The Outer Thames Estuary Special Protection Area (SPA) is designated for wintering red throated divers which are listed in Annex I of the European Union (EU) Birds Directive
(79/409/EEC).
The population of red-throated divers is currently designated as 6,466 individuals; 38% of
the Great British population. This figure was taken from visual aerial surveys undertaken
between 1989 and 2007. The aim of this project was to provide current information regarding the abundance and
distribution of a variety of wintering bird species, predominantly red-throated divers,
present within the Outer Thames Estuary SPA.
Two high resolution digital aerial surveys of the Outer Thames SPA were conducted
during January and February 2013. Each survey was flown on a series of transects
separated by 1.8 km, collecting abutting 3 cm resolution imagery. Average coverage was
15%.
Population estimates of red-throated divers were calculated using two methods 1) a
design-based method and 2) a model-based estimate using a Generalised Additive
Modelling (GAM) framework. Model-based estimates had tighter confidence limits
around the population estimates and precision was improved from the design-based
estimate.
From the model-based calculations, red-throated divers peaked in February 2013 at an
estimated 13,605 (12,712 â 14,489) individuals. This peak is more than double the
designated SPA total and represents 79% of the wintering Great British population.
Higher numbers of red-throated divers were recorded in the southern part of the Outer
Thames SPA during February 2013. In January 2013, higher numbers of individuals were
recorded in the northern part of the SPA. This suggests a movement further into the
estuary between the two surveys.
wide range of other bird species were recorded, including greylag geese, shelduck,
wigeon, scaup, common scoters, red-breasted mergansers, black-throated divers, great
northern divers, great crested grebes, fulmars, gannets, cormorants, shags, oystercatchers,
lapwings, redshanks, great skuas, kittiwakes, black-headed gulls, common gulls, lesser
black-backed gulls, herring gulls, great black-backed gulls, guillemots and / or razorbills,
little auks and puffins. After red-throated divers, common gulls were the next most
abundant species (12,403; 2,203-30,110).
9. A number of marine mammal species were also recorded during the aerial surveys
including dolphins and / or porpoises not identified to species level, harbour porpoises
and a phocid (seal) species. Marine mammals were most abundant in January 2013 (584;
356-880) and were distributed widely throughout the SPA.
10. Although it is necessary to treat modelling results based on two months of survey data
with great caution, red-throated diver distributions on the SPA appeared to be related to
various environmental valuables including: bathymetry, chlorophyll a, wave base, tidal
base, aspect of the sea bed, slope of the sea bed, average sea surface temperature, distance
from dredging operations and distance to coastline. The distributions of red-throated
divers may also have been affected by shipping activity and the presence of operational
and in-construction wind farms