Page 149 - Sjæklen2008

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has increased by 0.7°C and at the same time the sea level
has risen approx. 17 cm. Climate experts believe that this
trend will go on or even accelerate in the near future. Hence,
we might have to get used to a climate which is significantly
milder than it is today. Emissions of large quantities of the
greenhouse gas carbon dioxide (CO2) from the burning of
fossil fuels such as oil, coal and gas are causing these dra-
matic changes in climate.
There is no doubt that the warmer climate will affect life
on earth, both terrestrial and aquatic, in a number of ways.
The effects of increased water temperatures in the North
Sea are already starting to reveal. When looking at the geo-
graphical distributions of fish species in the North Sea, the
centre of distribution for a number of species has shifted
towards higher latitudes. These include cod, dab, grey gur-
nard, angler fish and whiting. Red fish and blue whiting are
on the brink of completely retracting from the North Sea.
At the same time, a number of species are now to be found
at greater depths than previously. These include plaice and
haddock.
While coldwater species are moving to higher latitudes,
exotic species are moving in. Recently, sea bass, striped red
mullet, red mullet and thicklip grey mullet have all become
increasingly abundant in Danish waters. The thicklip grey
mullet is probably even spawning in Denmark.
Resident species risk local extinction if temperatures
rise above their tolerance level. The eelpout is thus at risk
of disappearing entirely from the Wadden Sea. Rising tem-
peratures decrease the volume of dissolved oxygen in the
water. At the same time, the eelpout’s metabolic rate in-
creases. Thus at some point the quantity of oxygen needed
to sustain life exceeds what is available in the water. The
eelpout then simply suffocates. A team of German scientists
has found that this happens when the water temperature ex-
ceeds 25°C.
By opening a corridor to exotic warm water-adapted
species, global warming is intimately linked to biologi-
cal invasion. The invasion of the Wadden Sea by the giant
Pacific oyster has most likely been facilitated at least to
some extent by increasing water temperatures. The pacific
oyster originates from Japan and Southeast Asia, but was
introduced to Europe, including Denmark, in the 1960s by
aquaculture interests. The introduction was not expected to
lead to any settling in the wild due to the low temperatures
in Denmark. Unfortunately this assumption proved to be
wrong. In 1996 the first wild growing Pacific oysters were
found in the Wadden Sea, and in 2005 the biomass was esti-
mated at 1100 tons. One year later the population had grown
to 3300 tons, and in 2007 the estimate was 6300 tons.
When the climate changes, so does the timing of natural
phenomena such as migration, reproduction and flowering.
In the sea, the spring migration of common sole to shal-
lower water seems to occur earlier. The same seems to apply
to the migration of garfish. When different trophic levels
respond differently, the risk is that the functional links in
the ecosystem become decoupled. Altered species interac-
tions might change the entire ecosystem. The mud shrimp
Corophium voluntator is parasitised by two trematodes. The
level of infection, and hence mortality, is, however posi-
tively correlated with temperature. A model predicts that
a temperature increase of just 4°C will result in such high
mortality that the mud shrimp population in the Wadden
Sea will collapse. Being an ecologically important species,
a collapse of the mud shrimp population would also show
impact upon higher trophic levels, such as waders, as well
as sediment stability.
What the future fish fauna will be like is hard to predict.
However, some sort of answer might be found by looking in
the past.Around 7000-3900 BC, the temperature in Denmark
was approximately 2°C higher than today. Looking at fish
bones from kitchen middens during this warm period has
allowed a glimpse of the fish fauna at that time. It turns out
that sea bass, black sea bream, common stingray, thinlip
mullet and swordfish roamed the waters. A number of these
species have actually become more prevalent lately, and
they thus might give a hint of the future. Interestingly, the
kitchen middens also revealed that cod were abundant dur-
ing the warm period. Thus, given the species’ adaptability,
nothing excludes the possibility that cod might also thrive
in the future.