This study deals with the influence of benthos abundance when
assessing the potential impact of a small wind park on wintering sea
ducks. Using the Before-After-Control-Impact design, it was suggested
in a recent study (Guillemette et al. 1998) that the wind park provoked
a decline in the abundance and a change in the distribution of common
eiders Somateria mollissima and common scoters Melanitta nigra .
However, the observed decline in sea duck abundance occurred concomitantly
with a decline of benthic food supplies. We measured concomitant
food and common eider abundance for a fourth year at Tunø
Knob to test the hypothesis that, if food abundance increases again, we
should also observe an increase in duck abundance. The methods used
in this study are similar to those applied in the aforementioned study.
The results showed that the average number of common eiders increased
considerably in 1997-98 (3,361 individuals) compared to 1996-97 (458),
even surpassing the level observed during the baseline year in 1994-95
(1,821). A significant increase in the abundance of common scoters
occurred in 1997-98 compared to 1995-96 and 1996-97, but not in relation
to the baseline year. The abundance of food for sea ducks also
increased in 1997-98 where the frequency of occurrence of most potential
prey reached the level observed during the baseline year. The density
of blue mussels, a preferred prey species, was 1,113 individuals
m-2 in 1997-98 compared to 11,111 individuals m-2 during the baseline
year and only 100 individuals m-2 in 1996-97. Computations of the
amount of food supply eaten by wintering common eiders suggest that,
during the baseline year, prey were superabundant. This may explain
why we observed a great abundance of common eiders in 1997-98
despite the fact that mussel abundance did not reach the level observed
during the baseline year. Finally, the distribution of common eiders in
1997-98 on the study site as a whole was very similar to the distribution
observed during the baseline year. A similar observation was made
around the wind park. These results support the hypothesis that the
decline of sea ducks observed during the two After years was not caused
by the wind park. We conclude that without measuring the abundance
and the distribution of food supply, it will remain difficult to make any
reliable impact assessment of an offshore wind park on sea ducks.