Posts Tagged ‘reintroduction’

First fish-sampling after infestation procedure – 11/07/2012 –

Written by Ivan on . Posted in Host fish speices

Evaluation of potential host-fishes for Unio crassus needs information of glochidia load and survival on fish gills. Therefore, many fish individuals of different species, hatched and infested in the aquaria room at Lunds University (see previous blog text), were sacrificed and measured in weight and length on a Saturday afternoon/evening. Labeling the fishes was a very important procedure since each individual will be correlated to this data when checking for glochidia infestation rates using a stereomicroscope. Last week a few fish individuals could already be checked for glochidia infestation and many pictures could be taken. On some individuals an immune reaction was visible on the gills. On other individuals successful infestations was indicated by fish-tissue which had been growing over mussel glochidia. This information helps us now to choose fish species for the next UC4LIFE action: re-introduction of U. crassus in the streams Klingavälsån and Fyleån. For this experiment, let’s hope that we still can find gravid mussels in the steams of Skåne!

Aquaria experiment in Lund finished | Fish cage experiment started | AND | A third host-fish mapping under natural conditions realized | 07/08/2012 |

Written by Ivan on . Posted in Host fish speices

Having finished the aquaria experiment at Lunds University (see former blog posts) some three weeks ago, Laura and I started to focus on the third LIFE action: re-introduction of Unio crassus in the Rivers Klingavälsån and Fyleån. Therefore, a fish cage design was established and material was bought in hardware stores. The cage construction was carried out in Hemmerstorps Möllas’ garden during one and a half weeks of sunshine.

The purpose of the fish cages is to fit them with artificially infested fishes with mussel glochidia to finally hatch young mussels. Only fish species known as potentially good host fishes from literature and from own experience gained during the aquaria experiment this year are used therefore. For collecting mussel larvae (glochidia), the Rivers Bråån and Tommarpsån were checked for gravid mussels at the end of July. Unfortunately, none of the 200 checked Unio crassus individuals were gravid anymore. Therefore, I found a greatly deformed living Unio crassus in Bråån, which probably got its different shape due to a unknown physical impact.

Together with Niklas, who is very experienced in mussel gravidity physiology further mussels were checked after the sunny and warm time of period one week ago. But, the results did not change: Probably, the gravidity season of Unio crassus is over for this year. Anyway, we decided to build up the net cages to generally evaluate the method of fish cages in flowing water, together with fish survival. Gained knowledge from this experiment can be used for the re-introduction action in the following years.

Finally, last week was the week of many actions. On Monday Niklas traveled to Skåne while Laura and I started to span plastic nets on fish-cage-frames. On Tuesday and Wednesday a third electrofishing of this year was carried out in Bråån and Tommarpsån which serves the host fish mapping action under natural conditions. Thanks to Delphine’s help, we could realize this LIFE action again.

The evenings were spent with finishing the fish cages. On Thursday, the first fish cages could be transported to the Nature 2000 site “Fyledalen” and settled up in its rivers. Additionally, fishes were caught, measured and placed in the cages. The same for Klingavälsån on Friday.

Over the week-end I started to feed the fishes and to monitor fish survival plus physico-chemical parameters. Yesterday evening I found one cage lying on its side next to the mil in Klingavälsån. All fishes escaped :(. In contrast, one cage already proved great stability since it’s protection shield kept away heavy water plants  :).


Written by Ivan on . Posted in Re-introduction of Unio crassus


The Swedish Unio crassus populations are at the northern edge of its distribution. This highly threatened mussel species has very low population numbers and density in the few Swedish areas where they still persist. With such low population numbers and densities, few host-fish are being infected with the mussel larvae in their natural habitat. Therefore, artificial infection of mussel larvae on host-fish, whereupon the infected host-fish are released back into the rivers will increase the infection of mussel larvae on the host-fish manifold. This has been shown to have a high success of strengthening the mussel populations, for example in Germany. As the main causes for the unfavourable status of the target species have been eliminated, we regard the options for success to be high.

Water chemistry has improved dramatically during the last two decades at both sites where re-introduction of Unio crassus are planned for. Moreover, since the areas presently are under the Natura 2000 regulation, other disturbances fatal for Unio crassus (such as dredging) have been eliminated. In combination to this, several restoration measures will be accomplished in this project. In the two rivers where juvenile mussels will be re-introduced, re-meandering will be performed, which has been shown to have positive effects on recruitment when re-introducing mussel-infected fish.

As physical habitat restoration will be performed simultaneously with the re-introduction of mussels, we might end up with ideal conditions for re-introduction by Unio crassus. This is because very low density of adult mussels will probably not be enough to produce new juvenile mussels. However, by rearing juvenile mussels, and introduce them into the designed habitats (based on Unio crassus and host-fish requirements), the chance to survival of the species is thought to increase manifold.

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