Image: Oscar Nordahl
FEATURE There are over 27,000 fish species on earth, and the variation among different species is enormous. Not everyone is aware, however, how large the variations can be within species. Oscar Nordahl at Linnaeus University has studied pike populations, and also found that they are able to raise their body temperature by sunbasking.
Different populations of the same species can show variation in genetics as well as in appearance and behaviour. Understanding which factors affect population structure and intraspecific diversity is important not only from a management standpoint, but also helps us to understand how species are affected by environmental changes.
Oscar Nordahl received his PhD from Linnaeus University for his thesis work on coastal Baltic Sea pike. This species, which was initially considered to show little genetic variation, turns out to show a variety of genetically distinct subpopulations, even on fine spatial scale. This type of population structure has been studied in some Baltic Sea species, but not previously in a freshwater-adapted species such as pike. Because climate changes are expected to lead to decreased salinity in the Baltic Sea, knowledge about this freshwater species in brackish water will be vital for the future.
Oscar Nordahl at one of his experiment grounds.
ImageKristofer BergströmIt is easy to understand how geographically isolated animals can develop in different ways, but how can variations occur when there are no obvious physical boundaries?
An important factor for this to occur in coastal pike is their homing behavior. This means that they return to the same area to spawn. They have this behavior in common with a variety of fish species; the best-known example is salmon. Some coastal pike find their way into the waterways of suitable wetlands where they spawn in the spring. As soon as the spawning is over, the pike migrate back to the coast. Other pike stay on the coast and spawn there. During the remainder of the year, pike from the various subpopulations live together in the coastal area.
The pike in different subpopulations not only prove to be genetically distinct, but there are also differences in pike morphology and behavior. For example, the number of vertebrae and egg size differ, which in turn can affect the reproduction, growth and survival of the fish. It was also found that the pike can have local adaptations to their spawning areas, which means that they have the most reproductive success if they spawn in their home environment compared to using other spawning areas. Such adaptations can further reduce gene flow among neighbouring subpopulations.
Temperature is the most important environmental factor affecting fish by altering everything from behavior, reproduction, physiology, immune system to the potential if evading predators. Pike, like most fish, are ectothermic animals with a body temperature that closely matches that of ambient water. Even small changes in water temperature can have large impact on individuals and populations and fish are known to seek out areas with beneficial water temperatures with high sensitivity. Oscar has, however, shown that fish in fact can raise their body temperature to be higher than the surrounding water by sunbasking close to the surface. A process that previously have been considered impossible for fish due to the strong cooling effect in water. The temperature gain proved to be associated with the coloration of the fish and, more importantly, an increased growth rate. This novel mechanism for fish thermoregulation will thus greatly benefit our understanding of fish behavior, species interaction and the effect that light has on individuals and populations of fish.
The pike is one of Sweden's most common predatory fish. As a predator at the top of the food chain, it has an important ecological role in the Baltic archipelago environment. However, pike populations have declined dramatically, and despite extensive conservation efforts, populations are still weak. Knowledge of population structures and local adaptations is crucial to being able to work effectively with measures, such as fisheries management, habitat restoration and connectivity among and within habitats.
This article has earlier been published in the EcoChange annual report 2018, and is based on: