Oil pollution incidents constitute one of the greatest dangers to seabird populations in Danish waters. The enormous masses of seabirds present in these waters throughout the year, combined with the fact that Danish waters contain some of the heaviest shipping traffic in the world would give rise to anxiety for oil disasters. The majority of all tanker traffic from the Atlantic and the North Sea to the Baltic passes through the Cattegat and the narrow straits of the Sound, the Great Belt, and the Little Belt, to supply a population of about 100 million people. Up to 100,000 ships pass through these waters each year, half through the Sound.
There have been severe oil pollution disasters every year since about 1935, accompanied by enormous mortalities of seabirds, particularly marine ducks. The Danish Game Biology Station, which has studied these disasters (Joensen 1972a, 1972b, 1973b), has noticed that the number of seabirds involved has increased in recent years, in spite of increased control by Danish authorities.
Unfortunately, it appears that small amounts of oil in the sea, originating from cleaning the tanks of vessels, or from the release of a few tons of oil, are enough to create mass mortality of seabirds when large concentrations of birds are present in the vicinity. Such incidents have passed unnoticed in spite of control measures. In no case has the source of the pollution been traced (Joensen 1972b:27). There has not yet been a real "oil disaster" in the Danish waters similar to the Torrey Canyon catastrophe. If such a disaster takes place, the destruction of seabirds will be enormous and immeasurable.
| Species | Oil incident no. | |||||
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | Totals | |
| Gavia stellata | 1 | 9 | 1 | 4 | 15 | |
| G. arctica | 2 | 2 | 4 | 8 | 16 | |
| Gavia sp. | 4 | 1 | 5 | |||
| Podiceps grisegena | 4 | 1 | 8 | 8 | 21 | |
| P. cristatus | 1 | 1 | ||||
| Phalacrocorax carbo | 20 | 20 | ||||
| Anas platyrhynchos | 2 | 2 | 4 | |||
| A. clypeata | 2 | 2 | ||||
| Aythya marila | 6 | 2 | 8 | |||
| Clangula hyemalis | 35 | 2 | 26 | 6 | 4 | 73 |
| Melanitta nigra | 387 | 241 | 521 | 262 | 77 | 1,488 |
| M. fusca | 197 | 33 | 417 | 223 | 119 | 989 |
| Somateria mollissima | 1,683 | 1,081 | 947 | 1,713 | 19 | 5,443 |
| Bucephala clangula | 3 | 3 | 13 | 9 | 28 | |
| Mergus serrator | 48 | 28 | 28 | 2 | 106 | |
| Cygnus olor | 10 | 17 | 1 | 28 | ||
| C. Cygnus | 1 | 1 | ||||
| Fulica atra | 1 | 1 | 2 | 5 | 9 | |
| Larus sp. | 13 | 13 | ||||
| Alca torda | 1 | 12 | 1 | 14 | ||
| Uria aalge | 1 | 1 | ||||
| Cepphus grylle | 1 | 2 | 16 | 19 | ||
| Total birds examined | 2,380 | 1,362 | 1,996 | 2,324 | 242 | 8,304 |
| Estimated minimum number of birds killed | 10,000 | 5,000 | 12,000 | 15,000 | 1,500 | 43,500 |
| Percent of total birds contributed by three species[71] | 95.3 | 99.5 | 94.4 | 94.6 | 88.8 | 95.4 |
As a result of five of the major oil pollution incidents in the Cattegat from 1969-71, a total of 43,500 birds were killed, of which 8,304 were examined and enumerated (Table 4). Altogether, 21 or 22 species were involved, but 95% of all birds examined were diving ducks: common eider and black and velvet scoters. At present, it has not been possible to identify any decrease in the number of these ducks in Danish waters due to oil pollution. However, if these disasters continue, it can be expected that duck populations of northern Europe and the Baltic area will be severely reduced, and that an overall decline will take place from which the birds may not be able to recover.
A particularly disastrous year was 1972, when large numbers of ducks were killed as a result of rather small oil spills. A tanker disaster in March 1972 off the eastern coast of Jutland, in the northern Cattegat, and another in December 1972 in the Danish Waddensea, both took place in areas critical to major concentrations of sea ducks. A total of more than 60,000 birds were killed, of which about 95% consisted of the same three species of diving ducks mentioned above. These tragic events represent a further increase in the annual mortality of birds caused by oil, and there is reason to believe that a critical upper limit is rapidly being approached.
It appears, however, that the measures taken by pollution control and naval authorities have greatly improved in recent years. In January 1973, when a Polish merchant vessel collided with a Swedish tanker in the Sound, about 300 tons of heavy fuel oil were released into the sea. Several Danish and Swedish ships working in cooperation succeeded in dispersing the oil, and no serious effect on seabird populations took place (Joensen 1973b:118). It seems that the best way of cleaning up such oil disasters is through a mechanical 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 ultimately end up in the sea, and many large factories release their industrial wastes directly into the sea, the effects of this pollution on marine organisms is attracting a growing interest. 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 (polychlorinated biphenyls), a mixture of related chemical compounds often originating from industrial wastes. In addition, some mercury will always be found, sometimes in increased concentrations. 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.