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The decline of great Arctic charr in Lake Vättern: empirical and theoretical analyses of suggested causes
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Biology. Linköping University, The Institute of Technology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. , 32 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1447
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-77683ISBN: 978–91–7519–897–2 OAI: oai:DiVA.org:liu-77683DiVA: diva2:528417
Public defence
2012-06-01, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2012-05-25 Created: 2012-05-25 Last updated: 2012-05-25Bibliographically approved
List of papers
1. An invasive crayfish affects egg survival and the potential recovery of an endangered population of Arctic charr
Open this publication in new window or tab >>An invasive crayfish affects egg survival and the potential recovery of an endangered population of Arctic charr
2011 (English)In: Freshwater Biology, ISSN 0046-5070, E-ISSN 1365-2427, Vol. 56, no 12, 2543-2553 p.Article in journal (Refereed) Published
Abstract [en]

1.Many fish stocks have declined, because of overharvesting, habitat destruction and introduced species. Despite efforts to rehabilitate some of these stocks, not all are responding or are recovering only slowly.

2.In freshwater systems, introduced crayfish are often problematic, and it has been suggested that their egg predation could reduce recruitment in depleted stocks of native fish.

3.Here, we report the results of a field experiment, using experimental cages, on the extent of predation on eggs of great Arctic charr (Salvelinus umbla) in Lake Vättern, Europe's fifth largest lake. Here, the great Arctic charr has declined dramatically and is listed as critically endangered.

4.We were able to partition the total loss rate of eggs into background mortality, predation by introduced signal crayfish (Pacifastacus leniusculus) and predation by native fish. The mortality rate of charr eggs because of crayfish was estimated at more than five times that because of native fish. Of the total loss of eggs, 80% is believed to be caused by crayfish and 14% by fish, with 6% being natural background mortality.

5.In a worst case scenario, our data infer that only 25% of the original number of eggs would survive, compared with 75% in the absence of crayfish. This could impair recovery of the stock of the endangered great Arctic charr in Lake Vättern. 6.Contrary to earlier claims that crayfish predation on eggs of great Arctic charr is insignificant, our results indicate that crayfish predation may exceed fish predation and suggest that the abundance of signal crayfish on the spawning sites of great Arctic charr should be managed.

Keyword
Egg predation, field experiment, great Arctic charr, Lake Va¨ttern, signal crayfish
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-77676 (URN)10.1111/j.1365-2427.2011.02679.x (DOI)
Available from: 2012-05-25 Created: 2012-05-25 Last updated: 2017-12-07Bibliographically approved
2. Addressing catch mechanisms in gillnets improves modeling of selectivity and assessment of an endangered stock of Arctic charr
Open this publication in new window or tab >>Addressing catch mechanisms in gillnets improves modeling of selectivity and assessment of an endangered stock of Arctic charr
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Estimation of fish stock size distributions from survey data requires knowledge about gear selectivity. However, selectivity models rest on assumptions that seldom are analysed. Departures from these can lead to misinterpretations and biased management recommendations. Here, we use data on great Arctic charr (Salvelinus umbla) to analyse how these assumptions affect estimated selectivity curves, estimate the size frequency distribution of the stock, and estimate mortality in different years.

Initial selectivity curves, using the entire data, were wide and asymmetric, with poor model fits. Removing data for non-meshed fish and taking non-isometric growth into account, resulted in narrower and less asymmetric selection curves, with good model fits. Estimated relative abundance of charr across all sizes increased from 2006 to 2010 while estimated mortality rates indicate a decrease in mortality for medium-aged, but not for older fish. Likely causes for these changes are stronger fishery regulations implemented in 2007. Our study demonstrates an approach that increases the accuracy of estimates of fish size-distributions from survey data and leads to a better understanding of the dynamics of an endangered fish stock.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-77679 (URN)
Available from: 2012-05-25 Created: 2012-05-25 Last updated: 2012-05-25Bibliographically approved
3. Climate effects on an endangered stock of great Arctic charr
Open this publication in new window or tab >>Climate effects on an endangered stock of great Arctic charr
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Climate affects species in many different ways and climate change represent an important and increasing threat to single populations as well as the structure and functioning of entire ecosystems. Studies have indicated that water temperature may affect plankton spring biomass abundance in temperate lakes but does not seem to have much effect on the timing of plankton peak abundance. If plankton peak abundance is not significantly affected by water temperature, while the development time of eggs and fry is, and thus the predicted time of strong demand for food, there is a strong possibility for a trophic mismatch effect. Our data and analysis show that this indeed seems to be the case for great Arctic charr in Lake Vättern. The date when the yolk sac is predicted to be consumed is positively correlated with the standardized catches of great Arctic charr six years later. This suggest that warm winters, which result in early hatching of eggs and early date for when the yolk sac is consumed, affect survival of fry and subsequent recruitment to older size classes negatively. This leads to lower than expected catches a few years later. In addition, we also show that ice winters have a positive effect on Arctic charr with a time lag of 3-4 years and that this effect increase with the duration of the ice-cover. These effects are probably not due to trophic mismatch effects. However, all these effects are the result of a changing climate and when combined these effects of increasing water temperature, decreasing frequency and duration of ice winters are predicted to affect the stock of great Arctic charr negatively.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-77680 (URN)
Available from: 2012-05-25 Created: 2012-05-25 Last updated: 2012-05-25Bibliographically approved
4. Stomach content analyses to investigate longterm changes in a pelagic ecosystem
Open this publication in new window or tab >>Stomach content analyses to investigate longterm changes in a pelagic ecosystem
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This is the first serious step to describe a fish community food web of the off-shore ecosystem in Europe’s sixth largest lake, Lake Vättern. Stomach contents analyses from in total 777 fishes from nine different species were used to describe an updated off-shore food web of Lake  Vättern. Furthermore, a comparison with earlier made stomach content analyses were done 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. Our results indicate that diet shifts may have occurred for some species. For example, the diet during summer for great Arctic charr (Salvelinus umbla) and stocked Atlantic salmon (Salmo salar) in our study show a shift to a diet dominated by three-spined stickleback instead of smelt and vendace respectively. Still, 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. The updated off-shore food web for Lake Vättern presented here could be a valuable tool for using an ecosystem based approach and to better identify management strategies.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-77681 (URN)
Available from: 2012-05-25 Created: 2012-05-25 Last updated: 2012-05-25Bibliographically approved
5. The decline of great Arctic charr (Salvelinus umbla) in Lake Vättern: implications based on a sizestructured food web model
Open this publication in new window or tab >>The decline of great Arctic charr (Salvelinus umbla) in Lake Vättern: implications based on a sizestructured food web model
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Recent research has illustrated the explanatory power of body size for understanding ecosystem structure and function. Hence, many studies use body size as a tool to parameterize food web models. However, the body size of most species is not a static but dynamical trait that changes during the life time. Many other traits, including diet and vital rates, change in concern with body size. This indicates a need for food web studies to resolve the population structure of a species, rather than describing it by its total abundance or mean adult body size.

We here analyse a multispecies size-structured food web model of the pelagic community of Lake Vättern, Sweden. Here, the stock of great Arctic charr used to be commercially important but have decreased by more than 90% since the 1950s. Causes for the decrease have been debated and suggestions include (i) predation on eggs by introduced signal crayfish, (ii) overexploitation, and (iii) competition between introduced salmon and charr. As a tool for analysing potential impacts on the stock of great Arctic charr and evaluating different management options we present a dynamical food web model where the population of Arctic charr is size structured. The growth equations describe Lotka-Volterra type predator-prey interactions among non-structured species, and consumption-dependent mortality and transition rates of the size-structured Arctic charr. Mortality rates of consumers are allometrically related to body size and trophic interaction coefficients are estimated resulting in realistic abundances of all species. Results from our model analysis points in the direction of salmon stocking being the most important factor affecting the largest size class of great Arctic charr, followed by fishing intensity for the smallest size classes. We believe this multispecies modelling approach is one way of integrating size dependent traits in food web ecology and we discuss future developments of this modelling approach.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-77682 (URN)
Available from: 2012-05-25 Created: 2012-05-25 Last updated: 2012-05-25Bibliographically approved

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