Erik Degerman

Recruitment of Pelagic Fish in an Unstable Climate: Studies in Sweden's Four Largest Lakes

Abstract Pelagic fish population biology was studied in the large Swedish lakes Vänern, Vättern, Mälaren and Hjälmaren. It is crucial for fish fry in temperate regions to hatch early in the growth season to survive, and achieve large size before winter, and it is suggested that the key factors are to match the spring development of phyto- and zooplankton, but to avoid predation. This is more easily accomplished by the studied spring spawners smelt (Osmerus eperlanus) and pike-perch (Stizostedion lucioperca) than autumn spawners, such as vendace (Coregonus albula). It is shown that hatching of vendace fry shortly after ice-break-up is beneficial for year-class strength. In oligotrophic large lakes with few predatory species a rapid increase in water temperature after ice-break is also promoting recruitment, whereas this is not the case in eutrophic lakes where predation pressure from other species may become too high. The results indicate that autumn spawners will have difficulties in adapting to global warming and it is also suggested that the life history can explain the large variations observed in year-class strength between years.

Labile inorganic manganese : An overlooked reason for fish mortality in acidified streams ?

Acid deposition has caused acid episodes and elevated concentrations of metals like Fe, Al and Mn in streams in mountain regions of Sweden. In streams in the area of Lofsdalen, central Sweden, an episode in 1978 during snow melt caused massive downstream migrations of fish. Acid episodes have occurred yearly since then. pH of 4–4.5 and high concentrations of total Fe, Mn and Al were measured during these spates. Liming measures have been carried out in a number of catchments in order to increase the pH and try to decrease the leaching of metals. The lime has been delivered on wetlands, seepage areas and areas covered by meltwater during the spring. Cage experiments were carried out with yearling brown trout in the springs of 1982, and 1985–86. Fe, Al and Mn were analysed in water samples and on/in the gills of dead and live brown trout. There were no significant correlations between mortality or average survival time and total concentrations of Fe, Al or Mn in the water or on/in the gills. Average survival time was positively correlated to pH (p< 0.01). Metal speciation was carried out in 15 experiments in 1985 and −86. A canonical discriminant analysis showed that pH and the concentration of labile inorganic manganese in the water explained all the observed mortality (can. corr 0.89, p< 0.001). The rate of accumulation of manganese on/in the gills was correlated to the concentration of labile inorganic manganese in the water (p<0.01, r2=0.54, linear regression).

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

Abstract 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.

The effects of liming on aquatic fauna

Liming activities in Sweden have included extensive studies of fish fauna in lakes and running waters, with the result that it is now possible to make well-founded generalizations concerning the effects of treatment on this group of organisms. It is more difficult to make generalizations about lake zooplankton and benthic fauna, which have mainly been studied within the framework of research projects. Benthic faunas in running waters have almost exclusively been studied in conjunction with general monitoring of liming effects, via a large number of individual projects. Such projects have provided material to enable the effects to be described, but not the causal relationships. Little work has been done in this context on aquatic birds and only isolated studies have been performed on frogs.

Are fish populations in temperate streams affected by crayfish? - A field survey and prospects

SynopsisFish populations may be affected by predation and competition from various types of organisms, among which crayfish have been suggested as important actors. We here present results from stream surveys, suggesting that neither native noble, Astacus astacus, nor introduced signal crayfish, Pacifastacus leniusculus, necessarily affect fish population densities in temperate stream communities. Comparisons of fish densities within stream sites between years with absence and presence of crayfish showed no effect of either crayfish species. A further analysis of changes in fish densities between periods without and with crayfish in low, intermediate and high densities revealed that crayfish density did neither have an effect on fish densities. Our study is one of exceptionally few that consider the above aspects in long-term perspectives in natural systems, and we discuss that previously reported divergent results of crayfish effects on fish may be highly dependent on specific species and methods used, and that the effects of crayfish on fish populations deserve further attention to enable reliable predictions of community processes in streams.

The decline of fauna in small streams in the Swedish mountain range

Investigations in the southern part of the Scandinavian mountain range have shown a direct correlation between snowpack pH and the lowest pH in small streams. In streams with catchments <100 km2 a snowpack pH below 4.8 resulted in a stream pH below 5.5. As the snowpack pH in the southern mountain range is 4.0–4.6, lower values to the south and at high altitudes, large areas are affected by acid deposition. The acidity of the snowpack is released almost directly into the streams during thaw, due to the large snowpack, rapid thaw, steep terrain and thin soils. The acidification of the snow has lead to an extensive fauna depletion, especially in smaller streams without upstream lakes. The abundance of benthos in acidified streams in the municipality of Härjedalen (11.000 km2) is today only one tenth of the abundance before acidification. Fish populations have declined to the same extent, and several acid-sensitive species have been lost. It is concluded that mountain streams and fauna are extremely sensitive to acidification, and that even large reductions of emissions will be insufficient.

Liming strategies and effects: the River Högvadsån case study

The main stream of River Atran was developed for power generation in the 1920s. River Hogvadsan, a relatively unspoiled tributary, thereby became very important in the reproduction of the genetically unique population of Atlantic salmon (Salmo salar). River Atran and River Hogvadsan are also among the finest waters in western Sweden for recreational fishing (Lindgren 1992).