The largest colony of Cormorants is on St Margaret’s Island and this is one of the best studied in the UK, with almost continuous census data for 50 years and over 5,000 chicks ringed since the mid 1960’s. There are smaller colonies on Thorne Island (it relocated from Sheep Island in the early 1990’s), on the Mew Stone, Skomer, at times on Stack Rocks off Little Haven, on the Green Scar at Solva, around Dinas Island and at Cemaes Head. Occasionally pairs might nest at scattered mainland sites. There are many coastal drying out sites (including jetties etc in the Milford Haven Waterway) where sometimes good numbers of birds can be seen.
The Cormorant population fluctuates from year to year, as probably in poor springs some birds fail to breed. Overall counts show there was a tendency to a decrease during the 1990’s but a modest increase again in the early 2000’s. The total population in the county in 1984-88 survey was around 300 pairs and a census following the Sea Empress oil spill (1996) found 360 pairs, with 180 of these on St Margaret’s Island. The population during the most recent years appears to have been stable and may have increased slightly.
The highest county population was almost certainly in the early 1970’s, when around 450 - 500 pairs probably bred, 330 of them on St Margaret’s, but the reasons for the subsequent decrease are poorly understood. Breeding success varies from year to year but there are few years of substantial failure. Ringing has shown that considerable numbers are shot on rivers or killed in coastal fishing nets during the winter period. It seems likely that this is a major cause of population changes.
Steve Sutcliffe
Fieldwork 2003-07 (based on 490 tetrads)
Red = breeding confirmed = 13
Orange = breeding probable = 2
Yellow = breeding possible
Total tetrads in which registered = 15 (3.1%)