Freshwater Pearl Mussel

Caragh Freshwater Pearl Mussel (FPM) Population

Freshwater pearl mussels (FPM) were first recorded from the Caragh River in ca. 1906.  A full survey of the Caragh and its tributaries was undertaken in 1999 and FPM were found to be present in 22.6 km of the river channel within the catchment with an estimated 2.8 million adult mussels in total (Ross, 1999). This makes the Caragh one of the most important rivers in the world for this species. However, despite the high numbers of adults, the species is still likely to go extinct in this river because there aren’t enough young mussels surviving to maintain the population. Moorkens (2009) projected that the Caragh FPM population will be extinct within a generation unless habitat quality is improved.

The Caragh was chosen as a project area because:

  • It is ranked as one of the top three most restorable FPM populations in Ireland
  • The FPM population is large, widespread, has some recruitment and represents over 23 % of the total national FPM population
  • Without action, the population is likely to go extinct within the current generation due to the lack of juvenile survival
  • Actions are urgently required in the Caragh FPM catchment to reduce losses of fine sediment and nutrients from commercial forestry and farmland

Current conservation condition of FPM in the Caragh

The Caragh FPM population and habitat are currently in unfavourable conservation condition. The number of adults is in decline and there are not enough young mussels surviving to maintain the population.

S.I. No. 296 of 2009 sets out criteria for FPM population and habitat conservation condition and these criteria are shown for the Caragh below:

Freshwater pearl mussel population in the Caragh
Criterion Target Status
Numbers of live adults No recent decline Fail
Numbers of dead shells < 1% of population Pass
Mussel shell length ≤ 65mm ≥ 20% of population Fail
Mussel shell length ≤ 30mm ≥ 5% of population Fail
Freshwater pearl mussel habitat in the Caragh
Criterion Target Status
Macroinvertebrates EQR 0.90 Pass
Filamentous algae Absent or trace < 5% Fail
Phytobenthos EQR 0.93 n/a
Macrophytes Absent or trace (< 5%) Fail
Siltation No artificially elevated levels Fail

Records from Stelfox (Jackson 1925) indicate that FPM have been known from the Blackwater since the early 19th century, however, the size and importance of the Blackwater population was not recognised until the mid-1990s.  A comprehensive survey for FPM in the River Blackwater and its main tributaries, was undertaken in 1999 and concluded that the river supported a population estimated to be 2.76 million adult mussels (Ross 1999).  However, similar to the Caragh, there aren’t enough young mussels surviving to maintain the population (i.e. insufficent juvenile survival.

The Kerry Blackwater catchment was chosen as a project area because, similar to the Caragh, it had the following features:

  • It is ranked as one of the top three most restorable FPM populations in Ireland
  • It has an FPM population that is large, widespread, has some recruitment and represents almost 23 % of the national population
  • It is projected that the FPM population will decline appreciably if habitat quality is not improved rapidly
  • Actions are urgently required in the Blackwater catchment to reduce losses of fine sediment and nutrients from commercial forestry and farmland

Current conservation condition of FPM in the Kerry Blackwater

The FPM poulation and habitat in the Kerry Blackwater is currently in unfavourable conservation condition. S.I. No. 296 of 2009 sets out the criteria for evaluating conservation condition of FPM and these are shown for the Kerry Blackwater FPM population and habitat below:

Freshwater pearl mussel population in the Blackwater River 
Criterion Target Blackwater (Kerry)
Numbers of live adults No recent decline Fail
Numbers of dead shells < 1% of population Pass
Mussel shell length ≤ 65mm ≥ 20% of population Fail
Mussel shell length ≤ 30mm ≥ 5% of population Fail
Freshwater pearl mussel habitat 
Criterion Target Status
Macroinvertebrates EQR 0.90 Pass
Filamentous algae Absent or trace < 5% Fail
Phytobenthos EQR 0.93 n/a
Macrophytes Absent or trace (< 5%) Pass
Siltation No artificially elevated levels Fail