Parasite communities in pink salmon (Oncorhynchus gorbuscha) along their invasion gradient in Norway – a cause for concern?

Abstract

Globally, non-indigenous species (NIS) are increasing in both number and distribution. Sometimes NIS are intentionally introduced to provide food or financial gain. If NIS are moved to a new environment without their competitors, predators and parasites they may gain an advantage over native species. This is explained by the enemy release hypothesis, which also includes parasites, as NIS might lose their parasites as they move further into the new environment. In the 1950s, anadromous Pacific pink salmon (Oncorhynchus gorbuscha, Walbaum, 1792) were introduced to the White Sea, western Russia, and have since established and expanded along the whole Norwegian coastline. Since pink salmon were introduced as eggs, it is not likely the fish brought their own parasites. Thus, pink salmon may get an advantage from parasite release. However, pink salmon might still acquire parasites in the new environments over time. To investigate this, I have compared pink salmon caught in Varangerfjord and Agdenes in a north-south gradient in Norway, respectively. I hypothesised that 1) the northernmost location would have a higher parasite diversity, taxa richness and abundance, 2) location would have a higher effect on parasite communities than fish size, and 3) the parasite community I found would differ from previous studies. To do this, sea-caught adult pink salmon from the Varangerfjord and Agdenes were dissected, and parasites in the stomach and the intestines were counted and identified. In total, 13 different parasite taxa were revealed. One of these taxa has never been observed in pink salmon earlier. The findings did not reveal any differences in diversity (Margalef’s index), taxa richness nor abundance between Varangerfjord and Agdenes, however, there was a higher dominance (Berger-Parker index) in Varangerfjord. Location exhibited a higher effect on dominance, whereas size had a higher effect on the diversity. Therefore, no clear pattern of enemy release was proven. Although it is difficult to pinpoint any clear reasons, the lack of significant effects by location and fish size indicates that other factors play a crucial role in parasite acquisition. When comparing the parasite community of this study’s pink salmon with the parasite communities in pink salmon of other studies, it was revealed that there was no full overlap of parasite taxa. Thus, the community composition differs, emphasizing the need for further research due to the complexity of host-parasite interactions.