{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Environmental+Toxicology+Tox+2000+notes (2)

A case in point is the double crested cormorant

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: ng from a small colonizing population between 1913 and 1920, these birds increased in numbers until, by 1950, about 900 pairs were reported on the Great Lakes. At this point, populations began to decline, partially as a result of ―persecution‖ of Cormorants by humans but also because of nearly total reproductive failure from eg gshell breakage and egg loss which correlated with DDTr and PCB residues in the eggs. Numbers of eggs produced (4 -6 eggs/pair) did not decrease but survival of the eggs to hatch was 95% smaller than expected (Weseloh et al. 1995, Weseloh et al. 1983). Number of nesting pairs 5 From about 1970, when restrictions on 10 DDT use DDT use were implemented in the USA 4 and Canada, and releases of other Cormorant persecution 10 persistent organic pollutants began to decline, the Cormorant population started 3 10 to increase at a rate of about 29% per annum (Figure 3-28). Several reasons 2 10 contributed to this remarkable increase. The decrease in the input of DDT and 1 Alewife abundant other persistent organic pollutants into 10 the Great Lakes resulted in an increase in egg survival and increased hatch. In 0 10 addition, persecution by humans declined 1900 1920 1940 1960 1980 2000 and changes in the food web in the lakes Year made conditions more favourable for the Figure 3-281 Cormorants. Forage fish, such as Population size of Double-crested Cormorants in the Great Alewife (Alosa pseudoharengus) increased in numbers as predatory fish Lakes from 1913 to 1991. (Redrawn from data of Weseloh et al., 1995). populations declined in the Great Lakes. This decline in predatory fish was due to over -fishing by humans and to predation by the introduced Sea Lamprey, Petromyzon marinus. Cormorant populations in the Great Lakes are also biochemically less susceptible to the effects of planar PCBs and PCDD/PCDFs than other birds, further explaining their rapid recovery. The rapid recovery of populations of Do ubleCrested Cormorants in the Great Lakes was thus a combination of reductions in the use DDT, more effective metabolic detoxification of persistent organic pollutants, a large reproductive potential, and a unique availability of food sources (Ludwig et al. 1993). Developmental deformities in Double -Crested Cormorant eggs, chicks, and adults have been observed since the 1970s and include well known crossed -bill deformity seen in chicks and hand reared adults. This deformity is more common in dead eggs where many other deformities are also observed. However, in all eggs, the deformity has been observed at an incidence of about 1% and, in live chicks, it occurs at an even smaller incidence. Crossed -bill deformity had greater incidence in the late 1970s to late 1980s than from 1988 to 1993 and some locations showed greater declines than others. Although these deformities have shown a decline that is consistent with decreases in concentrations of persistent organic pollutants, th ey have always occurred at small rates of incidence in live chicks. As with all phases of ecological risk assessment, the impact of these responses must be viewed in the context of their effects at the population level. Clear...
View Full Document

{[ snackBarMessage ]}

Ask a homework question - tutors are online