4. Invasive Species in the Ocean
Case Study: The Pacific Oyster in the North Sea
Introduction
Blue mussels (Mytilus edulis) are important biological populations in the intertidal (eulittoral) zone of the Wadden Sea ecosystem. Beds of the blue mussel are important biogenic structures, serving as habitat and as food source for a number of species. They influence the hydrodynamic processes in reducing flow rates and retaining mud and sand.
Since the late 1990s, the Pacific oyster (Crassostera gigas) has begun to spread into the German Wadden Sea from the East (East Frisia). Today, the Pacific oyster is present in all blue mussel beds in all of the Wadden Sea from the Netherlands in the West up to Denmark in the North. Some mussel beds accommodate up to 1,000 animals per square meter. The Pacific oyster is thus an alien invader in the North Sea.
Development of Blue mussel populations in the North Sea
The area of the blue mussel beds in the Wadden Sea has been decreasing dramatically in the past decades. For example in Lower Saxony, Germany, the area of the mussel beds has decreased from 50 km2 in 1975 to 27 km2 in 1989/91. In spring 1996, the lowest documented level had been reached: only 1.7 km2 of mussel beds were left!
The total biomass of blue mussels decreased from 47,000 t in 1989/91 to 1,000 t in 1996. This equates to a loss of 60%, whereas the decrease in area in the same period was 47%.
Following a strong spatfall in the summer 1996, the total area of mussel beds increased to about 29 km2 (1999)
with a biomass of 110.000 t.
(Data source: Wadden Sea National Park of Lower Saxony)
The spread of the Pacific oyster in the North Sea
In the 1960s, the Pacific Oyster was brought to the Netherlands for farming. It was assumed that the oyster would not spawn in the wild because of relatively low water temperatures. However, the Pacific Oyster adapted quite well and spawn even faster than the native blue mussel. It can produce as much as 50 to 100 million eggs during one spawning period and this can happen several times in one season. The larvae escape from the oyster farms by transport of coastal currents. Thus apart from a successful spatfall, the wind conditions determine the spread.
In the late 1990s, the Pacific Oyster has been found in different places on the German islands of East Frisia. In 1986 it was introduced to Sylt for farming, a few years later the first specimens were discovered in the wild around the island.
Because of a good spatfall in 2003 many oyster beds have rapidly developed into solid reefs in many places. Today there are up to 1,400 Pacific oysters on one square metre (the mean abundance is 300 individuals per square metre). Younger oysters will settle on top of older ones. Today, there are no more mussel beds without oysters left.
How do Blue mussels and Pacific oysters get along?
Blue Mussels and Pacific oysters compete for food. Both species live in the intertidal (eulittoral) zone of the Wadden Sea and filter suspended particles from the sea water. However, the oyster is superior in its growth rate: it can grow up to 5 cm per year and reach a size (length) of up to 30 cm, more than double the maximum size of the blue mussel. Its larvae move much faster than those of the blue mussel thus having better chances of escaping possible enemies. The native mussel serves as food source for birds, crabs and sea stars. The invading oyster has no natural enemies in the North Sea. It settles on mussel beds and, by cementing itself and building reefs, to some extent kills the mussels. Thus the Pacific oyster must be considered a potential risk to the mussel beds of the Wadden Sea.
Sounds like a triumph for the Pacific Oyster?
Actually in the end the two species get along not too bad. They can form complex and biodiverse communities with algae, periwinkles and abundant mussel spatfall. Up till today there has been no evidence that the Pacific oyster has caused or will cause the extinction of the Blue mussel.
A management plan for the blue mussel in the Wadden Sea
The downward trend of the blue mussel population up to the mid 1990s and controversial discussions in the public about fishing being a possible cause for the decline lead to the political decision to set up a management plan for the blue mussel in the Wadden Sea. The two most important steps were
- Regulation of blue mussel fishing by law, and
- Constant monitoring of mussel beds.
As a result, considerable parts of the intertidal area of the Wadden Sea have been permanently closed for blue mussel fishing in the Netherlands, Germany and Denmark.
Monitoring Blue mussel beds
The most precise way of determining the location, size and shape of blue mussel beds in the southern North Sea would be comprehensive ground-surveys. However, considering the total size of the area this task would be impossible. Remote sensing can therefore be a valuable method for mapping mussel beds. Monitoring of Blue mussle beds is done by analysis of aerial photographs in combination with ground truthing. The latter is very important to verify the interpretation of the photograph.
Photographic surveys are carried out in a well-defined period of the year, because the surface covered by mussel beds can increase through spatfall during the summer months and will often decrease during autumn and winter due to storms and/or ice scour.
The following animation is the result of laserscanning which can be used to determine distances. The video shows a mussel bed in the backbarrier tidal flats of Juist and Memmert which is flooded two hours after low tide and again falls dry two hours before low tide. The height of that part of the mussel bed that falls dry is about 1 m.
Would you like to learn more about the laserscanning technique? Visit the tutorial about Remote Sensing Using Lasers.
And how about the Pacific oyster?
Up until today, there have been no management plans for the Pacific oyster. The invader is thus not constantly and comprehensively being monitored. However, there have been single studies where the development of oyster reefs have been mapped through ground-surveys.
Questions
- Between the monitoring period 1989/91 and 1996 the area of blue mussel beds in Lower Saxony decreased by 47% whereas the biomass decreased
by 60%. Why is there such a distinct difference between the two figures?
- Have a look at the map of the habitats of the Wadden Sea area above. Where do you think do the Blue mussels and Pacific oysters have their habitats?
- Would you consider the Pacific oyster to be a form of marine pollution? Remember that it was introduced to the North Sea
by people. Give reasons for your answer.
- On 26th June 2009 the Wadden Sea became a UNESCO World Heritage. From what you know so far about the Wadden Sea, what could have been the reasons for this decision?
