The Annual Mortality Rate of Black-Legged Kittiwakes in NE England from 1954 to 1998 and a Recent Exceptionally High Mortality

-Annual mortality rates of breeding Black-legged Kittiwakes (Rissa tridactyla) have been measured from 1954 to 1998 at colonies near the mouth of the River Tyne in NE England, with most annual values in the 1530% range. Over the period, mortality rates tended to increase progressively until the mid 1980s, when they briefly fell to the levels observed in the first ten years of the study. This was followed in 1997 and 1998 by the highest mortality rates recorded, reaching 39% and 58% respectively. No other species were involved. These high mortality rates, and those in 1984-1987, appeared to be caused by toxin-producing algal blooms. The toxin is extremely poisonous, killing birds away from the colony during a single feeding trip and affected significantly more females than males (possibly because of different feeding areas or methods between the sexes). Bodies floated ashore only when on-shore winds occurred. Birds died in several spring and summer months during each of three years and in the same restricted area about seven km off-shore and in, or close to, an area used to dump human sewage. Circumstantial evidence suggests that the nutrients from the sewage may have been responsible for larger than normal blooms of algae. As a result of the mortality, colonies in the immediate area, particularly at Marsden, have declined dramatically (43% during 1992-1997 and an additional 50% during 1997-1998). In 1998, over a quarter of the remaining occupied nests were frequented by males only. Received 30 November 1998, accepted 11 January 1999