Epsilon Archive for Student Projects

Abstract

The European eel has been a common species in our European waters, but today it is considered critically endangered and is on the International Union for Conservation of Nature´s (IUCN) red list. Several factors are believed to have prompted the decline of new eel recruits that has been observed since the 1980´s. One of these factors may be the invasive parasite Anguillicola crassus. The International Council for the Exploration of the Sea (ICES) has therefore called upon more research to better understand how this parasite affects eel fitness. This study examined how the eel's energy reserves are affected by the parasites and whether this may be due to a redistribution of energy resources to the immune system. In this study, 202 eels from two Swedish lakes were examined with no negative effects observed with regard to parasite prevalence or intensity. An enlargement of the spleen of eels with a slightly damaged swim bladder was, however, observed indicating a small increase in the immune system. This study found indications that multiple infestations by the parasite is needed in order to get a severely damaged swim bladder and that a high swim bladder degenerative index might lead to less energy being stored in the liver. However, no major impact on overall condition could be determined. Possibly, a damaged swim bladder makes it difficult to swim properly, in which case energy is drained from the eel. However, during the eel's growing life stage, it seems to be able to compensate for such a loss of energy. Based on the observations in this study, the damage that A. crassus causes might be of a greater concern when the eel starts to migrate to the Sargasso Sea. Then, the eel probably consumes much more energy if swimming with a not fully functioning swim bladder and no longer being able to compensate for an energy loss with food. This could in turn lead to failure in reproductive success and have consequences for the survival of the species. The index of the damage on the swim bladder seems to give a better indication of the negative effects of an infection by A. crassus than the number of adult parasites in the lumen, and is therefore proposed to be included in future monitoring programs.