The ecological communities we observe today are a product of the bidirectional interactions between ecological and evolutionary processes. Although the effects of ecological processes on population divergence and speciation have been studied extensively, far less is known about the effects of divergence and speciation on ecological dynamics. This is especially true for effects of ecological speciation processes on higher trophic levels. In this thesis I focus on how divergence in the European whitefish (Coregonus lavaretus) affects a guild of six piscivorous bird species. Previous studies have indicated that population densities of these species are higher on lakes with polymorphic whitefish than on lakes with monomorphic whitefish. Here I test the hypothesis that the high densities of piscivores is a response to the rich food resource provided by dwarf whitefish ecotypes, which are of suitable size and occur in very high abundance.To test this hypothesis I analyzed fecal samples from piscivorous birds in lakes with polymorphic whitefish, using samples from lakes with monomorphic whitefish as controls. With the method of ddPCR (digital droplet Polymerase Chain Reaction) the amount of DNA from different prey fish species in the droppings of six fish-eating birds was quantified and converted to proportional abundances. The results shows that the proportion of whitefish in the diet of the entire fish-eating guild was significantly higher in lakes with polymorphic whitefish (44%) than in lakes with monomorphic whitefish (18%). Species-level analyses showed that this result also holds for both black-throated loon (Gavia artica) and red-throated loon (Gavia stellata). Common merganser (Mergus merganser), red-throated merganser (Mergus serrator) and terns (Sterna paradisaea and Sterna hirundo) did not show any difference between the two lake categories. Thus, my study supports the idea that the evolution of small-sized whitefish ecotypes provides a profitable food source for piscivorous birds. However, the finding that only some species of piscivorous bird populations seem to rely heavily on dwarfed whitefish as food suggests that also some other aspect of the speciation process may favor these species. Thus, more studies are needed to further assess what effects polymorphic whitefish have on piscivorous bird populations.

Top predators in aquatic ecosystems often have strong top-down effects on the ecosystem. Northern pike (Esox lucius) has been documented to cause whitefish (Coregonus lavaretus) populations to diverge into different ecomorphs. This can facilitate piscivory in other predators as a novel resource becomes available to them in the form of dwarf whitefish. The aim of this study is to examine whether the presence of pike causes Eurasian perch (Perca fluviatilis) to shift their diet from insectivory to piscivory, and whether this is directly driven by whitefish polymorphism. Stomach contents of 147 perch from lakes with and without pikes were analyzed. The results show that the presence of pike has a clear influence on the diet of the perch. In lakes without pike, perch are mostly insectivorous, and in lakes with pike, they are mostly piscivorous. This diet shift appears to be driven by whitefish availability, as a majority of the diet of perch in pike lakes consisted of whitefish, while none of the fish eaten by perch in non-pike lakes was whitefish. In addition, the results showed that perch undergo the diet shift from insectivory to piscivory at a smaller size when coexisting with pike. This study can be added to the growing body of evidence for the ecological significance of pike.

A critical step in ecological speciation is the development of reproductive isolation. The processes leading up to reproductive isolation are difficult to study since they often occur over very long time periods. Populations of European whitefish (Coregonus lavaretus) has gone through reticulate divergent evolution in many Scandinavian lakes during the last 10 000 years. Some populations even exhibit morphological and genetic divergence that involves adaptations to different niches during the first 100-200 years after introduction. This observed rapid diversification into different ecological niches makes whitefish a useful model species for studying ecological speciation and early population divergence. By assessing divergence for three traits in recently introduced whitefish populations, this study aims to elucidate the processes that lead to adaptive phenotypic divergence and reproductive isolation. Whitefish populations of known age (71-170 years) were sampled on their spawning grounds and characterized with respect to, 1) length of spawning season, 2) temporal segregation based on body size, and 3) temporal segregation based on the number of gill rakers. I found that the length of the spawning season and body size differences between early and late spawners increased with population age. No such trend was observed for differences in gill rakers between early and late spawners, but significant divergence within some of the older populations was detected. I conclude that these young whitefish populations have taken the first steps toward reproductive isolation between ecotypes that differs in body size (a highly plastic trait) and gill raker numbers (a trait under strong genetic control).

The origin and maintenance of much of natural biodiversity can likely be explained by theecological speciation theory. Ecological speciation is defined as the process whenreproductive isolation arises through divergent selection caused by ecological factors, such asbiotic interactions and abiotic conditions. Many populations of European whitefish(Coregonus lavaretus) are currently going through sympatric speciation from a single littoralecotype to two or more ecologically separate ecotypes. It has been hypothesized that northernpike (Esox lucius) is promoting speciation in whitefish by exposing it to a trade-off betweenpredation risk and resource availability causing an initial divergence in size eventuallyleading to two or more ecologically and genetically distinct ecotypes. After initial divergencesmall whitefish ecotypes often abandon ancestral spawning sites to spawn in streams or deepwater suggesting that the ecotypes are at least partially reproductively isolated. The driver ofthis reproductive isolation could be predation risk as pike mainly forage in the littoral. If thisis true, we would expect to find significantly higher predation risk for small whitefish at thelittoral spawning sites than on alternative spawning sites. To test this idea, hooks baited withsmall whitefish was placed at different spawning sites during the fall spawning season andchecked every 12 hour for 36 hours. Relative predation risk for small whitefish wassignificantly higher at the ancestral littoral spawning sites than at stream spawning sites. Inaddition, there was a significant decrease in predation risk later in the spawning season. Thisindicates that predation is promoting reproductive isolation in European whitefish byselecting against small individuals spawning at littoral spawning sites, but potentially also byselecting for delaying spawning in small individuals in the littoral. To fully understand thespeciation process in this system more studies must be done considering the complexity ofthe speciation process. Because of the lack of studies investigating predation as a cause ofbehavioral changes, that in turn promotes reproductive isolation, future studies should putmore emphasis on predation as a possible driver of reproductive isolation.

The spawning time differs between sympatric ecotypes of lake-dwelling whitefish (Coregonus lavaretus) and this temporal divergence can be linked to the type of spawning habitat used (i.e., streams, littoral or profundal). It has been hypothesized that the divergence in spawning time between spawning habitats is due to differences in incubation temperature regimes, and a need to synchronize hatching with the spring zooplankton bloom. An alternative hypothesis is that the hatching time of eggs is controlled by cues such as rising temperatures or increasing day length, and the divergence in spawning time occurs due to later maturation in colder habitats. To test these hypotheses, whitefish eggs were sampled on different spawning grounds and the median hatching time was estimated using logistic regression. Temperature logger data and information based on interviews and earlier studies were used to study accumulation of thermal energy during incubation in the form of degree-days. According to this thesis, the hypothesis that hatching time is dependent on rising temperatures and day length is supported, due to high variation in the amount of accumulated thermal energy, and little variation in hatching time. The egg sampling was not successful on profundal spawning grounds, nor on littoral spawning grounds of old populations, which made the results of this study weaker. Further studies are needed to fully confirm either of these hypotheses regarding spawning time of mature whitefish and hatching time of eggs. Sampling methods need to be advanced in order to enable egg sampling on all types of spawning grounds.

Investigation of creatine- and adenylate phosphates involvement in fish spermatozoa metabolism is of high interest for fish spermatology. These compounds are necessary to support normal physiological state and motility of spermatozoa. The simultaneous changes in content of creatine- and adenylate phosphates in fish spermatozoa prior and during their motility are quite unclear. Therefore, studying and development of new methods for the quantification of creatine- and adenylate phosphates in spermatozoa of different fish species under such physiological conditions as maturation and in vitro manipulation are of high importance. One of the study outputs is the developed LC/HRPS (liquid chromatography coupled with high resolution product scan) method for the analysis of creatine- and adenylate phosphates content in fish spermatozoa (Chapter 2). Its main advantage is the possibility to detect and quantify several compounds (creatine, creatine phosphate (CP), AMP, ADP, ATP, and cAMP) simultaneously to obtain maximum information with minimum analytical effort. The method was validated taking into account such key parameters as limit of quantification, selectivity, recovery and repeatability. It represented an excellent performance allowing determination of target compounds in highly diluted fish sperm samples. Consequently, the method was applied for the quantification of aforementioned substances during sterlet (Acipenser ruthenus) spermatozoa maturation and in vitro manipulation with sperm of whitefish (Coregonus lavaretus maraena) and European eel (Anguilla Anguilla). The present study showed that immature sterlet spermatozoa are not able to initiate motility. Significant decrease of CP and stable levels of ATP and ADP content during their maturation were found. The critical importance of ATP regeneration system and oxidative phosphorilation for the maturation process of sterlet sperm as a prerequisite for successful fertilization was assumed (Chapter 3). Further experiments revealed that European eel spermatozoa were not able to initiate motility by activation medium (AM) at the start of the induced spermiation. They acquired the ability to be activated after the dilution with AM at the end of hormonal treatment. This accompanied by the increase of CP and cAMP levels in spermatozoa after activation. That allowed us to assume the involvement of ATP regenerating system and cAMP-dependent regulatory pathways in the process of hormonally induced spermiation (Chapter 4). Current study represents a first successful estimation of cAMP in fish spermatozoa during the motility period using the LC/HRPS. Important issues concerning the short-term storage of European eel sperm were rised. Storage at 4 °C was accompanied by higher marcoergic phosphates content and higher motility in comparison to the storage at 20 °C. It suggests the involvement of macroergic phosphates metabolism in short-term storage. (Chapter 4). Obtained results could contribute to the development of new effective methods for improving of spermiation and short-term sperm storage in European eel aquaculture. Various degrees of energy consumption in response to environment composition were found in whitefish spermatozoa. Energy consumption was significantly higher in motility activating conditions. No effect of osmolality was found on this process. The content of CP and ATP was significantly higher when cells were in motility inhibiting medium comparing to activation medium. No relationship between content of CP, ADP, and ATP and spermatozoa motility parameters in AM of different osmolality was found. Isotonic conditions favor the spermatozoa with longer motility period, higher linearity, and fast velocity without increase in ATP content (Chapter 5). This suggests that whitefish sperm energy management is more efficient after activation in isotonic conditions. Obtained results are of high interest for elaboration of new sperm motility activating media for fisheries practice.

An important question in determining species composition in a lake is what species has had the chance to reach the lake in the first place. The aim of this study was to examine natural stream connectivity limitations for whitefish (Coregonus lavaretus) between lakes, and to contribute to the development and evaluation of GIS-based methods to answer this question. 497 lakes with identified downstream source lakes, were classified as naturally colonized or introduced whitefish populations, reflecting the possibilities for whitefish to reach the lake naturally. Data of maximum stream slope and stream length between each lake pair was generated by Stefan Blumentrath, NINA. Maximum stream slope was measured on two different resolutions of stream length for comparison, one on slope over 10 m and one over 150m. A subset of lakes was manually examined in geographic information systems and compared to maps and aerial photos to evaluate the data and compare the results with the full dataset. False slope maximums were corrected and streams with much human alteration around a possible natural slope maximum was removed from the small dataset. The two datasets were analyzed using logistic regression models. Akaikes information criterion (AIC) showed that the optimal model, for both datasets, was the one using only slope maximum as predictor, and slope over 150m gave better results than 10m. A k50-coefficient, the value of a predictor that results in 50% probability of colonization, was introduced as an approximate of when the predictor forms a connectivity hinder. The k50-corefficient was estimated to 2,08±0,22° (±standard error) for slope over 150m for the big dataset and 2,58±0,20 for the small. Weaknesses in the data were distinguished and improvements for future fish connectivity studies are suggested.