Human Impact on the Fish Diversity in the Four Largest Lakes of Sweden

Erik Degerman, Johan Hammar, Per Nyberg and Gunnar Svärdson

Human Impact on the Fish Diversity in the Four Largest Lakes of Sweden The four largest Swedish lakes, Vänern, Vättern, Mälaren, Hjälmaren, host important commercial fisheries for char, salmon, trout, whitefish, vendace (cisco), perch, pike-perch, pike and eel, i.e. highly diverse biological resources. Case studies illustrate physical, chemical and biological impacts on some of these commercial species caused by constructions of dams and ship canals, eutrophication, and overexploitation. Although some original species have been lost and a few new species have been added, the recent human interference has basically caused major shifts in dominance of the fish community structures because of eutrophication, alterations in the abundance of eel or crayfish, and due to overfishing. The latter is in some cases caused by the Great Lake Fishery Paradox—in an environment with several predators and competitors, but with ample food resources, especially salmonid fish but also species like pike-perch may adapt a life history favoring growth over sexual maturation. If harvested at a conventional size these populations will decline rapidly due to too small spawning stocks.

Figure 1. Location of the large Swedish lakes with ship canals and places mentioned in the text. Asterisks denote rivers with extinct landlocked Atlantic salmon strains in L. Vänern. © National Land Survey M2001/5200.

INTRODUCTION Human impact on water quality and quantity has affected freshwater fish biodiversity on a global scale (1). In the large lakes of industrialized regions the human impact often occurred early and documentation of impact on fish diversity is often lacking. It has been said that the industrialization of Sweden began in the 12th century when hydropower was put to use in the mining industry (2, 3). The mining industry developed around the four large lakes (Fig. 1) as the region combined accessible iron ores with arable land and productive lakes (food), large forests and rivers (energy) and means of transportation (rivers and lakes). Dam construction most probably affected fish stocks already at this time. Some lake-to-river migrating species, e.g. landlocked Atlantic salmon (Salmo salar), brown trout (S. trutta), whitefish (Coregonus spp.), ide (Leusicus idus), asp (Aspius aspius) and river lamprey (Lampetra fluviatilis), must have disappeared or been reduced in numbers early in time. The human influence on the fish fauna progressed in medieval times, as fish became the main source of protein in the then catholic Sweden. It has been estimated that 145 kg fish were consumed annually by each person at the Castle of Gripsholm on L. Mälaren in 1555 (4). Today, the average consumption for the whole Swedish population is approximately 13.5 kg yr–1. The medieval figure reveals the fishery of those days to be extensive, and to a very large extent exceeding the present harvest levels in the large lakes. The difference is of course due to the fact that medieval man utilized a large variety of species, while modern man harvests only a few. The human impact on fish populations can be divided into 4 categories: i) climatic; ii) physical; iii) chemical; and iv) biological. Depending on the fish diversity, the location, and the physical characteristics of the water, the combination and dominance of the various components of human impact may differ in significance. In this paper, we present examples of the 3 latter categories. Effects of climatic variations on the large lakes fish 522

fauna have been suggested (5), but whether or not these climatic variations are brought about by man still needs to be proven. Among the physical impacts addressed in this paper are altered migration routes, e.g. changes resulting from the construction of dams and ship canals. Chemical effects may comprise acidification, eutrophication, and pollution from various toxic compounds. Although early signs of acidification have been noted in, e.g. L. Vänern (6), and certainly acidification has affected L. Vättern by reducing the spawning success of brown trout in the tributaries (7), the most prominent chemical impact has been from plant nutrients (8, 9). The biological impact has mainly been from fisheries and fisheries management, i.e. mainly overexploitation and introduction of new species. This paper focuses on the former. The various effects of human impacts on the fish diversity will be discussed in the context of comparisons with studies in other large lakes, especially those in North America. THE SCENE Around 10␣ 000–11␣ 000 yrs B.P. the ice-shield of the last Ice Age melted from the four large Swedish lakes. For thousands of years the water of the present Baltic Sea emptied into the Atlantic Ocean via the depression where these four lakes are situated today. Initially, the lakes were, therefore, colonized by the same aquatic species. L. Vättern was the first to be isolated from the others around 9500 B.P., L. Vänern was closed from the Atlantic Ocean around 9000 B.P., but was still connected with the “upstream” Mälaren and Hjälmaren. The next 2 successive stages of the Baltic Basin, i.e. the late-glacial Ice-sea (Yoldia Sea) and the Ancylus Lake, emptied through Lakes Mälaren-Hjälmaren into L. Vänern for approximately 1000 years. During the latter stage several warmwater cyprinid species and pike-perch (Stizostedion lucioperca) entered the lakes from the east. Thus, the latter species could not colonize L. Vättern. About 7000 B.P.

© Royal Swedish Academy of Sciences 2001 http://www.ambio.kva.se

Ambio Vol. 30 No. 8, Dec. 2001

THE PRESENT FISH FAUNA The fish species diversity is naturally dominated by salmonids (including coregonids) in the cold and oligotrophic L. Vättern (Fig. 2), which was early isolated from the other lakes. L. Vänern holds 6 out of 8 species of Coregonus (10, 11). Together with salmon, brown trout and occasional occurrence of grayling, salmonids also dominate this lake, but cyprinids dominate in the warmer, more eutrophic and shallow bays. In Mälaren and Hjälmaren cyprinids are the dominating family, as is to be expected from the higher nutrient levels, warmer water and shallower depths. Dominating species in the pelagic area are smelt and vendace (cisco, Coregonus albula) in all lakes, except in L. Hjälmaren where the lack of a cold hypolimnion in summer makes the environment unsuitable for vendace (Table 2). L. Vänern: Two stocks of landlocked salmon occur naturally in the lake. Three stocks have been lost due to hydropower development during the 20th century (Fig. 1). Along with salmon, lake-run brown trout is common in this lake. These are the major fish predators in the pelagic, with pike (Esox lucius) and pikeperch being important in more littoral areas and eutrophic bays. Although various strains of salmon, brown trout, and whitefish have been lost over the last 500 yrs in the lake, no species as such has been documented as lost. Introductions in upstream lakes have, however, led to occasional occurrence of carp (Cyprinius carpio) and rainbow trout (Oncorhynchus mykiss). Today, 90 commercial fishermen utilize the lake, i.e. 1 fisherman per 63 km2. The average fish yield is low in this oligotrophic lake (Table 1). L. Vättern: The major fish predators are lake-run brown trout, stocked Atlantic salmon of the L. Vänern strain and relict Arctic char. The relict fauna includes vendace, smelt, fourhorn sculpin (Triglopsis quadricornis), and 7 coldwater crustaceans (12). Warmwater predators like pike and especially pike-perch are confined to warmer bays. The lake is too cold and oligotrophic for cyprinid species to develop large populations outside shallow, sheltered and warm bays (13, 14). Ide occurred into the 20th century, but probably not today. The cause of extinction was most likely the construction of dams in several of Ambio Vol. 30 No. 8, Dec. 2001

the inlet streams to the lake. With the construction of a dam in the outlet at Motala (Fig. 1) in 1928–1929 also the extremely large-sized and downstream spawning brown trout population was lost (15). In 1925, pike-perch was introduced and presently spawns in 3 areas (13, 14, 16). During the 1960s rainbow trout, splake (Salvelinus namaycush x Salvelinus fontinalis) and kokanee (Oncorhynchus nerka) were stocked but this did not result in self-sustaining populations. American brook trout (Salvelinus fontinalis) populations, however, were established in a few small streams within the watershed. Since 1959, Atlantic salmon is stocked annually, originally as an experiment because of trophy-sized returns of accidental stockings, but today as part of the management program. Today, 20 commercial fishermen fish in the lake, i.e. 1 fisherman per 94 km2, and the fish yield is extremely low (Table 1). L. Mälaren: The major fish predators are pike and pike-perch, with the latter dominating in the pelagic. Predators thriving in cold oligotrophic waters, i.e. salmonid fish, are missing. Some cyprinid predators occur, e.g. asp and chub (Leuciscus cephalus). Wels (Siluris glanis) and brown trout have been lost. The former species, a relict from warmer periods, spawned in upstream L. Hjälmaren and was lost when this lake was lowered. Large-sized lake-run brown trout occurred in at least four streams and declined successively from the 16th century, due to dam construction and habitat destruction. Since the eel migration to Northern Europe has declined (17), the lake has been annually stocked with eels since the beginning of the 1960s. Stocking of salmon and brown trout takes place in the sea area immediately downstream of the lake, and previously in the lake or in upstream lakes and tributaries. In upstream lakes and streams, rainbow trout and carp are stocked. No introduced exotic species has established itself in the lake. Today, 40 commercial fishermen fish in the lake, i.e. one per 28 km2. L. Hjälmaren: This is smallest, shallowest and most eutrophic of the lakes (Table 1). Pike-perch is the dominating fish predator, with pike being subdominant. The cyprinid predator asp also

Table 1. Some important limnological features of the four large Swedish lakes. Total-phosphorus is given as an average for surface layers in the whole lake. Fish yield (kg ha–1 yr–1) applies to commercial catch during 1995–2000 (National Board of Fisheries). Altitude m a.s.l. Lake Vänern Lake Vättern Lake Mälaren Lake Hjälmaren

45 88 0.7 22

Area km2 5650 1890 1120 480

Depth (m) mean max 25 39 13 6

Tot-P (µg L–1)

Fish yield

7 5 39 51

1.5 0.5 2.4 4.0

106 120 66 20

Figure 2. Proportion of the number of reproducing fish species in each lake divided into major taxonomic groups. The lakes are arranged from cold oligotrophic on the left to warm eutrophic on the right.

100 Number of species, %

the land bridge between Sweden and Denmark broke, and the Litorina Sea and later the present Baltic Sea were formed. L. Hjälmaren was isolated from L. Mälaren and the Baltic Sea 3500–4000 B.P. L. Mälaren was for thousands of years part of the low saline Baltic Sea. In the beginning of the 13th century, however, the isostatic uplift created a threshold in L. Mälaren’s outlet where Stockholm is situated today (Fig. 1) and most fish migration between L. Mälaren and the Baltic Sea ceased. Thus, following the 13th century all the large lakes may be considered as freshwater bodies, although intrusions of brackish water occurred frequently in L. Mälaren until 1943, when a dam was built in the outlet. Throughout the postglacial period freshwater fish mainly invaded the large lakes from the east. Few species, e.g. eel (Anguilla anguilla), twaite shad (Alosa fallax) and some anadromous salmonid species including Arctic char (Salvelinus alpinus sp. complex), Atlantic salmon, brown trout, whitefish and smelt (Osmerus eperlanus) could, however, colonize from the marine habitat of the Atlantic Ocean as well. As has been pointed out in other papers in this issue of Ambio the lakes differ considerably in their major limnological features, and the lakes present an extensive gradient in size, depth and trophical state. The species diversity is highest in the largest and the lowest situated lakes, where the ecological niches are most diverse and plentiful (Table 1), and where immigration has occurred over long time and through several stages of the present Baltic Sea.

80 60 40 20 0 Vättern Vättern Salmonidae


Vänern Vänern Varia

Mälaren Cottidae

Percidae

Hjälmaren Cyprinidae

523

Table 2. List of fish species in the four Swedish large lakes in year 2001. Abbreviations: O = occurring and reproducing, I = introduced and reproducing, E = extinct species, M = migratory species not reproducing, (U) = species stocked in upstream lakes, rarely caught in the lake in question, (D) = species stocked in downstream sea area with occasional upstream migrations. The updated coregonid taxonomy are derived from Svärdson (10) and Svärdson (11). Common name

Scientific name

L. Vänern

L. Vättern

L. Mälaren

L. Hjälmaren

River lamprey Brook lamprey

Lampetra fluviatilis Lampetra planeri

O O

O O

O O

E O

M, rare

M, also stocked

M, also stocked

O O O O O O O (U)

O O O

O O M, rare O O O O O

E E

E O O O, rare, stocked (D) E, (D)

E O O O, rare, stocked

European eel

Anguilla anguilla

M, also stocked

Twaite shad

Alosa fallax

M, very rare

Zope (blue bream) White bream Bream Zanthe (Vimba) Bleak Asp Crucian carp Carp Dace Chub Ide (Orfe) Sichel (Chekhon) Minnow Rudd Roach Tench Spined loach

Abramis ballerus Abramis bjoerkna Abramis brama Abramis vimba Alburnus alburnus Aspius aspius Carassius carassius Cyprinius carpio Leuciscus leuciscus Leuciscus cephalus Leuciscus idus Pelecus cultratus Phoxinus phoxinus Rutilus erythrophthalmus Rutilus rutilus Tinca tinca Cobitis taenia

O O O O O O O (U) O O O

O, rare E?

O O O O O

O O O O O

Sheatfish (wels) Northern pike European smelt Rainbow trout Salmon Brown trout Arctic char Grayling Vendace (cisco) Northern densely-rakered w Large sparsely-rakered w. Lesser sparsely-rakered w. Southern densely-rakered w River whitefish Burbot Three-spined stickleback Nine-spined stickleback Bullhead (Millers thumb) Siberian bullhead (Alpine b.) Fourhorn sculpin Perch Pike-perch Ruffe Flounder

Silurus glanis Esox lucius Osmerus eperlanus Oncorhynchus mykiss Salmo salar Salmo trutta Salvelinus salvelinus Thymallus thymallus Coregonus albula Coregonus peled Coregonus pidschian Coregonus widegreni Coregonus nilsoni Coregonus lavaretus Lota lota Gasterosteus aculeatus Pungitius pungitius Cottus gobio Cottus poecilopus Triglopsis quadricornis Perca fluviatilis Stizostedion lucioperca Gymnocephalus cernuus Platichthys flesus

O O O, rare, stocked O O

O O O, rare, stocked O, stocked O O O O

Total Reproducing annually

M, rare O O O O O, rare O O O O O O O O O O M, very rare 42 36

occurs. Five fish species have been lost in modern time; wels, ide, chub, brown trout, and river lamprey. No new species have been introduced, but eel is stocked annually, and in tributaries also rainbow trout. Today, 40 commercial fishermen fish in the lake, i.e. 1 fisherman per 12 km2, and the lake has a comparably high fish yield (Table 1). In all the lakes, the noble crayfish (Astacus astacus) occurred until the detrimental crayfish plague (Aphanomyces astaci), a fungi, was introduced in 1907 with infested crayfish in L. Mälaren. All the lakes had lost their native crayfish populations by 1960 and stocking of the American signal crayfish (Pacifastacus leniusculus) as an alternative began (18, 19). Today, a commercial fishery for signal crayfish is established in L. Hjälmaren, it is expanding in L. Vättern, but insignificant in the other two lakes. PHYSICAL IMPACT Ship Canals The Trollhätte Ship Canal and sluices were ready for traffic in the year 1800, allowing ships from the Atlantic Ocean to reach L. Vänern 82 km from the river mouth and 40 m a.s.l. (Fig. 1). 524

O O O O

O O O

O, rare O O O O

E

O

O, rare O O O O O O O O O I O 33 30

O, rare O O O O O

O O O O O

O O O O M, very rare

O O O

36 29

24 22

For the first time in 9000 years, eel and twaite shad could enter the lake and its tributaries. Very soon people around the lake noted that the stocks of the noble crayfish declined drastically, even several kilometers upstream in inlet streams (20). In 1872, Steffenburg concluded that “In L. Vänern the crayfish is gone today” (21), which was an exaggeration, but the population was apparently insignificant. The eel fishery has since been important in the lake. The eel catch today is 0.035 kg ha–1 while the catch of crayfish is practically nil. Thus, the colonization, and the subsequent predation by eel on crayfish (22), and perhaps other species, has had an immense effect on the biodiversity not only of this large lake, but also its adjoining rivers and lake systems. It clearly illustrates the ecological and economical antagonism between eels and crayfish (23). The ship canals in Vättern, Hjälmaren, and Mälaren have not resulted in the establishment of new fish species. The former two lakes are not directly connected to the sea by the canals. L. Mälaren is connected to the Baltic Sea by 3 canals, each with only one sluice. However, since the freshwater fish fauna of the brackish Baltic Sea highly resemble the fauna of L. Mälaren, no new species have colonized through these passages. Occasional reports of the extremely rare cyprinid sichel (Pelecus cultratus)



and the euryhaline flounder (Platichthys flesus) are the only observed effects of the ship canals in this lake. Dams in the Outlet In contrast to canals, the establishment of dams for water-level regulation impedes migratory activities of diadromous fish species, and also access to natal spawning areas of local stream spawners of other fish taxa. Internationally, the severe effects of hydropower development and dams in inlet streams have been given much attention. Certainly, the obstruction of inlet streams has influenced the fish diversity of the large lakes. As was mentioned previously 3 strains of landlocked salmon have been lost in L. Vänern (Fig. 1) and many strains of large sized lake-run brown trout in Vättern and Mälaren have vanished. However, in this paper we will focus on dams in the outlet of the large lakes as these are common worldwide. In fact few large lakes have been left unregulated. In L. Hjälmaren, in contrast to L. Vänern, eel occurred naturally. Here, however, human activities were detrimental to the eel and favorable to the crayfish. To facilitate shipping between Hjälmaren and Mälaren, the river between the two lakes was levelled and widened to a ship canal in 1596–1610. The canal was difficult to maintain, however, and a second new canal with 14 sluices was dug in 1629–1639 in another place. As the new canal was made deeper than the original outlet, the water level of L. Hjälmaren was lowered by approximately 0.3–0.6 m, making upstream migration in the original outlet more difficult (24). Successively, the return of eels decreased, as illustrated by the declining catches of eel in the outlet, and records from some abandoned eel traps in 1738 (24). However, Uggla Hillebrandsson in 1786 informed the Royal Swedish Academy of Sciences that still ‘eel was plentiful in the lake’ (25). The eel catches continued to drop in the outlet, thus confirming that immigration to the lake was physically prevented, now also by dams. The catches in one of the outlet eel traps dropped from 4000 eels in 1840 to 1000 eels in 1860 (23). In 1879–1886, L. Hjälmaren was lowered another 1.3 m (reducing the surface area by 55%) and the dams were allowed to shut off the outflow completely, i.e. eel could only with great difficulty reach the lake. In 1880, Hofberg (26) confirmed that eel was ’less common’. In only 94 years, the eel abundance had gone from being ‘plentiful’ to ‘less common’. The population continued to decline, and official statistics revealed that the annual eel catches were down to 0.037 kg ha–1 during 1914–1923. Very likely, the catches had

Figure 3. Average total phosphorus (µg L–1) in the surface layer (0.5–3 m) of Mellanfjärden Bay in L. Hjälmaren 1965–1986 during March–May (data from Swedish University of Agricultural Sciences, Uppsala). Bars indicate 95% confidence intervals.

200

6 n=4 6

Total-P, µg L–1

150

6

100

6 6

4 4

4

50

4

0 1965–1968

1971–1972

1969–1970


1975–1976

1973–1974

1979–1980

1977–1978

1983–1984

1981–1982

1985–1986

been at least 10 times as high during the best years, but proper early statistics are lacking. In the year 1942, as the Commercial Fishermen Association of L. Hjälmaren had their annual meeting, a fisherman said: “I have caught only two eels in my whole life, and that was in 1917. It was great fun and I would like to experience this event again”. Thus, from the first cut with a spade in 1629 for a canal, to the deliberate lowering of the water level of L. Hjälmaren in 1879–1886 a highly profitable eel fishery was lost. As the eel population in L. Hjälmaren declined, an immense expansion of the noble crayfish population took place. The crayfish fishery originally started to develop around 1730. However, it was not until the 1850s that the crayfish fishery was mentioned to be of some significance, and after the lake lowering operation fishing for crayfish became extremely important. It has been estimated that the annual yield of crayfish was ca 500 tonnes (i.e. 10.4 kg ha–1) during the best years at the end of the 19th century (27). Crayfish from L. Hjälmaren was widely exported to central Europe via railroad and the fishery was an important industry. Unfortunately, soon afterwards, 1907–1908, the crayfish plague eradicated the entire population. Hence, minor physical alterations and manipulations of the outlet of our large lakes have led to gigantic ecosystem shifts and also economical collapses. CHEMICAL IMPACT: EUTROPHICATION Densely populated communities and extensive agricultural districts surround the large lakes of southern Sweden. The eutrophication history of the lakes is thoroughly described in other papers in this Ambio issue (28–30). In general, it can be concluded that eutrophication accelerated in the mid-20th century in all the lakes. L. Hjälmaren, however, had already previously been characterized as what we today consider eutrophic (31). In 1786, Uggla Hillebrandsson (25) reported a visual depth of about 2 m, and mentioned an algal bloom. The vicar Strandberg in 1772 (32) reported that fish kept in chests died during summer (likely because of oxygen deficit following algal bloom). The fish yield in the lake has since medieval times been high, considering the latitude and nutrient level, and the dominating species have been those indicating eutrophic conditions, e.g. pike-perch, asp, and other cyprinids (31, 33). Especially cyprinids such as roach (Rutilus rutilus), rudd (R. erythrophthalmus), bream (Abramis brama) and white bream (A. bjoerkna) are regarded as favored by eutrophication, since they generally increase synchronously with nutrient enrichment (9, 34). The lowering of L. Hjälmaren in 1879–1886 increased the agricultural areas and the nutrient leaching to the lake. In addition, the reduced depth increased the effect of wind-induced resuspension of bottom sediments thus increasing the internal nutrient load. The establishment of a paper mill in 1902 resulted in additional organic effluents, and after that a general increase of nutrient loading came from the municipal sewage. A sewagetreatment plant was built 1957. But it was not until the mid-1970s that a major improvement of the wastewater treatment reduced the phosphorus levels in the western part of the lake considerably (Fig. 3). A test-fishing program in the bay of Mellanfjärden (average depth 1.5 m) was carried out during 1955–1978 and 1984–1987. Annually, 60 efforts with gillnets, of mesh sizes ranging from 30 to 67 mm, were set (9). The results illustrated a classical response of the fish community to initial eutrophication and the successively improved wastewater treatment (Fig. 4). The biomass of cyprinids peaked along with the highest recorded levels of phosphorus in the area (Fig. 3). In both lakes Mälaren and Vänern eutrophication was evident during the same period, but the effect on fish fauna was obscured by decreased fishing, partly due to increased mercury levels in


525

Figure 4. Catch in numbers per unit of effort (60 gillnets per year) during a test-fishing program in Lake Hjälmaren in 1955–1978 and 1984–1987. The sharp increase in cyprinid abundance (blue line) in the early 1960s coincides with increased eutrophication (Fig. 3). In the early 1970s, the wastewater treatment was improved and the abundance of cyprinids decreased, and so did other species, mainly percids (red line).

Individuals, gillnets–1

30

Other Cyprinids

20 10 0 1955

1960

1965

1970

Figure 5. The commercial catch of whitefish (Coregonus spp.) in L. Vättern 1914–1999. The sharp catch increase in the early 1950s coincides with increased eutrophication and fishing gear efficiency, the latter due to a shift from cotton to nylon nets. Successful wastewater treatment resulted in a return to previous catch levels by the 1990s.

1975

180 ● ●

● ●

Whitefish

160

●

● ● ●

140

160

●

140

●

100

● ●●

0

●

● ● ●●● ● ● ● ● ● ● ● ●●●● ● ● ● ● ●●●● ● ● ● ● ● ● ● ●● ● ● ●●

● ●●

●

● ●

●

●●

80

20

●

●

● ●●

Catch, kg 103

Catch, kg 103

120

● ● ●

●

● ●●

● ●●

●

●●● ● ●● ● ●

120 100 80 60 40 20

1920 1930 1940 1950 1960 1970 1980 1990 2000

fish and to industrial effluents from paper mills in the latter lake. However, in the oligotrophic L. Vättern an obvious increase in the catches of whitefish (Coregonus spp.) during 1955–1975 coincided with eutrophication of the lake (Fig. 5; 8, 35, 36). The Secchi-depth in 1888 was 17 m in the open lake, but decreased during the 1950–1970s to 10–12 m indicating increased nutrient levels and productivity (36), but fortunately a large effort was put into treatment of municipal wastewater during the early 1970s. The effect of eutrophication on the whitefish stock was also indicated by improved fish growth. During the period 1924–1931 a 5-yr old whitefish averaged 311 mm (298–323 mm; 95% confidence interval) and in 1951–1962 343 mm (330–356 mm, Oneway Anova, p = 0.006). In addition to the sole effect of eutrophication, monofilament nylon-nets were introduced in the fishery around 1953–1955. This could account for part of the increased catch during the 1950s, but not for the significantly increased growth (35). BIOLOGICAL IMPACTS: OVERHARVESTING OR THE GREAT LAKE FISHERY PARADOX In the 18th century, the connection between the deterioration of the environment and the fish stocks, and also the impact of overfishing became obvious. In 1785, Johan Fischerström, an economist and member of the Royal Swedish Academy of Sciences interpreted the changes reported from the fisheries in L. Mälaren: “There is a general complaint that the catches of fish have been declining seriously. However, as more people fish today than before, as tributaries and streams are more and more cut off, as the disturbance of the spawning activities is increas526

Arctic char

● ●

40

1985

Figure 6. The commercial catch of Arctic char (Salvelinus salvelinus) in L. Vättern during 1914–2000 (statistics for 1924 are missing). Today, the catch has become so low that commercial fishing for the species in some areas is close to unprofitable.

180

60

1980

■ ■ ■■ ■■ ■ ■■ ■ ■ ■■ ■■ ■ ■■ ■ ■ ■ ■■■ ■ ■ ■■ ■ ■ ■■ ■■ ■ ■■■ ■ ■ ■■ ■ ■ ■ ■■■ ■■ ■ ■ ■ ■ ■ ■ ■■ ■ ■ ■■ ■ ■■■ ■ ■ ■ ■ ■ ■ ■ ■■ ■ ■■ ■■ ■ ■ ■■■ ■

0 1920 1930 1940 1950 1960 1970 1980 1990 2000

ing, as the forests are incautiously cut down along islets, spits, and inlets, where the fish thrive, and when too large and finemeshed seine nets are used, removing the sexually immature fish and fish fry, the cause of these changes are easily identified” (37). Overfishing, however, was not an isolated problem, neither in time nor place. Being cold and oligotrophic L. Vättern has a low fish production and is particularly prone to overfishing. The Arctic char, and 2 taxa of whitefish, have been the most important commercial fish species in the lake during the 20th century. For the last 75 years char has probably been commercially overexploited. Although more efficient gear (shift from gillnets made of cotton to modern monofilament nylon gillnets) have been used, the catch has declined (Fig. 6). This is due to the fact that the large-sized char of L. Vättern, and also other marginal char populations in southern Sweden, have very different life-history traits to those of the smaller char of more central populations of northern Sweden (38, 39). The commercial fishery in L. Vättern used to land char as small as 35 cm, a size which would normally have allowed appropriate spawning to have taken place among more northern populations. Early investigations, however, showed that 50 cm was the modal length of spawning female char in L. Vättern (40), thus indicating that the majority of females were caught before they had had a chance to spawn. The legal minimum size of 36 cm of the mid-1930s, was eventually increased to 38 cm, and is today only 40 cm. Since this means that still only fractions of the female char are allowed to spawn before capture, the decline of the char stock is not surprising. What we can see in L. Vättern is just another example of the Great Lake Fishery Paradox. Man inevitably doubts that fish



in such a large lake can be overharvested. The irony is that in an environment with several predators and competing species, but with ample food resources and eventually a large cold-water refugia in the hypolimnion, salmonid fish may adapt a life history favoring growth over sexual maturation. The same life history is adopted by pike-perch in the large lakes. Big game fish in these large lakes are thus large and fast growing, but immature. This reflects a selective response to predation and competition from littoral pike, perch, burbot (Lota lota), eel, etc. In order to be able to reproduce along shores and inlets in late fall, these fish have to grow large. In the pristine Great Lakes of North America, perhaps 50% of the biomass of the fish community was contributed by individuals over 5 kg in mass (41). In most large lakes of temperate regions such fish are rare today, and in L. Vättern extremely few Arctic char larger than 5 kg (~74 cm) have been landed during the last 30 years. GENERAL DISCUSSION AND INTERNATIONAL COMPARISON Colonization of alien organisms via man-built water system connections do not come as a surprise. It has been shown all over the world that ship canals function as migration routes for new species to landlocked lakes and sea areas (42). The canals also open up for species to disperse via ballast water carried by ships on international routes (43). The extensive ship canal system in inland England has facilitated the dispersal of the introduced pike-perch (44). Perhaps the most widely known example is the invasion of sea lamprey (Petromyzon marinum) and alewife (Alosa pseudoharengus) to the upper North American Great Lakes. Alewife successfully outcompeted the pelagic ciscoes (Coregonus spp.) for food, and the sea lamprey in its parasitic stage predated heavily on lake trout (Salvelinus namaycush) and other species (45). Most interesting is the rapid decline and eradication of the lake trout population of Lake Michigan in the late 1940s. Although commercially exploited, the weak negative trend of the lake trout catches between 1890 and 1944 ranged from approximately 0.64 to 0.55 kg ha–1 per year. In 1936, sea lamprey was first observed in the lake, and in 1945 the catch of lake trout declined abruptly, and was by 1949 only 0.03 kg ha–1. In 1956, the species was probably lost. Obviously overexploitation by the fisheries was not the reason (46). After the introduction of a lamprey control program lake trout has been successfully reintroduced. Accidental introductions of new species, intentionally through stocking or unintentionally through waterway modifications or shipping, can be irreversible and fatal to the original fauna. Effects of dams also show that no lake is large enough to be independent of its tributaries and outlets as migration routes for fish and other biota. Eutrophication has been recognized as restructuring fish communities. The general trend along a productivity gradient demonstrates a fish community dominance shift from salmonids to percids to cyprinids in European lakes, and from salmonids to percids to cyprinids and centrarchids in North American lakes (34, 47). In this context, L. Hjälmaren could be compared to L. Erie, as being the most productive of the large northern lakes in its region. With increased nutrient load to L. Erie the salmonids tended to fade away, while coregonids went through a period of large population fluctuations before their final extinction (41). Warm-water species tend to increase or remain unchanged in eutrophicated waters, as illustrated by the development of carp and freshwater drum (Aplodinotus grunniens) in L. Erie, and bream and white bream in L. Hjälmaren. These cyprinids spawn in shallow littoral areas in spring, i.e. in periods of good oxygen conditions, and the eggs are stuck to plants, avoiding the sediments, suggesting an adaptive response to eutrophic conditions. The freshwater drum eggs float near the water surface, and Ambio Vol. 30 No. 8, Dec. 2001

thus avoid potentially deleterious conditions on the lake bottom. Pelagic eggs are otherwise rare in freshwater fish in temperate regions. A comparison between lakes Hjälmaren and Erie has one important drawback, the nonexistence of stagnant deep water in L. Hjälmaren. This is where the species suffered most in L. Erie (45, 48). The limnological features of L. Hjälmaren should rather be compared with those of, e.g. L. Peipsi-Pskov (3560 km2) located between Estonia and Russia. Both these lakes lack a summer thermocline. The latter lake has an average depth of 7.1 m, which is comparable to L. Hjälmaren’s 6 m (Table 1). In spite of even “hypertrophication” in parts, the fish yield remains high in L. Peipsi-Pskov (49). The fish fauna is dominated by cyprinids, including asp, but also wels, pike-perch and smelt, i.e. a fish community identical to L. Hjälmaren’s, except for the wels, which went extinct after the lake lowering process of L. Hjälmaren in 1879–1886 (Table 2). All these species live in the upper water layers, spawn in shallow areas, and have a short phase of larval development. In spite of high nutrient loading no species have been lost due to eutrophication in these lakes. Hence, it can be emphasized that shallow lakes, and the fish fauna of lakes inhabited by cyprinids/centrarchids are more resilient to further nutrient input than deep oligotrophic lakes given a similar nutrient load. Especially deep lakes inhabited by salmonids are sensitive (47). In the latter type of lake eutrophication within decades can lead to irreversible losses of species. Evidence of overexploitation of commercially important fish stocks of large lakes are well documented from the North American Great Lakes (45, 46), and also from Europe (50) and Asia (51). For almost every major species that decreased in the North American Great Lakes overfishing was responsible at least at some stage. But there is no evidence that fishing in the Great Lakes has eradicated species, instead the term ‘commercially extinct’ has been used for stocks that have become so small that commercial fishing has been unprofitable (52). The situation for the lake trout populations of the Great Lakes has several parallels to the conditions of the Arctic char in L. Vättern. Both species live at the southern limits of their distribution, are in summertime confined to cooler deep water layers, they mature late at a large size and have to compete with introduced game fish for prey. Coexisting species in these deepwater layers are, e.g. burbot, whitefish, crayfish, and sculpins of the genera Cottus, Myxocephalus or Triglopsis. Pink salmon (Oncorhynchus gorbusha) was introduced to L. Superior and kokanee was introduced into L. Ontario, while Atlantic salmon is continuously stocked into L. Vättern. It is known that introductions of new game fish to large temperate lakes lead to reductions in prey population size, diet, and habitat shifts of game fish (53). It is to be expected that increased stocking of salmonids in oligotrophic lakes will lead to a decreased growth rate (54), which probably will lower survival. The pattern has been repeated in L. Vättern where initial high yields from the stocking of salmon has declined considerably since the 1960s in spite of unchanged stocking numbers (55). LESSON LEARNED Although few original species have been lost and few new species have been added to our Swedish large lakes, the recent human interference has caused major shifts in dominance of the fish community structures, and serious reductions of vulnerable populations and strains of native crayfish, salmonids and river spawning cyprinids. These large changes may also involve large, anonymous and genetically irreversible losses. Many of the impacts described above have proven reversible, e.g. the effects of eutrophication in non-salmonid lakes and possibly the effects of overfishing. However, examples of irrevers-


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ible effects have been shown, especially when new species have been introduced or migration routes blocked. With this in mind it is surprising that so little attention is addressed to these issues in our national environmental legislation. In Sweden, as the only country within the European Union, it is now allowed to import live crayfish of any species from any part of the world, although fishery biologists have warned against this for over 40 years (56). As has been described previously it is still allowed to stock genetically deprived rainbow trout in all the large lakes and their tributaries, despite the fact that this species carries its specialized parasites and diseases (57), and competes with red-listed

original salmonid species. Ships are allowed to empty their ballast water where ever it pleases the crew. To compensate for the loss of brown trout we stock salmon in a lake that never had any salmon. Salmon, a species that competes with a relict redlisted Arctic char population on the verge of commercial overexploitation. Clearly, as we write this today we are not looking to the past to learn from the long period of mistakes, made by our ancestors. There is no passed history of human impact in fisheries management—we are certainly in the midst of repeating as well as achieving new mistakes.

References and Notes 1. Goudie, A. 1993. The Human Impact on the Natural Environment. Fourth edition. Blackwell, 454 pp. 2. Mumford, L. 1949. Technics and Civilization. Vinga Press, 400 pp. (In Swedish, US edition printed in 1930). 3. Hansson, S. 1996. Technical History—On Technical Knowledge and Its Importance for Individuals and the Society. Studentlitteratur, 446 pp. (In Swedish). 4. Heckscher, E.F. 1935. The Economic History of Sweden from 1523. Albert Bonniers Förlag, Stockholm, 265 pp. (In Swedish). 5. Nyberg, P., Bergstrand, E., Degerman, E. and Enderlein, O. 2001. Recruitment of pelagic fish in an unstable climate; studies in Sweden’s four largest lakes. Ambio 30, 560–565. 6. Odén, S. and Ahl, T. 1970. Acidification of Scandinavian surface waters. Ymer, pp. 103–122. (In Swedish). 7. Nyberg, P. and Markusson, K. Commercial fishing on Lake Vättern. The Lake Vättern Society for Water Conservation, Report. (In press, in Swedish). 8. Grimås, U., Nilsson, N.-A. and Wendt, C. 1972. Lake Vättern: effects of exploitation, eutrophication, and introductions on the salmonid community. J. Fish. Res. Bd Can. 29, 807–817. 9. Svärdson, G. and Molin, G. 1981. The impact of eutrophication and climate on a warm water fish community. Rep. Inst. Freshw. Res., Drottningholm 59, 142–151. 10. Svärdson, G. 1979. Speciation of Scandinavian Coregonus. Rep. Inst. Freshw. Res., Drottningholm 57. 95 pp. 11. Svärdson, G. 1998. Postglacial dispersal and reticulate evolution of Nordic coregonids. Nordic J. Freshw. Res. 74, 3–32. 12. Svärdson, G., Filipsson, O., Fürst, M., Hansson, M. and Nilsson, N.-A. 1988. The significance of glacial relicts for the fish fauna of Lake Vättern. Inf. Inst. Freshwater Res., Drottningholm 15, 61 pp. (In Swedish, English summary). 13. Filipsson, O. 1983. The fish stocks of lake Vättern as seen from bottom-set gillnets. Inf. Inst. Freshwater Res., Drottningholm 1, 61 pp. (In Swedish, English summary). 14. Degerman, E. and Nyberg, P. The fish fauna of lake Vättern. The Lake Vättern Society for Water Conservation, Annual Report. (In press, in Swedish). 15. Alm, G. 1929. On occurrence of trout in the Lake Vättern and in the upper part of Motala river. Meddel. Kungl. Lantbruksstyrelsen 276, 69 pp. (In Swedish, with extended English summary). 16. Filipsson, O. 1994. Fish stocks established through introductions. Inf. Inst. Freshwater Res., Drottningholm 2, 1–65. (In Swedish, English summary). 17. Moriarty, C. and Dekker, W. 1997. Management of the european eel. Fish. Bull. 15, 110 pp. 18. Svärdson, G. 1965. The American crayfish Pacifastacus leniusculus (DANA) introduced into Sweden. Rep. Inst. Freshw. Res., Drottningholm 46, 90–94. 19. Svärdson, G. 1995. The early history of signal crayfish introductions into Europe. In: Freshwater Crayfish VII. Romaire, R. (ed.). Eight Internat. Symp. Freshw. Crayfish, Baton Rouge, Louisiana. pp. 68–77. 20. von Schéele, F. 1854. On the Fisheries in the County of Wermland in 1854. 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Freshwater Res., Drottningholm 3, 62 pp. (In Swedish, English summary). 28. Persson, G. 2001. Phosphorus in tributaries to Lake Mälaren: analytical fractions, anthropogenic contribution and bioavailability. Ambio 30, 486–495. 29. Wilander, A. and Persson, G. 2001. Recovery from eutrophication; experiences of reduced phosphorus input to the four largest Lakes of Sweden. Ambio 30, 475–485. 30. Willén, E. 2001. Four decades of research on the Swedish large lakes Mälaren, Hjälmaren, Vättern and Vänern: the significanse of monitoring and remedial measures for a sustainable society. Ambio 30, 458–466. 31. Willén, E. 1999. Lake Hjälmaren—water quality and biota. From: Från Bergslag och Bondebygd, pp. 216–223. Örebro läns museum. (In Swedish). 32. Strandberg, O. 1772. Notes on the fishery in Lake Hjälmaren. Roy. Acad. Sci. 33. (In Swedish). 33. Bagge, J.F. 1785. Description of Örebro town. 324 pp. (In Swedish). 34. Persson, L. 1994. Natural shifts in the structure of fish communities: Mechanisms and constraints on perturbation sustenance. In: Rehabilitation of Freshwater Fisheries Chapter 39, pp. 421–434. Cowx, I.G. (ed.). Fishing News Books, 486 pp. 35. Svärdson, G. 1963. The dominance shift between char and whitefish in Lake Vättern. Svensk Fiskeritidskrift 72, 149–152. (In Swedish). 36. Persson, G., Olsson, H., Wiederholm, T. and Willén, E. 1989. Lake Vättern, Sweden: a 20-year perspective. Ambio 18, 208–215. 37. Fischerström, J. 1785. A Draft on the Description of Lake Mälaren. J.C. Holmberg, Stockholm. 467 pp. (In Swedish). 38. Hammar, J. 1994. Split-routine strategies in competitive bodybuilding: Species and size selective predation in marginal Arctic char. International Charr Symposium, Trondheim, 1994, Book of Abstracts. 39. Hammar, J. Management Program for Arctic char, Salvelinus salvelinus. National Board

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Prof. em. Gunnar Svärdson is a former director of the Drottningholm Institute of Freshwater Research. He has mainly worked with speciation, species interactions, and once introduced the concept of ecology in Sweden. Johan Hammar, PhD, works mainly at Drottningholm with the evolutionary ecology and species interactions of Arctic char in northern and southern lakes. Their address: Institute of Freshwater Research, SE-178 93 Drottningholm, Sweden. E-mail: [email protected] Per Nyberg, PhD, is a former director of the Drottningholm Institute of Freshwater Research. He is currently the office head of a research division supervising assessment and management of fish in the southern large lakes in Sweden and coastal migrating Salmo species. Erik Degerman, BSc, works mainly on the assessment and ecology of fish communities in streams and large southern lakes. Their address: National Board of Fisheries Regional Office, Pappersbruksallén 22, SE-702 15 Örebro, Sweden. E-mail addresses: [email protected] [email protected] All four authors are senior scientists within the National Fishery Administration.

