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Freshwater Pearl Mussel Population in a Norwegian River by Means of Liming

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Freshwater Pearl Mussel Population in a Norwegian River by Means of Liming Successful restoration of the freshwater pearl mussel population in a Norwegian river by means of liming BJØRN MEJDELL LARSEN ([email protected]) Norwegian Institute for Nature Research (NINA), NO-7485 Trondheim, Norway INTRODUCTION The freshwater pearl mussel (Margaritifera margaritifera) is a rare and threatened species throughout its range, and is the subject of recovery programmes in many countries (cf. Degerman et al. 2009). The main reasons for the decline are anthropogenic influences on aquatic systems. The recruitment is failing and many populations are now without juveniles. During their life cycle the freshwater pearl mussel is dependent on the presence of salmonid fish as hosts for its larvae. Acidification has wiped out Atlantic salmon (Salmo salar) populations in 25 rivers in Norway. In addition, several salmon populations in south-western and western Norway have been in different stages of decline. One of these rivers is the river Ogna in the south-western part of Norway. In 1982-1990 periodical fish mortality was observed in the river, usually following sudden increases in discharge, and decline in river pH, and the freshwater pearl mussel population was reduced to a low level. STUDY AREA FRESHWATER PEARL MUSSEL The catchment area of River Ogna is 115 km² (Fig. 1). The area consists mainly of Recruitment to the freshwater pearl mussel population was completely absent during slowly weathering rocks. The width of the river ranges from 15 to 30 m, and the many years resulting in a predominance of older individuals (length 110 – 135 mm). average discharge at the river mouth is 6.6 m3 s-1. The three main species of fish in Mature specimens, however, have been found and the glochidia survive, attaching the river are Atlantic salmon, brown trout (Salmo trutta) and eel (Anguilla anguilla). themselves to the gills of the salmon host fish. Finds of a few young mussels in 1997 - 1999 (4% of the total number of mussels found) indicated that recruitment was in a stage of recovery (Fig. 4). In 2002, the number of young mussels increased to 36%, and in 2005 and 2008 about two thirds of the mussels were found to be younger than 15 and 18 years respectively (less than 95 mm). Still, only a small number of salmon were infected with mussel larvae. However, the prevalence is increasing as a higher fraction of young mussels are becoming mature. The distribution of the freshwater pearl mussel has increased from 4.0 to 4.8 km of the river. In 2008, young mussels were found at ten of the twelve study sites in the river, and the mean density of mussels has increased by more than 100% from 1999 to 2008 (cf. Fig. 5). 25 25 1997-99 2002 20 20 15 15 10 10 Frequency, % Frequency, Frequency, % Frequency, 5 5 Fig. 1. Location of River 0 0 4,4 4,4 - - 24,4 34,4 44,4 54,4 64,4 74,4 84,4 94,4 14,4 14,4 64,4 74,4 84,4 94,4 24,4 34,4 44,4 54,4 - - - - - - - - 0 - - - - - - - - - - Ogna showing the 0 114,4 124,4 134,4 144,4 154,4 164,4 104,4 104,4 114,4 124,4 134,4 144,4 154,4 164,4 - - - - - - - - - - - - - - 9,5 9,5 19,5 29,5 39,5 49,5 59,5 69,5 79,5 89,5 19,5 29,5 39,5 49,5 59,5 69,5 79,5 89,5 99,5 catchment area with the 99,5 109,5 119,5 129,5 139,5 149,5 159,5 109,5 119,5 129,5 139,5 149,5 159,5 lime dosers and limed Shell length, mm Shell length, mm lakes. 25 25 2005 2008 20 20 LIMING PROJECT 15 15 In an attempt to restore the Atlantic salmon stock in River Ogna, a liming project was initiated in 1991. The river was limed from two lime dosers controlled by river 10 10 Frequency, % Frequency, discharge. In addition, a smaller tributary at Eikeland was limed from a d o s e r, and % Frequency, 5 liming occurs at several of the lakes within the catchment area (Fig. 1). 5 0 0 4,4 4,4 - - 14,4 24,4 34,4 44,4 54,4 64,4 74,4 84,4 94,4 14,4 24,4 34,4 44,4 54,4 64,4 74,4 84,4 94,4 - - - - - - - - - 0 - - - - - - - - - 0 104,4 114,4 124,4 134,4 144,4 154,4 164,4 104,4 WATER QUALITY 114,4 124,4 134,4 144,4 154,4 164,4 - - - - - - - - - - - - - - 9,5 9,5 19,5 29,5 39,5 49,5 59,5 69,5 79,5 89,5 19,5 29,5 39,5 49,5 59,5 69,5 79,5 89,5 99,5 99,5 109,5 119,5 129,5 139,5 149,5 159,5 109,5 119,5 129,5 139,5 Prior to liming (1980 – 1987) the mean annual pH level was 5.2 – 5.8 (Fig. 2). Liming 149,5 159,5 Shell length, mm resulted in a gradual increase in the mean annual pH level to 6.6 in the late 1990s. Shell length, mm An increase in calcium content was also evident after liming, and the annual average exceeded 2.2 mg/l in the first years of liming. Fig. 4. Length frequency diagram of living freshwater pearl mussels in River Ogna in 1997–99 (N = 154), 2002 (N = 194), 2005 (N = 295) and 2008 (N = 318). Fig. 2. Data on pH from 8,0 the monitoring programme 7,5 1,40 7,0 in River Ogna above 6,5 Hetland power station in 1,20 pH 6,0 1971 - 2008. Permanent 1,00 5,5 liming was started in 1991 5,0 (marked with the arrow). 0,80 4,5 From Saksgård & Schartau 0,60 (2009). 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 0,40 Fig. 5. Relative density of 1999 living freshwater pearl 0,20 FISH Numberof mussels minute per 2002 mussel in River Ogna based 0,00 2005 on time-restricted counts In spite of fish mortality in the 1980s, the river Ogna sustained a low level of 1 2008 2 3 (given as number of 4 5 recruitment of salmon throughout the period (Fig. 3). From 1994 there was a notable 6 7 8 9 mussels per minute) in 10 11 increase in the density of salmon fry to more than 60 individuals per 100 m² (Fig. 3). Study site 12 1999 - 2008. This resulted simultaneously in an increase in the density of older salmon parr. 100 Salmon fry (age 0+) Older salmon parr (age 1+, 2+, ...) 2 80 60 Fig. 3. Mean densities per 100 m2 for 0+ and ≥1+ 40 age groups of Atlantic salmon (Salmo salar) in River Ogna before liming, Numberof fish 100 per m 20 1983 - 1988, and after liming, 1991 - 2008. Data 0 from Larsen et al. (1992), 1983 1984 1985 1986 1987 1988 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 Larsen et al. (2006) and Year Saltveit et al. (2009). CONCLUSION Liming has been an important measure in River Ogna. The development has been positive for the water chemistry, fish and freshwater pearl mussels in the first years of liming. The survey indicated that populations of the freshwater pearl mussel have the potential to recover if the conditions are improved. But the watercourses are still sensitive to acid water and continue to be dependent upon a continual supply of lime. REFERENCES Degerman, E., Alexanderson, S., Bergengren, J., Henrikson, L., Johansson, B-E., Larsen, B.M. & Söderberg, H. (2009). Restoration of freshwater pearl mussel streams. WWF Sweden, Solna. 62 pp. Larsen, B.M., Hesthagen, T. & Lierhagen, S. (1992). Vannkvalitet og ungfisk av laks og aure i Ogna, Rogaland før kalking. NINA-Oppdragsmelding 130: 1-37. [In Norwegian]. Larsen, B.M., Hårsaker, K., Kleiven, E., Kvellestad, A. & Simonsen, J.H. (2006a). Ogna. 3 Fisk. Kalking i vann og vassdrag. Effektkontroll av større prosjekter 2005. DN-notat 2006-1: 142-146. [In Norwegian]. Saksgård, R. & Schartau, A.K.L. (2009). Ogna. 2 Vannkjemi. Kalking i vann og vassdrag. Effektkontroll av større prosjekter 2008. DN-Notat 2009-3. [In Norwegian on www.dirnat.no]. Saltveit, S.J., Brabrand, Å., Bremnes, T., Kleiven, E. & Pavels, H. (2009). Ogna. 3 Fisk. Kalking i vann og vassdrag. Effektkontroll av større prosjekter 2008. DN-Notat 2009-3. [In Norwegian on www.dirnat.no]. www.nina.no Cooperation and expertise for a sustainable future.
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