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Spread of the Pacific Oyster continues
Leading experts from all Wadden Sea countries and
England met in Wilhelmshaven at a workshop at the Senckenberg
Research Institute on Thursday 22 March 2007 on invitation of
the Common Wadden Sea Secretariat (CWSS). It was the first
trilateral workshop to discuss the recently observed intensified
spreading of the Pacific Oyster in the entire Wadden Sea. Dr.
Gerald Millat (National Park Administration Lower Saxon Wadden
Sea) emphasized the importance of this scientific workshop as
input for management decisions on nature conservation.
Recent results from scientific projects in The
Netherlands, Germany, Denmark and England were presented which
focus on the bioinvasion of this alien species and the possible
consequences for the Wadden Sea ecosystem, for example, by
replacing native blue mussel beds and changes in the species
composition. In particular birds, which depend on blue mussel as
food resource, may be affected by the spreading of the Pacific
oyster.
Over the last five years the Pacific oyster has
achieved a continuous distribution throughout the entire Wadden
Sea. Main sites of introduction were near the island of Texel
(Netherlands) and near the island of Sylt (Germany) where an
oyster culture was started in 1986. The spreading of the Pacific
oyster is also a global phenomenon: aquacultures on all
continents have caused the establishment of wild populations on
almost all coasts.
The participants of the workshop agreed that the
spreading of the oyster in the Wadden Sea is still in an early
stage. The process is regarded as irreversible. Prof. Karsten
Reise (Alfred-Wegener-Institute for Marine and Polar Research,
Sylt): “This makes it very difficult to predict the future
development and consequences for the ecosystem. It will,
however, be an aggravating change which has to be observed very
carefully.”
On behalf the Common Wadden Sea Secretariat, a
comprehensive data compilation for the entire Wadden Sea area
was carried out to assess this bioinvasion. Although the Pacific
oyster has been observed sporadically in some Wadden Sea regions
for over 20 years, a rapid increase affecting the entire area
has been documented only for the last 5 years. The total oyster
biomass increased 10-fold to about 70,000 tons fresh weight over
that period (The Netherlands: 21,000, Lower Saxony: 23,000,
Schleswig-Holstein: 23,000; Denmark: 3000).
Dr. Georg Nehls (BioConsult SH, Husum), who
compiled the assessment report, expected that these figures
would go up within the next years. He also pointed out regional
differences: the highest occurrence of oysters had been observed
in areas with already decreased abundances of blue mussels.
The increased summer temperatures over the last
10-12 years have been identified as one of the main factors of
the recent increase of the Pacific oyster. A few days in July
and August with water temperatures above 18-20 °C can trigger
the reproduction of the oyster. One oyster can produce about 100
million larvae. On the other hand, the Pacific oyster is able to
survive cold winters better than previously expected. The oyster
development is probably also promoted by an extension of rich
phytoplankton occurrence in late summer.
The Pacific oyster has no natural predator in the
Wadden Sea. No starfish, crabs, birds or parasites can stop the
oyster population from spreading rapidly.
Prof. Wim Wolff (University of Groningen) was
able to show from experiments that oyster larvae swim up to 4
times faster than blue mussel larvae and therefore can better
escape from those animals preying on larvae.
From experiences in the Dutch Delta region (Oosterschelde),
where the Pacific oyster showed a continuous increase over the
last 20 years, it can be assumed that the spreading in the
Wadden Sea will continue further. Therefore, it may be possible
that the typical blue mussel beds will disappear from the Wadden
Sea in future. Dr. Achim Wehrmann (Senckenberg Research
Institute) already detected this development in Lower Saxony:
“Almost all natural blue mussel beds have already been colonized
by Pacific oysters.” A complete displacement of blue mussels by
oysters, however, is not expected. There are several examples of
co-existence of blue mussels and oysters on mixed beds. In
regions with good blue mussel spatfall the occurrence of oyster
reefs is relatively small. However, a spreading of the Pacific
oyster into the subtidal area cannot be excluded.
Oyster reefs may also cause changes in
biodiversity. Heike Büttger (BioConsult SH, Husum) und Alexandra
Markert (Senckenberg Research Institute) documented a different
species composition of the associated fauna on oyster reefs
compared to blue mussel beds. However, negative effects on the
biodiversity have not been detected.
Most of the bird species seem to be able to adapt
to the appearance of oyster reefs because they feed mainly on
the associated fauna, such as worms and crabs. However, birds
with food preference for blue mussels, like Eider and
Oystercatcher, are not able to use oysters as food resource.
“The Oystercatcher may learn to feed on oysters” said Dr. Bruno
Ens (SOVON, NL), ”but the Eider does not have this chance”. A
research project by the Institute of Avian Research
(Wilhelmshaven) will address this issue in detail in field
surveys on the Lower Saxon tidal flats near the mainland coast
of Dornum.
The spreading of the Pacific oyster will probably
also affect the blue mussel fishery because mixed beds with blue
mussels and oysters can no longer be exploited. Another topic,
yet to be addressed, is whether fishery on Pacific oyster would
be possible taking into account the requirements of nature
conservation.
Dr. Karel Essink, chairman of the workshop,
underlined the urgency of further research in order to better
assess the ecological consequences of the Pacific oyster
invasion. “We have to prepare a good scientific foundation now
and in this phase of oyster spreading, to be able to better
assess future changes.” This implies a more intensive assessment
of the data for the entire Wadden Sea. “The Trilateral
Monitoring and Assessment Program (TMAP) is the instrument to
provide results for the entire Wadden Sea area in a harmonized
way and faster than in the past” stated Dr. Harald Marencic of
the Common Wadden Sea Secretariat (CWSS, Wilhelmshaven). |
