| Natural England |
S |
Species (terrestrial) |
S1029 |
Freshwater pearl mussel. (Margaritifera margaritifera) |
3.1.3 |
3.1.3 Trend of population size within the network (short-term trend) |
natura2000_population_trend |
Long-term population trend estimates have been calaculated for the SAC populations in England where repeated baseline surveys have been undertaken:
1. Dubb's Beck in Cumbria was surveyed in detail in 1999 when a population of 1,940 individuals was recoreded (Killeen 2000). Since 1999 there have been a number of occassions where mussels die-offs have been recorded (English Nature per comms). In 2006 a full survey was undertaken for English Nature. These results show that there has been a catastrophic reduction in the Dubb's Beck mussel population. Between the 1999 and 2006 surveys, the numbers of individuals had fallen from 1,940 to 175, equivalent to a 91% loss (Killeen, 2006). In 2007 and 2009 a proportion of the remaining mussels were rescued and moved to a hatchery facility to safeguard the population (in total 50 mussels in captivity). Natural England continues to regulatly monitor the remaining mussels and it is estimated there are ~40 remaining musels and that the population continues to decline (>5%/year) (Measures, 2008-2012).
2. River Clun, Shropshire: baseline surveys have been carried out in 2000-2002 and in 2007-2008. In the 7-8 year period between the two surveys the population has declined by 26% from 3,334 to 2,500 individuals (Killeen, 2008). The greatest losses were from sections upstream of Jay Bridge where there has been a 34% loss and from Jay Bridge to the tributary where there has been a 36% loss. It is estimated that during this 7-8 period an average of approx. 100 mussels per year has been lost (or >3% per year loss). Recruitment of juvenile pearl mussel has not occurred within at least the last 30 years on the Clun and our current estimate (based on extrapolation of extinction curves) is that the mussels will only survive for another 25 years if nothing is done to improve these conditions.
3. River Ehen, Cumbria: In 2006 a full repeat survey was undertaken on the Ehen. The age profile of the mussels were assessed from a series of quadrats from all areas of the mussel population. These results showed that there has been a shift in the entire population to a higher size/age class since the 1996 baseline survey, with the adults now 10 years older. Mussels less than 65mm were found to comprise less than 1.5% of the overall population. Juveniles less than 10mm (4yr old) were rare and only foun din the most oxygenated riffles. The lack of young mussels in the population of this size shows that there is barely any recruitment. Overall the 2006 survey results showed that the river was in unfavourable condition for FWPM?s as there is insufficient recruitment, resulting in an aging population. Further work undertaken in 2007/2008 showed similar results to the 2006 survey. The results shows that numbers of mussels up to 90mm in length were present in higher numbers in the 1996 survey, and mussels greater than 90mm in length were present in higher numbers in the 2006 and 2007 surveys. The change at 90mm is important as it signifies a loss of mussels in the younger age groups (0-35 years old), compared with older specimens. Surveys from other sites demonstrate that younger mussels, not just small juveniles, are more vulnerable to stress and death than older mussels. Thus the loss of 0-90mm mussels in 2006 corresponds to a loss of the mussels that were 0 to 25 years old in 1996.
A recent incident on the River Ehen in 2012 as a result of low flows resulted in significant stress to the majority of the mussel population in the upper reaches (where 90% of the total Ehen population are located). Juvenile searches were undertaken after the incident and no juvenile mussels less than 30mm were found in any of the wider juvenile habitat areas where the vast majority of adult mussels are found. The results of monitoirng transects mussel counts carried out in 2012 show a decline with respect to pearl mussel numbers in the same transects in earlier years. The average estimated decline was 31.5% loss of adult mussels in these transects. However, mussels count were carried out during the time of very poor habitat conditions. The transects will be carried out under good conditions in order to detremine whether this level of loss is genuine. |
| Natural England |
S |
Species (terrestrial) |
S1029 |
Freshwater pearl mussel. (Margaritifera margaritifera) |
1.1.2 |
1.1.2 Method used - map |
distribution_method |
The map is based on all recent survey data collected by the Environment Agency and Natural England and shows the current known distribution of pearl mussels in England. A number of populations have not been surveyed since 2006 and the distrubution map is based on the lastest full baseline survey available for each population. For some historic populations, we need to undertake new surveys to confirm the status of pearl mussels and these have not been included in the distribution map (classed as data deficient) - e.g. River Eden - main river. |
| Natural England |
S |
Species (terrestrial) |
S1029 |
Freshwater pearl mussel. (Margaritifera margaritifera) |
2.4.2b.s |
2.4.2 Population size estimation b) |
population_alt_minimum_size |
Only one river in England is classed as having a viable population (i.e. juvenile recruitment). |
| Natural England |
S |
Species (terrestrial) |
S1029 |
Freshwater pearl mussel. (Margaritifera margaritifera) |
2.4.2c.s |
2.4.2 Population size estimation c) |
population_alt_maximum_size |
Only one river in England is classed as having a viable population (i.e. juvenile recruitment). |
