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Japanska ostron i svenska vatten - Främmande art som är här för att stanna

Report
Authors Åsa Strand
Susanne Lindegarth
ISBN 978-91-637-6174-4
Publisher University of Gothenburg
Place of publication Göteborg
Publication year 2014
Published at Department of Biological and Environmental Sciences, Tjärnö Marine Biological Laboratory
Language sv
Links www.vbcv.science.gu.se/
Keywords Pacific oysters, Crassostrea gigas, Scandinavia, bio- invasion, spreading, risk assessment, climate, habitats
Subject categories Marine ecology

Abstract

Where do the Pacific oysters come from? The oyster Crassostrea gigas originates from the western Pacific. It was imported to Europe in the 1960s for aquaculture purposes. Since then, the oysters have dispersed by natural reproduction from the farm areas and established wild populations in many places. In Sweden, the species was disco- vered in large quantities during the summer of 2007. Surveys along the Swedish west coast down to Öresund identified specimens of oysters from the Norwegian border down to Falkenberg in the south. It is not clear how the oyster larvae came to the Swedish west coast in 2006, but modelling conducted within the project show that the most plausible explanation is that oyster larvae were transported by ocean currents from wild oyster stocks in Denmark. Genetic studies are currently undertaken and will provide further insight as to the origin of the Swedish populations. Environmental preferences and habitat choice The oysters can settle on virtually all types of substrates, ranging from muddy sediments with small shell fragments or grains of sand to hard substrates. They are also found in environments with the brown algae bladderwrack. The oysters prefer limited wave action but areas with good water circula- tion. Shallow sandbanks and straits with relatively high water flow is an optimal environment. Oysters thrive in similar environments such as blue mussels and they often occur together. The dense oyster stocks are commonly found at about 0-1 m depth but there are observations from several places where oysters occur deeper. Sites with established oyster populations have increased in den- sity between 2007 and 2013, from 4 individuals/m2 to over 100 individuals/m2, and at some sites the density of oysters can even reach more than 1 000 individuals/m2. Optimum salinity for growth and survival is approximately 25‰ but adult oysters occur in salinities down to 5 and above 40‰, and the temperature span for survival range from below freezing point to about 40°C (depending on exposure time). Suitable conditions for reproduction and larval stages are more limited than for the survival of adult individuals. Our experiments have shown that oysters are extremely cold-resistant and can survive several days at –22° C, thus oysters can survive air exposure during cold winter days. Despite this high tolerance to low temperatures, the winter 2009/2010 caused high mortalities of the oyster population (approximately 87%), most likely as a consequence of low water levels in combi- nation with a prolonged period with extremely low temperatures and heavy ice formation. Condi- tions on the Swedish west coast are thus suitable down to Öresund, although reproduction south of Gothenburg may be limited. It is unlikely that the oysters will become established in the Baltic Sea due to the low salinity. Our conclusions are that climate conditions in the region may temporarily reduce the oyster population, but extinction of the species by natural causes is highly unlikely. Present distribution in Sweden and population structure Since 2007, the presence of oysters has shifted northward and the largest quantities of oysters are now found in northern Bohuslän. This may be a result of the low salinity and high wave exposure along the coastline south of Gothenburg. The number of locations with live oysters in southern and central Bohuslän has decreased between 2007 and 2013. However, at sites where oysters occur in dense aggregates, the numbers are expected to increase as new oyster larvae are attracted to sites with oysters already present. Changes in densities, recruitment, age structure and growth rate have been investigated at five loca- Åsa Strand och Susanne Lindegarth 11Japanska OstROn i sVenska Vatten english summary tions visited annually since 2007. Recruitment has occurred annually since the initial invasion, albeit with varying degrees of success. The oysters that arrived and settled in Sweden in 2006 have been dominating the population structure until the summer of 2011, when a very successful reproduction was observed. Winter mortality has been recorded to be very limited over the years, except for during the winter 2009/2010. Despite the high winter mortality that year, several large oyster populations still existed with the ability to reproduce. The oyster population has now recovered and continues to increase in density. The growth rate of individual oysters in Sweden is high and comparable to oysters in other countries.

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