| dc.description.abstract | Benthosema glaciale (Reinhardt, 1837) is one of the most abundant fish species found globally. The species plays an important role in the food-web of marine ecosystems, as a link between the trophic levels. In west Norwegian fjord systems, B. glaciale have been observed to be infected by the parasitic copepod Sarcotretes scopeli (Jungersen, 1911), however, there is limited knowledge on how parasitic infections affect the biology of this species. Furthermore, west Norwegian fjords have been experiencing more frequent oxygen loss in the basin water over the last decade and it is not known if such changes could impact the parasite-host relationship between these two species. A cross-sectional study was conducted with infected and uninfected B. glaciale from four west Norwegian fjords, which differed in dissolved oxygen. This study was performed in order to investigate how dissolved oxygen affects S. scopeli prevalence. In addition, the effect of infection on gonadosomatic, hepatosomatic and heartsomatic indices, Fulton’s condition factor and host growth were studied separately for the host sexes. Furthermore, the site-specificity of S. scopeli on host exterior and interior, and the relative parasitic volume in relation to the host were investigated. The outcome of the cross-sectional study showed that S. scopeli greatly reduces the gonadosomatic index for both host sexes. The hepatosomatic index was similar for infected and uninfected females in all fjords except Sørfjorden, where the hepatosomatic index was higher for infected females. Infected males in all of the fjords had higher hepatosomatic index than uninfected ones. Furthermore, infected fish were found to be in poorer condition than the uninfected ones in all of the investigated fjords. However, infection did not seem to compromise the growth rate. Inconclusive results were presented on the effect of infection on host cardio function. The data presented did not show an effect of dissolved oxygen concentration on the parasitic prevalence but did show that Sørfjorden had a lower prevalence than the other three fjords. Furthermore, the results from the parasitic attachment selectivity showed that S. scopeli attach its feeding apparatus randomly both inside and outside of the hosts visceral cavity, while the parasites external part most often located on the hosts dorsal side. Overall, the data suggested that S. scopeli acts as a parasitic castrator on the host; areas of reduced oxygen concentration do not affect S. scopeli prevalence; and that the parasitic attachment site is randomly distributed in the host internal, while the parasite has a high affinity for dorsal attachment on the host external. | |
