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.