Human activity is affecting species and ecosystems all over the world. In aquatic systems negative trends can be seen for many fish stocks and potential consequences of this, for ecosystem structure and functions, are of particular concern. Overexploitation is often suggested as a major driver behind these changes but other factors such as acidification, habitat destruction, eutrophication, pollution, introduction of alien species and climate change are also considered important. Fisheries biologist are now faced with the challenge of finding suitable management for affected fish stocks but the task is difficult because the causal connections tend to be complex, involving many factors and synergistic effects as well as interactions among species that may lead to cascades of indirect effects within communities. Thus, to fully understand, ameliorate and predict the complex effects of disturbances and environmental change on ecosystems, knowledge of species and how they interact with each other and the environment is required. This has led to an increased demand for multispecies management of fisheries and ecosystem-based management and food webs are central to both these approaches. This thesis is an attempt to use a food web approach to increase our understanding of an endangered fish stock in Europe’s sixth, and Sweden’s second largest lake: Lake Vättern.
Lake Vättern is a deep, oligotrophic lake in south-central Sweden that harbours some 30 species of fish, among these a large-bodied form of Arctic charr: great Arctic charr (Salvelinus umbla). The stock of great Arctic charr in Lake Vättern used to be of great importance for the commercial fisheries but today the stock is considered critically endangered. Suggested causes for the decline and/or problems for the stock to recover include overexploitation, decreased nutrient loading, climate change and introduction of Salmon (Salmo salar) and signal crayfish (Pacifastacus leniusculus). The focus of this thesis is great Arctic charr in Lake Vättern and the dramatic decline of this important fish stock during the second part of the 20th century. In a series of papers we combine field experiments, analyses of climate, commercial harvest and stock survey data, stomach content analyses and model simulations to study several of the suggested causes for the decline of the stock of great Arctic charr and discuss implications of the results for future management of the stock.
In Paper I we investigate the potential effect of the introduced signal crayfish on the stock of great Arctic charr, using a controlled field experiment. More precisely, we investigate the extent of predation on eggs of great Arctic charr. We are able to partition the total loss rate of eggs into background mortality, predation mortality from introduced crayfish and predation from native fish. It has earlier been suggested that predation on eggs of great Arctic charr by fish is more important than by crayfish. However, we find that the mortality rate due to signal crayfish in our experiment is more than five times that because of native fish. We thereby conclude that crayfish predation are at least of the same magnitude, or even greater, than fish predation and that high abundance of signal crayfish on spawning sites could impair the recovery of the stock of great Arctic charr in Lake Vättern. Thus, suggests that targeted reductions of signal crayfish on selected spawning grounds are potential management options that should be considered.
In Paper II we use survey data from 2006-2010 of the stock of great Arctic charr to first estimate the selection curves for the gillnets used in the survey and subsequently estimate the size-frequency distribution and relative abundance of the stock. We begin by analyzing some of the assumptions behind the so called SELECT-model, which is used to estimate selection curves, and suggest how data can be treated to better conform to these assumptions. We show that by removing potentially nonmeshed fish from the data and taking non-isometric growth into account, our approach results in narrower and less asymmetric selection curves with a significantly better model fit. Next, using the obtained selection curves, we estimate the size frequency distribution and relative abundance of great Arctic charr in different years and find that mortality of medium-sized fish have decreased and abundance of fish is increasing slightly. Likely causes for this are the new fishery regulations that were implemented in 2007. Generally speaking, our study demonstrates an approach that is expected to increase the accuracy of estimates of fish size-distributions from survey data and more specifically, this is expected to lead to better understanding the dynamics of the endangered stock of great Arctic charr in Lake Vättern.
Paper III uses records of commercial catch data since 1914 to analyse the potential effects of climate change on great Arctic charr in Lake Vättern. We find that there is a positive effect of winters with ice on the stock of Arctic charr that can be seen as increasing commercial catches that peak four years after an ice-winter. Furthermore, the positive effect increases with the duration of the ice winter. It is unclear however, if this is a direct or indirect effect of ice on the stock of great Arctic charr. To analyze this, the date of different development stages in hatching for eggs of great Arctic charr is estimated using water temperature data since 1955. The results show that there is a positive correlation between the predicted date of fully consumed yolk sac and standardized catches six years later. This suggests that warmer winters, which result in early hatching of eggs and early date for when the yolk sack is consumed, will affect survival of fry and subsequent recruitment to older size classes negatively. Thus, lending support for a strong possibility for a trophic mismatch. Our study show that climate do appear to affect the stock of great Arctic charr in several ways and underscore the fear that future climate change will have negative consequences for the stock.
Paper IV uses stomach content analysis to (i) describe the diets of fish and thus identify and quantify links in the pelagic food web in present day Lake Vättern, and (ii) compare the results with older diet data to see if observed changes in Lake Vättern in the last 30-40 years have led to any changes in the trophic interactions between the species. Overall, we conclude that the investigated food web structure of Lake Vättern has remained largely intact and stable during the last 50 years even if there have been introductions of non-native species and environmental changes in Lake Vättern. However, when comparing the old and new data there appear to have been some diet shifts for some species. For example, the diets during summer for both great Arctic charr and Atlantic salmon in our study suggest a possible shift to a diet dominated by three-spined stickleback, thus, indicating support that an increased interspecific competition between these species may have occurred.
Finally, Paper V develops and analyzes a size-structured model of the pelagic food web of Lake Vättern. The aim is to analyze the combined effects of some of the suggested causes for the decline of the stock of great Arctic charr in Lake Vättern. We incorporate results from preceding papers to quantify trophic links in the food web and define a realistic starting size distribution of great Arctic charr. In the model we vary the stocking of salmon, the fishing pressure and the abundance of signal crayfish and study the effects on different size classes of great Arctic charr. We find that a decrease in salmon stocking into the lake has the greatest positive impact on large great Arctic charr while a decrease in fishing intensity has the greatest positive impact on smaller sizes of great Arctic charr.
Linköping: Linköping University Electronic Press, 2012. , 32 p.
2012-06-01, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)