The Dynamics of a Stressed River Fish Community
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Water Air Soil Pollut: Focus DOI 10.1007/s11267-006-9073-y Invasives, Introductions and Acidification: The Dynamics of a Stressed River Fish Community Bjørn Mejdell Larsen & Odd Terje Sandlund & Hans Mack Berger & Trygve Hesthagen Received: 16 June 2005 /Accepted: 23 June 2006 # Springer Science + Business Media B.V. 2006 Abstract We describe the development of the fish been a simultaneous major increase in the occurrence community in the acidified and limed river Litleåna in and density of European minnow since 1997. Our southern Norway, and describe how chemical resto- results show that both brown trout and European ration, compensatory introductions of exotics, and minnow increase after liming. Minnow densities are accidental invasion of exotics interact to influence the negatively affected by low pH episodes in the river. population of the naturally occurring brown trout The growth rates of brown trout fry are negatively (Salmo trutta). The river Litleåna is a tributary to the correlated to minnow densities, indicating competi- river Kvina in Vest-Agder County, southern Norway. tion between the species. Brook trout densities have During the years 1996–2004, annual mean pH was decreased since liming started, and during the brown 4.9–5.0 and 6.1–6.4 above and below the liming trout recovery. facility, which was installed in 1994. Originally, brown trout was the only fish species in the river, Keywords brown trout . European minnow. but brook trout (Salvelinus fontinalis) have been brook trout . introduction . acidification . liming intentionally introduced, whereas European minnow (Phoxinus phoxinus) was introduced by accident. Fish densities were recorded by means of electrofishing 1 Introduction annually over the ten year period 1995–2004. Al- though close to extinction before liming was initiated, Pollution and introduced species are among the major brown trout fry densities increased from 1995 to threats to natural biodiversity. In southern Norway, 1999, with subsequent varying densities. There has acidification due to atmospheric deposition (acid rain) has caused local extinction of thousands of inland fish : : stocks. Simultaneously, intentional or unintentional B. M. Larsen (*) O. T. Sandlund T. Hesthagen introduction of non-native fish species has taken place Norwegian Institute for Nature Research, in most water courses. Over the last decade, reduced Tungasletta 2, 7485 Trondheim, Norway levels of acidifying emissions and various mitigation e-mail: [email protected] measures, in particular liming of water courses, has improved living conditions for fish. The introduced H. M. Berger species, however, are still there, and constitutes a Felt-BIO, Flygata 6, major hindrance for a full restoration of the aquatic 7500 Stjørdal, Norway ecosystem. Water Air Soil Pollut: Focus The development of a river fish community during chemical restoration after acidification has previously been documented (e.g. Larsen & Hesthagen, 2004). The impact of introduced fish species has also been described (e.g., Sandlund & Bongard, 2000; Museth, Borgstrom, Hame, & Holen, 2003). However, the development of a riverine fish community during chemical restoration, with the simultaneous compli- cating presence of non-native fish species has not been described. This paper analyses the development of the fish C 5 LF community in the river Litlåna in southern Nor- 21 way, and discusses how environmental conditions, na compensatory introductions of exotics, and acci- River Litle dental invasion of exotics interact to influence the River Kvina 22 population of the naturally occurring brown trout Lake Galdalsvatnet (Salmo trutta). 23 2 Study Area 24 The river Litlåna is a tributary to the river Kvina in C 8 Vest-Agder County, southern Norway (Fig. 1). The N calcium content in bedrock and sediments is low, and 10 km the capacity to buffer acidifying depositions is poor. Originally, brown trout was the only fish species in Fig. 1 Location of the study area in River Litlåna with electrofishing localities (21–24), the liming facility (LF), and the river. Since 1980, when atmospheric pollution was localities for chemical analysis (C5–C8). The arrows indicate about to wipe out the brown trout populations, the the upper part of the river sections with anadromous salmonids more low pH tolerant brook trout (Salvelinus fonti- nalis) have been introduced several times. Natural reproduction of brook trout has been recorded. environment for some fish species. These drops in pH European minnow (Phoxinus phoxinus) was detected are usually associated with high precipitation, fol- in the river for the first time in 1997 (Berger, 1999), lowed by high water flows in the river. Calcium levels probably as a result of an accidental introduction in a in Litlåna above the liming facility has over the years tributary at least 10–12 years earlier (cf. Hesthagen & 1996–2004 only occasionally been above 0.5 mg Ca Sandlund, 1997). l−1. Below the liming facility, calcium levels are normally around 2 mg Ca l−1, but with occasional 2.1 Water Chemistry drops to just above 1 mg Ca l−1 (Kaste & Skancke, 2004; personal communications). To chemically restore the river, a liming facility was installed in 1994 (Fig. 1). Subsequently, water quality shows the typical trend of acidified rivers in this part 3 Sampling Methods of Norway, with mean pH at 4.9–5.0 above (station C5; Fig. 1) and 6.1–6.4 below (station C8; Fig. 1) the Sampling was performed annually with a portable liming facility (Fig. 2; Kaste & Skancke, 2004; back-pack electric fishing apparatus at four localities personal communication). Although pH around 6 below the liming facility. Fishing was performed in normally is adequate for brown trout and other August every year over the 10-year period 1995– sensitive fish species, the observed drops in pH 2004. Each locality was fished three times (removal values to below 5.5 (Fig. 2) indicates a precarious method; Bohlin, Hamrin, Heggberget, Rasmussen, & Water Air Soil Pollut: Focus Fig. 2 pH above and below 8 the liming facility in Above liming Below liming Litlåna, from January 1996 to January 2005 7 6 pH 5 4 01.01.1996 01.05.1996 01.09.1996 01.01.1997 01.05.1997 01.09.1997 01.01.1998 01.05.1998 01.09.1998 01.01.1999 01.05.1999 01.09.1999 01.01.2000 01.05.2000 01.09.2000 01.01.2001 01.05.2001 01.09.2001 01.01.2002 01.05.2002 01.09.2002 01.01.2003 01.05.2003 01.09.2003 01.01.2004 01.05.2004 01.09.2004 01.01.2005 Date Salveit, 1989; Zippin, 1958), and all fish was creased to a maximum of 40 fish 100 m−2 in 1999. identified to species. All body lengths were measured The dramatic increase was mainly caused by fry, in the field. which reached a density of 30 fish 100 m−2 that year. The density of older parr showed a more steady increase up to 1999. In 2001, however, brown trout 4 Results density decreased dramatically to only 7 fish 100 m−2. Again, the density differences were more dramatic for Brown trout was close to extinction and very few fry (0+) than for older fish. Since 2001 brown trout anglers took an interest in fishing in Litlåna before densities have increased steadily to nearly 30 fish liming was initiated (Kvinesdal kommune, 1999). 100 m−2. The overall tendency for brown trout over During the first two years of sampling (1995–96), the the period 1995–2004 is a significant density increase 2 total density of brown trout fry (0+) and older parr only for older parr (≥1+) (F1,8=10.36; r =0.56; was at only 6 and 5 fish 100 m−2, respectively P<0.05) (Fig. 3). Brown trout densities vary in a (Fig. 3). Subsequently, brown trout densities in- similar pattern at the four sampling stations. Fig. 3 Densities of 50 brown trout fry (0+) (ns) Brown trout Fry (0+) Older parr and older parr (≥1+) (F1,8= 10.36; r2=0.56; P<0.05) in Litlåna, 1995–2004. The 40 sampling localities 21–24 were pooled. Fitted regres- sion line is indicated 30 20 Density per 100 m² 10 0 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Year Water Air Soil Pollut: Focus Brook trout was first introduced to several small minnow at the upper sampling locality (by 1999), lakes in Litlåna in the beginning of the 1980s (Berger, densities of minnow has been 12–75 times higher 1999), and later spread to the main stem of Litlåna. than age-0 brown trout at this locality. Downstream The purpose of this introduction was to compensate migration of minnow has occurred in conjunction for the loss of acid-sensitive brown trout by establish- with the population expansion at the upper locality. ing a population of the acid-tolerant brook trout. Minnows were detected at locality 22, which is When the monitoring programme started in 1995, the 6.5 km downstream from locality 21, in 2002. The mean total density of brook trout was 9 fish 100 m−2, initial density at this locality was 11 fish 100 m−2, but and the population density reached a maximum in already one year later densities were above 200 fish 1998, at 11 fish 100 m−2 (Fig. 4). Brook trout has 100 m−2. In 2004, the first minnows were recorded at mainly been found at the three upper sampling locality 24, which is 8 km below the outlet of the stations, in particular at the station upstream of Lake 4.5 km long lake Galdalsvatn. This indicates a Galdalsvatn. Spawning has also been observed at this downstream migration rate of 19.0 km over seven locality. At the river stations, more than 90% of the years, i.e.