Geoscience Reference
In-Depth Information
to assess and quantify the amount of stable and
potentially suitable habitat for pearl mussels, so
that efforts have the greatest chance of success
as well as to comply with government guidance
on reintroductions. What is striking from this
(unpublished) work is that while many rivers
appear outwardly to have changed little since the
last pearl mussels were recorded (e.g. they have
host fish, are not polluted, nor heavily engineered),
only four rivers out of 11 still have enough suitable
habitat. It appears that stable and suitable pearl
mussel substrate habitats are diminishing across
many Scottish rivers.
for this general decline since the 1950s have
been attributed to a number of factors, including
climate change, over-fishing, increased predation,
sea-lice infestations associated with marine fish-
farming and pollution (Anon, 1993; Walker, 1993;
Hastie and Cosgrove, 2001). It is likely that similar
migratory host fish declines have affected non-
designated west-coast sites, where little fisheries
research has been conducted and changes have
not been quantified. Host fish declines in east-
coast rivers has been far less significant and recent
evidence suggests that host fish populations in
designated east-coast sites tend to exceed critical
density thresholds (Langan
et al.,
2007).
Although a number of studies into the
relationship between pearl mussels and their
hosts have been carried out (Young and Williams,
1984; Bauer, 1987, Ziuganov
et al.,
1994), more
research is needed. Since non-migratory brown
trout are suitable hosts, it is possible that some
trout-dependent pearl mussel populations will
remain, despite declines in migratory components
of salmonid populations. However, migratory
salmonids produce more eggs and juvenile fish
than resident trout (being able to use marine
derived resources unavailable to stream-bound
trout), so their decline is particularly worrying. The
west-coast populations of migratory salmonid fish
are now at their lowest levels ever recorded and
this has coincided with recruitment problems in
most west-coast pearl mussel populations studied
(Hastie and Cosgrove, 2001).
To compound matters, depleted salmonid
stocks may be affected detrimentally by climate
change, being sensitive to extremes in temperature
rise. Thermal stress from elevated temperatures
may cause critical temperature thresholds to
be exceeded in small (warmer) streams. The
upper critical limits for successful growth and egg
development for
S. salar
and
S. trutta
is below 20
◦
C.
Dissolved oxygen content is inversely related to
water temperature, and lowered levels associated
with temperature rises is an important factor in
juvenile fish survival (Elliott, 1995). Therefore,
the UKCP09 high temperature projections of up
to 5
◦
C increases may significantly affect host fish
populations adversely in many watercourses,
Sea level rise
Although Scotland is rebounding as a result
of isostatic (post-glacial) uplift, UKCP09 high
projections indicate that by 2080 sea level is likely
to rise between 5-50 cm on the west coast of
Scotland. This potentially (in combination with
more storm events e.g. higher spring tides, storm
surges, greater on-shore winds - Dawson
et al.,
2001) means that mussels in the lowest reaches
of catchments are at risk from immersion in salt
water. Freshwater pearl mussels cannot tolerate
saline conditions, so potentially some mussels
may be directly killed by saltwater intrusion.
Given the known distribution of mussels, it is
thought that few mussels would be killed in most
Scottish catchments and that such impacts might
affect
10 extant mussel populations. However,
effects may be more severe in some low-lying
small catchments, where the majority of both
mussels and suitable habitat occur within 1 m
height of current sea levels, as these watercourses
may be disproportionately affected by sporadic
salt/brackish water intrusions.
Decline in host salmonid stocks
The present declines in stocks of host migratory
salmonid fish threaten Scottish pearl mussel
populations (Hastie and Cosgrove, 2001), with
host densities below critical threshold levels in
many west-coast populations, including several
of the largest and most important (designated)
mussel populations, where numbers of juvenile
mussels are low (Langan
et al.,
2007). The causes
