1:a NT-Forskningskonferensen Karlstad 5 maj 2015
Measures to preserve a threatened species – The
freshwater pearl mussel (Margaritifera margaritifera)
in the River Västerdalälven
S. Gustafsson
Institutionen för miljö- och livsvetenskaper.
Tel.:054-7002076, E-mail: stina.gustafsson@kau.se
The freshwater pearl mussel is one of the most threatened species in Europe. It has a
complex life cycle including both free-living and parasitic life stages. The obstruction of
one stage may impede life cycle completion, resulting in recruitment failure. In River
Västerdalälven measures are being taken to preserve the freshwater pearl mussel. The
work includes recreating habitats for the mussel and its host fish, the brown trout, as well
as laboratory testing to identify suitable host fish strains used for re-stocking.
Keywords: Conservation, nature-like fishways, biodiversity, Brown trout, host-parasite interactions
BACKGROUND
Freshwater pearl mussels (FPM) (Margaritifera
margaritifera L.) are long-lived organisms with a
complex reproductive strategy. The FPM in Europe
spend their first 10-12 months as obligate gill parasites
on brown trout (Salmo trutta) or Atlantic salmon (Salmo
salar), whereupon they metamorphose to juvenile
mussels (Bauer, 1987). The post-parasitic juvenile life
stage has been identified to be particularly sensitive to
anthropogenic land use changes as juvenile mussels
transform to adults while being buried in the stream
sediment (Osterling et al., 2008). In recent decades, the
recruitment of FPM has decreased in Sweden, which can
be directly linked to factors negatively affecting juvenile
survival. However, the close relationship to their host
fish may imply that factors affecting the parasitic stage
also contribute to the impaired mussel recruitment. Dam
construction, for example, not only changes flow
regimes and causes sedimentation of stream substrates,
but it also disrupts connectivity in running waters
(WCD, 2000). This may prevent host fish from reaching
areas with FPM, impeding mussel life cycle completion,
and obstructing mussel dispersal by larvae attached to
fish (Hastie and Cosgrove, 2001; Watters, 1999).
CONSERVATION EFFORTS
Creating habitats
In south-central Sweden a hydroelectric power plant was
constructed in the River Västerdalälven in 2009. The
inundation caused by the dam flooded FPM habitats and
the mussels in the area had to be relocated. To mitigate
the effects of the dam construction a 500-m long naturelike fishway, called the biocanal, was built. The fishway
was fit with a range of different habitats, some
specifically designed to host a population of FPM and
brown trout (Figure 1). Plans have been made to
reintroduce the FPM in these habitats. Since the County
Board in Dalarna consider the original population of
FPM from River Västerdalälven to rare and valuable to
move, mussels from Tansån (a nearby river with natural
recruitment of FPM) will be used.
Fig. 1. Eldbäcken biocanal. Photo: Anders Bruks
Relocated mussels should be monitored for survival and
gravidity, as gravidity is a sign of wellbeing. The
monitoring is done by carefully opening the mussels
approximately 1 cm with special tongs. Mussels are
considered gravid when the gills have glochidia (mussel
larvae) in their marsupium, which can be seen as a lightyellowish colored mass on the gills (Figure 2).
Fig. 2. Gravid freshwater pearl mussel. Photo: Stina
Gustafsson
Host fish compatibility
To compensate for lost catches of brown trout, due to the
power plant construction in the River Västerdalälven, the
power plant company is obliged to release 50 000
stocked brown trout (0+) individuals of the Siljan strain
in the area. It has been shown that host fish origin may
affect the survival, growth and metamorphosis rate of
juvenile mussels (Taeubert et al., 2010). Before
compensatory stocking can commence it is therefore
important to identify the brown trout strain most
compatible to the mussel. An infestation experiment
containing FPM from Tansån (the steam from which
mussels will be taken to re-stock the biocanal), its
sympatric trout strain and three allopatric trout strains,
the Trettonjällbäcken strain (from the vicinity of the
power plant) and two hatchery reared, Siljan and
Gullspång strains, took place in 2014. Fish and mussels
were kept in the aquarium facility at Karlstad University.
A well-mixed suspension of glochidia from four FPM
was used during the infestation. The concentration of
larvae was 25 000 L and the fish were exposed to the
suspension for 45 minutes. To quantify the encystment
rate, glochidia number of four gill arches/fish were
determined 1, 3 and 40 days after infestation. All fish
were weighed and measured. To study the growth of the
larvae on the different fish strains, the diameter of
encapsulated larvae was measured 1 and 40 days after
infestation.
RESULTS AND CONCLUSION
Preliminary results indicate that there were no
differences in weight normalized encystment rate
between any of the trout strains. However, larvae seemed
to grow faster on the Siljan strain, which may imply a
higher survival for the remaining parasitic period and for
the relatively large juveniles starting their benthic life. In
conclusion, it seems important to investigate the
compatibility between FPM and its potential host fish
strains before re-location or stocking of either fish or
mussel. In the present case, the stocked trout strain
seems to be functioning with the mussel strain that is
planned to be introduced, at least for the parasitic stage
of the mussel.
REFERENCES
Bauer, G. (1987). Reproductive strategy of the
freshwater pearl mussel Margaritifera
margaritifera. The Journal of Animal Ecology.
691-704.
Hastie, L., Cosgrove, P. (2001). The decline of migratory
salmonid stocks: a new threat to pearl mussels
in Scotland. Freshwater Forum.
Osterling, E.M., Greenberg, L.A., Arvidsson, B.L.
(2008). Relationship of biotic and abiotic
factors to recruitment patterns in Margaritifera
margaritifera. Biol Conserv. 141, 1365-1370.
Taeubert, J.E., Denic, M., Gum, B., Lange, M., Geist, J.
(2010). Suitability of different salmonid strains
as hosts for the endangered freshwater pearl
mussel (Margaritifera margaritifera L.). Aquatic
Conservation-Marine and Freshwater
Ecosystems. 20, 728-734.
Watters, G.T. (1999). Freshwater mussels and water
quality: a review of the effects of hydrologic
and instream habitat alterations. Proceedings of
the First Freshwater Mollusk Conservation
Society Symposium.
WCD (World Commission on Dams). (2000). Dams and
development. A new framework for decisionmaking. Earthscan Publishing, London.