Epsilon Archive for Student Projects

Abstract

Many fish populations in the north have divided into coexisting ecotypes that differ in body size. While such polymorphic fish populations have been studied for many years, the understanding of how different types of selection contribute to the formation of dwarf- and giant ecotypes remains limited. Previous findings suggest a positive relationship between body size polymorphism in European whitefish (Coregonus lavaretus) and standing biomass of littoral benthic invertebrates. As this observation may hold important clues to the mechanism that underlies body size divergence, it is important to find out if it represents a general phenomenon. In this study, I compare data on biomass and community composition (both estimated using catch data from invertebrate traps) of littoral invertebrates from north-swedish lakes with- and without body size polymorphism in the resident populations of Arctic charr (Salvelinus alpinus). In a related follow-up question, I wanted to explore the potential mechanism behind enriched littoral invertebrate communities, i.e., that the habitat specialization associated with body size divergence will cause most Arctic charr to actively avoid feeding on (and thereby supressing) the littoral resource. From this mechanism follows that the standing biomass of littoral invertebrates should be negatively correlated to the degree of littoral benthivory in Arctic charr. I therefore performed stomach content analysis of charr from the lakes of the invertebrate study to test if my invertebrate biomass estimates were correlated to the proportion littoral invertebrates in charr stomach contents. The result of this study reveals no general positive relationship between body size polymorphism in charr and standing biomass of littoral invertebrates. Moreover, Arctic charr preferred to eat zooplankton in all lakes and I found no correlation between littoral invertebrate biomass and the amount of littoral benthic prey found in Arctic charr stomachs. While my results revealed few significant trends overall, my findings still indicate that lakes with polymorphism could be associated with high biomass of predation-sensitive invertebrate taxa. These findings include significantly higher biomass of amphipods and on average (although non-significantly) higher biomass of large-bodied Trichopterans (Phryganidae and Limnephilidae) in lakes with polymorphism. To determine whether charr polymorphism is indeed associated with enriched littoral invertebrate communities, more studies involving more lakes are needed.