CONSERVATION OF MARINE BIRDS IN THE DANISH MONARCHY 



275 



persing the oil, and no serious effect on sea- 

 bird populations took place (Joensen 

 19736:118). It seems that the best way of 

 cleaning up such oil disasters is through a me- 

 chanical removal of the oil, but this is a very 

 expensive and difficult procedure. 



Pollution by Toxic Chemicals 



Chemical pollution is probably the most 

 ominous threat to seabirds at present. Since 

 all toxic chemicals used in agriculture ulti- 

 mately end up in the sea, and many large fac- 

 tories release their industrial wastes directly 

 into the sea, the effects of this pollution on 

 marine organisms is attracting a growing in- 

 terest. Many students have worked on these 

 problems, and the results that concern birds 

 were summarized by Bourne (1972:205). It is 

 known that organochlorine residues have 

 been found in seabirds in all the oceans of the 

 world, including Antarctic waters and Arctic 

 seas (Bogan and Bourne 1972:358). The 

 chemicals most often found in birds are DDE 

 (a metabolite of DDT) and PCB's (polychlori- 

 nated biphenyls), a mixture of related chemi- 

 cal compounds often originating from indus- 

 trial wastes. In addition, some mercury will 

 always be found, sometimes in increased con- 

 centrations. The present restrictions on the 

 use of DDT and PCB in Denmark have not yet 

 resulted in a corresponding decrease in the 

 amount of these pesticides in birds. 



It is well known that marine pollution 

 reaches a peak in the Baltic. This high level of 

 pollution is reflected in seabirds. For example, 

 analyses have shown that eggs from the 

 colony of common murres on Christians0 in 

 the Baltic contain about 100 times as much 

 DDE and 50 times as much PCB as eggs of 

 murres from the Faroe Islands in the Atlantic 

 Ocean (Dyck 1975). 



A similar difference exists in the mercury 

 content in birds examined in the two areas. 

 Feathers of a large sample of black guillemots 

 and murres from the Cattegat and the Baltic 

 had higher mercury levels than those from the 

 Faroe Islands and Greenland. It is interesting 

 that this difference existed over a hundred 

 years ago, as evidenced by the analysis of 

 feathers in museum specimens. The Baltic 

 populations of both species show very signifi- 

 cant increases in the mercury content in 1965- 



70, as compared with the values earlier in this 

 century. Since 1970 there has been a sharp de- 

 crease in mercury content, and in 1973 the 

 level was almost as low as it was early in the 

 century. These results indicate that the strict 

 control of mercury discharges enforced in 

 Sweden has resulted in a quick recovery of 

 nearly normal conditions in the Baltic (Somer 

 and Appelquist 1974). However, recent 

 studies by Koeman et al. (1975:286) appear to 

 show that mercury does not accumulate to the 

 same extent in seabirds as it does in seals. 



High concentrations of chlorinated hydro- 

 carbon residues accumulate in carnivorous 

 birds and upset the normal breeding behavior 

 by making the eggshells too thin and fragile 

 to survive (Peakall 1970:73; Mueller and 

 Leach 1974:289). In Denmark, shells of 

 herring gull eggs from the Baltic population 

 were thinner, lighter, and more heavily con- 

 taminated with DDE and PCB than were 

 shells of eggs from other colonies (J0rgensen 

 and Kraul 1974:173). This further emphasizes 

 the pollution of the Baltic Sea. 



Massive mortalities of common murres, 

 such as the one reported in the Irish Sea in the 

 fall of 1969 which was apparently caused 

 partly by malnutrition and PCB poisoning 

 (Parslow and Jefferies 1973:87), are unknown 

 in Danish waters. 



It should be added that the pollution of sea- 

 water with cadmium, so very dangerous for 

 man, has been high in recent years owing to 

 the increased use of this element in industry, 

 but no analysis of its importance for seabirds 

 in Danish waters has yet been made. 



It should also be mentioned that pollution 

 of fresh water in lagoons or lakes near the sea 

 can often cause serious declines in numbers of 

 certain seabirds. This is well illustrated by re- 

 cent events in the sanctuary Nakskov Indre- 

 fjord on the island of Lolland. This landlocked 

 fjord once supported numerous breeding 

 populations of ducks, grebes, and terns, but in 

 recent years a number of species (e.g., eared 

 grebe; common teal; garganey, Anas querque- 

 dula; pintail; and black tern, Chlidonias nigra) 

 have failed to breed and practically all other 

 species have declined in numbers. The main 

 reason for these changes is a severe pollution 

 from the admission of raw sewage from tribu- 

 taries (Bloch et al. 1972). After several out- 



