Density and biomass of smelt (Osmerus eperlanus) in five Finnish lakes

Fisheries Research 73 (2005) 353–361

Density and biomass of smelt (Osmerus eperlanus) in five Finnish lakes Juha Jurvelius a, ∗ , Heikki Auvinen a , Irma Kolari a , Timo J. Marjom¨aki b a

b

Finnish Game and Fisheries Research Institute, Saimaa Fisheries Research and Aquaculture, Laasalantie 9, FI-58175 Enonkoski, Finland University of Jyv¨askyl¨a, Department of Biology and Environmental Science, Fish Biology and Fisheries, P.O. Box 35, FI-40014 Jyv¨askyl¨a, Finland Received 24 February 2004; received in revised form 17 January 2005; accepted 17 January 2005

Abstract Hydro-acoustic stock assessment and exploratory sampling with small mesh-sized trawls and seines have sometimes suggested that the importance of smelt (Osmerus eperlanus) in pelagic fish communities might be greater than sampling from commercial fishery and gillnetting indicate. We studied the proportion of smelt in the total fish density (fish ha−1 ) and biomass (kg ha−1 ) of pelagic fish with echo sounding and trawling in five southern boreal lakes. Fish density varied between 460 and 2000 fish ha−1 in the study lakes. Smelt and vendace (Coregonus albula) accounted for more than 95% of the exploratory trawl-catches. The total fish biomass in the study areas varied between 3 and 13 kg ha−1 . In four lakes the proportion of smelt was more than 60%. In one lake the proportion of vendace was over 85%. These estimates indicate the importance of smelt in the study lakes. Studies of the co-occurrence of smelt with other fish species in the pelagic area of boreal lakes are needed to get a less biased picture of the fish community. This underlines the need for relevant sampling methods for the species involved. © 2005 Elsevier B.V. All rights reserved. Keywords: Biomass; Boreal lake; Density; Smelt; Vendace

1. Introduction The smelts of the genus Osmerus have an almost circumpolar distribution and over this range they live under various ecological conditions, e.g. in temperatures from below 0 ◦ C to more than 20 ◦ C (Nellbring, ∗ Corresponding author. Tel.: +358 205 751 612; fax: +358 205 751 609. E-mail address: [email protected] (J. Jurvelius).

0165-7836/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2005.01.016

1989). Detailed studies of the co-occurrence of smelt (Osmerus eperlanus) with other fish species are needed to get a good picture of its impact on these species and on fishery, especially, in pelagic areas of boreal lakes. However, the sampling of pelagic smelt has proved to be problematic. This may create difficulties, e.g. in regard to “The Water Framework Directive” of the European Union (2000/60/EC), which demands the evaluation of the quality and status of fish stocks in lakes.

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Vendace (Coregonus albula) is economically the most important fish in Finnish lakes. The commercial and recreational value of smelt is low in these lakes. Hence, the commercial fishermen try to avoid catching smelt by trawling at appropriate times and by fishing with a large mesh-sized, usually larger than 13 mm (from knot to knot) cod-end, thus focusing on vendace. During recent years, special sorting panels have been developed for vendace trawls to avoid catching smelt and small vendace (e.g. Suuronen, 1995). Prevailing practices in lake fishery have in many cases led to “a protection” of smelt, and hence to an underestimate of its stock size when the assessment has been based on commercial fish catches. Because of its low value, smelt is also often insufficiently reported in the catch statistics. In recreational lake fishery the smelt catch is ca. 1% of that of vendace (Anon, 2002). However, exploratory trawling with smaller mesh, e.g. 10 mm, cod-end, has often resulted in large smelt catches in comparison with vendace (e.g. Jurvelius et al., 2000a). Also the stomach analyses of pikeperch (Sander lucioperca), landlocked salmon (Salmo salar) and brown trout (Salmo trutta) suggest the large importance of pelagic smelt in the food chain in lakes (Peltonen et al., 1996; Valkeaj¨arvi et al., 1997; Auvinen et al., 2004; Keskinen and Marjom¨aki, 2004). Additionally, the numbers of smelt has been undervalued, especially when fishing with gillnets was the only method to assess the species composition of pelagic fish stocks (e.g. Jurvelius et al., 1988). As Olin and Malinen (2003) have shown, smelt is very often missing from gillnet catch, and, furthermore, Peltonen et al. (1999) stated that gillnet CPUE was not suitable for analysing the dynamics of the smelt stock. Most smelt studies in Finnish lakes are done in eutrophicated areas like Lake Vesij¨arvi (e.g. Jurvelius and Sammalkorpi, 1995; Horppila et al., 1996; Turunen et al., 1997; Peltonen et al., 1999). In very few occasions, e.g. in Lake Ouluj¨arvi smelt is intentionally trawled or caught in large amounts as by-catch in vendace trawling in order to keep the supposed balance between smelt and vendace in the fish assemblage (Sutela et al., 2001). In Lake Pyh¨aj¨arvi (SW, Finland) commercial seine-netters have been subsidized for catching smelt (Partanen, 1997). Echo sounding is a rapid and fishery-independent method of fish stock assessment in lakes (e.g. Jurvelius et al., 1988; Marjom¨aki and Huolila, 1995). It has

often revealed in oligotrophic lakes many small targets whose identity has remained unknown because the sampling of the echo-surveyed fish has been based on commercial trawl-catches or gillnet fishing (e.g. Jurvelius et al., 1984; Jurvelius et al., 1988). When the sampling was done with small mesh-sized seine, the length distribution of fish caught and those estimated by hydro-acoustics had a fairly close correspondence (Jurvelius et al., 2000b). “The Water Framework Directive” of the European Union (2000/60/EC) demands the evaluation of the quality and status of the fish stocks in lakes. There are plans to base much of this work on gillnet fishing (e.g. Appelberg et al., 2000; Malmquist et al., 2001). In this paper, we assess the density and biomass of smelt and vendace with echo sounding and trawling in five boreal Finnish lakes. On the basis of the results we discuss the possible ecological role of smelt in meso- and oligotrophic lakes.

2. Materials and methods 2.1. Study area The study lakes are located in eastern Finland and they belong to the Vuoksi water system flowing through Lake Ladoga to the Baltic Sea (Fig. 1). The water area of the lakes ranges from 110 to 700 km2 (Table 1). The study areas were between 6 and 12 km2 and they were concentrated in the most popular trawling areas, i.e. the deepest parts of the lakes. Water quality was good or excellent in the study areas (Niinioja et al., 1999). Commercial trawling for vendace is done in every lake when the strength of the fluctuating stocks makes it possible. Because of the scarce vendace stocks, trawling was forbidden in our study year in Lake H¨oyti¨ainen and restricted in Lake Paasivesi. The trawl-catches were weak in Lake Etel¨a-Saimaa and moderate in Lake Pyh¨aselk¨a. However, trawling in a strong vendace stock was going on in Lake Pihlajavesi. 2.2. Estimation of the density and biomass of fish species A vertically aimed 120 kHz split-beam echo sounder (Simrad EY500, ES120-7F transducer, 7◦ beam width, 2 pulses s−1 ) was used during the exploratory trawling

J. Jurvelius et al. / Fisheries Research 73 (2005) 353–361

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Fig. 1. The five study lakes in eastern Finland.

to estimate the fish density (fish ha−1 ) in the study areas in 1999–2003. The number of sampling units (EDSU) was 10 in each trawling in lakes Pyh¨aselk¨a, Pihlajavesi and Etel¨a-Saimaa. Furthermore, each survey transect in lakes H¨oyti¨ainen and Paasivesi was divided also into 10 units. The length of the units was equal within each trawling and survey transect, although there was some variation in its length between the study areas. The length of the unit was most often between 200 and 400 m. The length of the echo-survey varied from 3 to 25 km in the study areas (Table 1). The fish densities (fish ha−1 ) were analysed in one layer between 5 m and the bottom in every EDSU. Al-

most all targets in this depth layer were seen as single echoes on the echogram. Therefore, the densities could be estimated by echo counting with TVG 40 logR. In trace density estimation the minimum target strength was −56 dB. Thus we aimed on adult fish. Small fish (<4 cm) and all fish living between the surface and 5 m were excluded from the analysis. The densities were estimated by post-processing program EP500, Simrad, and averaged for each trawling and survey transect. Fish in the echo-surveyed area were sampled with one-boat exploratory trawling in all areas except in L. H¨oyti¨ainen where two-boat trawling was done. Hauling took place between the surface and 45 m

Table 1 Parameters of the study lakes and the length of the echo-survey in 1999–2003 Lake

A tot. (km2 )

Depthmax (m)

Study area (km2 )

Study area depthmax (m)

Length of the survey (km)

H¨oyti¨ainen Pyh¨aselk¨a Paasivesi Pihlajavesi Etel¨a-Saimaa

293 246 110 550 700

55 67 75 67 64

6 10 10 6 12

40 45 45 45 45

3 12 10 25 4

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Table 2 Trawl-sampling in the five boreal lakes Study area

Trawl Date

Hour

Hauling depth (m)

L. H¨oyti¨ainen

1 1 1 1

10.8.2000 10.8.2000 10.8.2000 10.8.2000

21.00–22.00 21.00–22.00 23.00–24.00 23.00–24.00

0–5 5–10 10–15 15–20

L. Pyh¨aselk¨a

3 3 3

24.9.2003 24.9.2003 25.9.2003

11.20–12.20 15.14–15.54 11.00–12.00

5–20 15–30 5–20

L. Paasivesi

1 1 1 1 2 2 2 2

1.10.1999 1.10.1999 1.10.1999 1.10.1999 10.10.2001 10.10.2001 10.10.2001 10.10.2001

17.09–18.07 17.09–18.07 9.47–10.45 9.47–10.45 16.23–17.04 11.16–12.17 13.08–14.09 15.07–15.47

10–15 15–20 20–25 25–30 0–10 7–17 25–35 35–45

2 2 2 2 2 2 2 2 2

6.8.2001 6.8.2001 6.8.2001 7.8.2001 7.8.2001 8.8.2001 8.8.2001 9.8.2001 9.8.2001

19.30–20.20 21.02–21.34 22.55–23.15 20.05–20.47 21.42–22.08 18.36–19.36 22.16–22.52 18.50–19.49 20.48–21.48

0–10 10–20 10–20 0–10 10–20 0–10 10–20 0–10 10–20

L. Etel¨a-Saimaa 1 1 1 1

22.9.1999 22.9.1999 23.9.1999 23.9.1999

14.27–14æ51 14.27–14.51 12.41–13.20 12.41–13.20

20–25 25–30 20–25 25–30

L. Pihlajavesi

1 = 36 m trawl, 2 = 151 m trawl and 3 = 212 m trawl.

depth. (Table 2). The hauling speed varied from 0.7 to 1.1 m s−1 and the time between 30 and 60 min haul−1 in each depth. The trawl depth was regulated by altering the length of the ropes from each otter-board to the buoys at the surface. The exact hauling depth was decided according to the vertical distribution of fish found by hydro-acoustics. Our 151 m and 212 m trawls had a conventional structure, being 10 and 15 m in height and 66 and 91 m in width respectively. Our 36 m trawl had a two-storey structure, the height of each part being 5 m with 13 m width. The cod-end mesh size for the 36 m trawl was 5 mm from knot to knot. The largest two trawls had an extra 5 mm cod-end around their 10 mm cod-end. Total fish density (fish ha−1 ) was divided into species according to the proportion of different species

Table 3 Smelt and vendace yield (kg) in exploratory trawling in the study lakes Lake

Smelt

Vendace

Others

Tot.

H¨oyti¨ainen Pyh¨aselk¨a Paasivesi 1 Paasivesi 2 Pihlajavesi Etel¨a-Saimaa

26.6 379.0 5.3 54.0 0.8 8.7

0.6 24.0 3.5 15.7 169.8 2.6

2.9 18.0 0.5 5.0 0.5 2.3

30.1 421.0 9.3 74.7 171.1 13.6

Total

474.4

216.2

29.2

719.8

Paasivesi 1 = October 1999, Paasivesi 2 = October 2001.

in the exploratory trawl-catches. The mean length and weight for smelt and vendace was estimated from samples. Thereafter the density estimate of each species was multiplied by its mean weight in the study area to get an estimate of the biomass (kg ha−1 ) of the species.

3. Results Smelt and vendace accounted for more than 95% of the total fish yield (720 kg) in the exploratory trawling (Table 3). The other species caught were whitefish (Coregonus lavaretus), perch (Perca fluviatilis), brown trout and pike-perch. About 90% of the catch in lakes H¨oyti¨ainen and Pyh¨aselk¨a were smelt. The catches in lakes Etel¨a-Saimaa and Paasivesi were to a minor extent dominated by smelt. Only in the catches of L. Pihlajavesi the biomass of vendace was dominating. The average weight of smelt in the study lakes varied between 3 and 10 g and that of vendace from 7 to 60, respectively (Table 4). The average weight of all smelt caught in L. Paasivesi was 6.0 g in October 2001 (Table 5). Smelt specimens caught by the 5 mm Table 4 Mean weight of smelt and vendace in the trawl-catches Lake

H¨oyti¨ainen Pyh¨aselk¨a Paasivesi 1 Paasivesi 2 Pihlajavesi Etel¨a-Saimaa

Smelt

Vendace

Mean (g)

N

Mean (g)

N

6.4 6.0 9.9 6.0 3.1 8.5

1650 30 177 267 16 200

12.0 31.7 60.1 36.1 7.4 54.6

50 761 70 45 30 47

For specifications see Table 3.

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Fig. 2. Length distribution (mm) in numbers of smelt caught with 10 and 5 mm cod-end in trawling in four depth layers in L. Paasivesi in October 2001.

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Table 5 Mean weight of smelt and vendace in four depth layers in trawl-catches in Lake Paasivesi in 2001 Depth (m)

Smelt

Vendace

10 mm cod-end n

5 mm cod-end

Mean (g)

S.D. 2.8 0.8 4 5.1

0–10 7–17 25–35 35–45

15 30 50 50

6.5 4.3 7.2 10

Total

145

7.5

n

Total smelt mean

Mean (g)

S.D.

32 30 30 30

3.9 3.6 4.2 5

2.1 0.7 0.6 0.8

122

4.2

10 mm cod-end n

Mean (g)

S.D.

4.7 4.0 5.8 8.1

17 17 9 2

32.6 31.1 50.1 45.5

17.8 20.2 15.6 10.6

6.0

45

36.1

Sampling was done between surface and 45 m from fish determined acoustically. Every vendace was caught from the 10 mm cod-end, n is the number of fish specimens.

cod-end were significantly (p < 0.0001) smaller (mean weight 4.2 g, mean length 90 mm) than the ones (7.5 g, 106 mm) caught by the 10 mm cod-end in all depth layers (Fig. 2). On average the weight and length of both smelt (p < 0.0001) and vendace (p < 0.05) was greater below 20 m depth than above it. Acoustically estimated fish density varied from 460 to 2000 fish ha−1 in the study areas in the lakes (Fig. 3). The fish biomass was 3 kg ha−1 in L. H¨oyti¨ainen (Fig. 4). In L. Pyh¨aselk¨a the corresponding figure was ca. 13 kg ha−1 and in L. Pihlajavesi 9 kg ha−1 . In four lakes the proportion of smelt was more than 60%. The

biomass of vendace was larger than that of smelt only in L. Pihlajavesi. It was almost as large as the smelt biomass in the three densest lakes.

4. Discussion On average smelt was 20–100 times more abundant (specimens ha−1 ) than vendace in lakes H¨oyti¨ainen, Pyh¨aselk¨a, Paasivesi and Etel¨a-Saimaa. This situation was comparable with that in the three largest Swedish lakes in 1987–1999 (Nyberg et al., 2001). In L. Pihla-

Fig. 3. Acoustically estimated smelt and vendace density (fish ha−1 ) ± S.E. in the study area. The total fish density was divided into species according to exploratory trawl-catches.

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Fig. 4. Smelt and vendace biomass (kg ha−1 ) ± S.E. in the study area.

javesi the situation was inverse; the density of vendace was six times higher than that of smelt, i.e. vendace density was at the same level as the smelt density in lakes Paasivesi and Etel¨a-Saimaa, as well as in two of the three largest Swedish lakes. Also the biomass of smelt was greater than that of vendace in four of our study areas. Our results together with those from the Swedish lakes (Nyberg et al., 2001) indicate that smelt is an important species also in the pelagic areas of oligotrophic lakes. In the current work, only in lakes H¨oyti¨ainen and Paasivesi biomass estimates were representative for a larger part of the lake. In the other lakes the estimate was strictly restricted to the hauling area. In contrast to the estimates of the density and biomass of vendace in several other lakes, the density of smelt was in the same category in four of our study lakes. In L. Paasivesi in 1984–1991 the density of adult vendace was assessed at 59–949 specimens ha−1 (Auvinen and Jurvelius, 1994). The respective figure for age 0+ vendace has varied between 0 and 5000 in Finnish boreal lakes in 1971–1997 (Helminen et al., 1993; Auvinen, 1994; Auvinen et al., 2000; Karjalainen et al., 2000). However, because vendace specimens are generally larger than smelt, the biomass of vendace is usually greater than that of a smelt population of the same abundance. The smelt catch has exceeded 20 kg ha−1 in two successive years in a bio manipulation experiment of a eu-

trophic lake (Jurvelius and Sammalkorpi, 1995). Based on present results, it is likely that the annual sustainable smelt catch could be ca. 5 kg ha−1 in many oligotrophic boreal Finnish lakes. However, because of the noise caused by the waves, and the small sample volume and blind zone of the transducer we had to exclude from our analysis fish living between surface and 5 m. Thus the sustainable yield might be larger. To give less biased recommendations for sustainable smelt fishery an up-looking and/or horizontal beaming echo sounding should be needed (e.g. Jurvelius et al., 1996; Knudsen and Saegrov, 2002). It is known that the annual vendace catch has exceeded 20 kg ha−1 , at least in lakes S¨akyl¨an Pyh¨aj¨arvi (Sarvala et al., 1998) and Onkamo (Auvinen et al., 2000). According to Kurkilahti et al. (1998), for smelt lengths between 8.9 and 17.0 cm the gillnet catches have to be corrected for gillnet selectivity. In the current work, it might have been difficult and very laborious to catch a representative smelt sample with gillnets from the surface to 45 m depth in L. Paasivesi. In L. Pyh¨aselk¨a it is necessary to sample smelt with trawling or seining (Karjalainen, unpublished working paper in Finnish). Smelt eats during its first summer initially rotifers, copepods and successively larger zooplanktons (Nellbring, 1989). With increasing size and age, its food changes to larger crustaceans and in some cases to

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fish (Nilsson, 1979; Sv¨ardson et al., 1988). According to Sterligova (1979) smelt also eats vendace, whitefish larvae and fry. On the other hand, smelt is an important food item for predatory fish species like pike-perch, brown trout and landlocked salmon (Peltonen et al., 1996; Heikinheimo et al., 2002; Keskinen and Marjom¨aki, 2004). Hence, the strong smelt stocks in lakes H¨oyti¨ainen and Pyh¨aselk¨a might support strong pike-perch stocks (e.g. Jurvelius et al., 2000a). Smelt forms the majority of the diet of landlocked salmon and brown trout, especially, when the vendace stock is scarce (Auvinen et al., 2004). In this respect, neglecting the assessment of smelt leads to an underestimation of the food resources of pelagic fish predators. The amount of prey species is useful information, e.g. for fish stockings. In relation to fishery, smelt remains a protected species. In recreational fishery, gillnet, trap, hook and line, and spinning rod are the gear most often used and they do not catch smelt. In commercial trawling and seining, catching smelt is usually effectively avoided. Fishermen know that smelt have a nocturnal and vendace have a twilight type of vertical migration, and hence trawling for vendace is not done in the midnight hours when smelt ascend into the surface water layers (e.g. Bagenal et al., 1982). Recently, because of its growing export potential, professional fishers have increased their interest in exploiting smelt stocks. The biomanipulation experiments, however, indicate that smelt stocks have rather limited compensation ability towards high fishing mortality (e.g. Horppila et al., 1996) which suggest low sustainable exploitation and yield level. This together with the important role that the smelt has in the pelagic ecosystem emphasize the need for development of valid methods for its stock estimation. For future studies it would be important to monitor over several years the abundances of smelt and other pelagic fish species.

Acknowledgements We thank Mr. Raimo Riikonen for all-round technical work in the construction of trawls and trawling. The Ministry of Agriculture and Forestry of Finland financed most of the study. Study in L. Pihlajavesi was funded by the European Commission (con-

tract No. EVK2-CT2000-22001, www.dur.ac.uk/imew. ecproject).

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