Amble-Hauxley. For the latter species, analyses of the survival rates of colour-ringed birds suggested that the decline in numbers at that site may have been the result of an increase in adult mortality. 

Recommendations

The declines of Purple Sandpiper and Turnstone in the study area – and, in particular, that of Turnstone at Amble-Hauxley – are of concern given that they began immediately following the changes to sewage treatment. Although numbers of Turnstone and Purple Sandpiper over the study area as a whole and at Amble-Hauxley were similar in 2004/05 to those in the previous winter, it is still too early to affirm that numbers have now reached a new equilibrium. 

The following recommendations are made with the aim of providing a more complete understanding of the impacts of the changes in sewage disposal on the waterbirds wintering on the Northumbrian coast:

• Firstly, waterbird counts should continue during the winter of 2005/06. This would show whether numbers of Turnstone and Purple Sandpiper have now stabilised or whether there might be continued decline either over the study area as a whole or at Amble-Hauxley. By collecting sightings of colour-ringed Turnstone for an additional winter it would also be possible to improve the accuracy of existing survival estimates and thus determine more fully the significance of the change in survival seen following the changes to sewage treatment at Amble. These data could be collected while undertaking waterbird counts.
• Secondly, it is recommended that stable isotope analyses are undertaken in order to establish the importance of nutrient inputs from sewage along the Northumberland coast to invertebrates and the waterbirds that feed upon them and the extent to which these inputs have changed following the improvements to discharges.