Thesis Defense: Patterns of co-occurrence of parasites in Eastern Baltic cod stocks within...

In the coming weeks, UW master's students will defend their theses. The subjects are varied and the students are from both master's programs, Coastal Marine Management and Coastal Communities and Regional Development. The defenses are accessible to everyone through zoom links but they are also open to the public, at the University Centre of the Westfjords in Ísafjörður.

Student: Elisa Janssen
Date & time: September 12th at 9:00
Thesis title: Patterns of co-occurrence of parasites in Eastern Baltic cod stocks within different depth strata
Program: Coastal Marine Management
Zoom link: https://eu01web.zoom.us/j/61157747546

Abstract: The cod (Gadus morhua) stock in the eastern Baltic Sea is in a poor state and lacks signs of recovery, despite historically low fishing pressure in recent years. Poor body condition is a major feature of these cod, but the reasons for this are still unclear. The present study investigates whether infection pressure from parasites could be a cause of poor body condition. Three types of parasites were analyzed: the nematode Contracaecum osculatum, which infects the cod liver; the microsporidian parasite Loma morhua, which infects the gills and other organs; and acanthocephalans, which infect the cod intestines. Earlier analyses suggested that the water depth at which cod forages might influence parasite infestation rates. Our hypothesis is that the prevalence, as well as the intensity of infection, differs with depth stratum. In this study, 125 cod (August 2023) were analyzed in both the Bornholm Basin and the Gdansk Deep for differences between infestation rates in different depth strata. The samples were caught with bottom trawls at two depths in those two areas (approximately 43m and 67m water depth). Biological parameters were taken from each cod, and numbers of nematodes on the liver, of Loma morhua on the gills, and acanthocephala in the intestines were counted, and stomach contents were analyzed. The two sampling depths had no influence on any of the factors tested, and the data were pooled. Total length and age of cod ranged between 21 – 50 cm (average ± standard error: 34.5 ± 0.7) and 1 – 5 years (average ± standard error: 3.1 ± 0.1), respectively. The body condition of cod was relatively low (mean: 0.79 ± 0.01). All the cod examined in this study were infected with at least one of the three parasites. The prevalence and mean intensity of infection of Loma morhua as well as acanthocephala in cod examined were generally higher than Contracaecum osculatum. Cod were mainly feeding on crustaceans and less on fish. The stomach fullness index was low (mean: 0.61% in a 31-40 cm cod). Adverse changes in environmental conditions like trophic lengthening and habitat loss due to anoxia and hypoxia may have led to a lack of food supply, impairing the food web structure. High parasite loads seem to be a sign of poor health of the Baltic cod stock rather than the key reason for the poor body condition of the fish.

Defense schedule for this defense season: