Eirik Haugstvedt Henriksen

Mercury (Hg) bioaccumulates in aquatic organisms and biomagnifies through food webs, potentially reaching concentrations that affect fish, wildlife and human health. Because Hg concentrations generally increase with trophic position, ontogenetic shifts towards piscivory can increase dietary Hg exposure as fish begin consuming prey with higher Hg concentrations than lower‐trophic invertebrates. Here, we investigated drivers of Hg accumulation in European perch ( Perca fluviatilis ) from two subarctic lakes by combining stomach‐content data, carbon and nitrogen stable isotopes, and total mercury (THg) concentrations. Fish length and trophic position (indicated by δ ¹⁵ N) were the strongest predictors of THg concentrations, and length‐based models performed substantially better than age‐based models. Increasing fish size was accompanied by an ontogenetic dietary shift from predominantly invertebrate prey towards piscivory. Mean THg concentrations were higher and the relationship between THg and δ ¹⁵ N was steeper in lake Skrukkebukta than in lake Vaggatem. Males had higher THg concentrations than females after accounting for other predictors, while body condition was negatively associated with THg. Overall, our results indicate that ontogenetic changes in trophic ecology are important drivers of Hg accumulation in perch, while sex, body condition and lake‐specific conditions contribute additional variation. The findings highlight the importance of considering species‐specific trophic ecology and life‐history variation, particularly differences in somatic growth, when assessing Hg dynamics and fish‐consumption risks in freshwater ecosystems.

Pink salmon ( Oncorhynchus gorbuscha ) has rapidly expanded throughout northern Europe, raising concern that continued warming may enable establishment even in high Arctic ecosystems. Svalbard has experienced increasing occurrences of pink salmon in recent years, yet it remains unclear whether these individuals reflect successful local reproduction or straying from mainland populations. We combined field surveys and population genomics to assess the origin and establishment potential of pink salmon on Svalbard. Five rivers around Isfjorden were surveyed in 2024 following observations of unusually high pink salmon abundance and suspected spawning in 2023. Despite extensive sampling of spawning substrates, no eggs or fry were detected. Because most Svalbard rivers freeze to the bed or dry out during winter, current environmental conditions are likely insufficient to support successful egg incubation and emergence . Genomic analyses of 24 adults captured in 2023, integrated with a 3RAD SNP reference dataset (6174 SNPs), revealed that Svalbard individuals retained genetic diversity within the observed range of the mainland populations but formed no distinct genetic cluster. Instead, they exhibited admixture and greatest affinity to sampling populations in the Varanger Fjord region, indicating recurrent long-distance dispersal from established invasion hotspots. Together, the absence of detected recruitment and the lack of a distinct Svalbard genetic cluster indicate that pink salmon in Svalbard currently represent a demographic and genetic sink sustained by recurrent immigration from mainland populations rather than a self-sustaining population. However, the genetic diversity of incoming strays suggests that, if future environmental change weakens current barriers to egg incubation and juvenile emergence, establishment may become increasingly possible.

Host density, host body size and ambient temperature have all been positively associated with increases in parasite infection. However, the relative importance of these factors in shaping long‐term parasite population dynamics in wild host populations is unknown due to the absence of long‐term studies. Here, we examine long‐term drivers of gill lice (Copepoda) infections in Arctic charr (Salmonidae) over 32 years. We predicted that host density and body size and water temperature would all positively affect parasite population size and population growth rate. Our results show that fish size was the main driver of gill lice infections in Arctic charr. In addition, Arctic charr became infected at smaller sizes and with more parasites in years of higher brown trout population size. Negative intraguild interactions between brown trout and Arctic charr appear to drive smaller Arctic charr to seek refuge in deeper areas of the lake, thus increasing infection risk. There was no effect of host density on the force of infection, and the relationship between Arctic charr density and parasite mean abundance was negative, possibly due to an encounter‐dilution effect. The population densities of host and parasite fluctuated independently of one another. Water temperature had negligible effects on the temporal dynamics of the gill lice population. Understanding long‐term drivers of parasite population dynamics is key for research and management. In fish farms, artificially high densities of hosts lead to vast increases in the transmission of parasitic copepods. However, in wild fish populations fluctuating at natural densities, the surface area available for copepodid attachment might be more important than the density of available hosts.

Competition is assumed to shape niche widths, affecting species survival and coexistence. Expectedly, high interspecific competition will reduce population niche widths, whereas high intraspecific competition will do the opposite. Here we test in situ how intra‐ and interspecific competition affects trophic resource use and the individual and population niche widths of two lacustrine fish species, Arctic charr and brown trout, covering a 40 year study period with highly contrasting competitive impacts prior to and following a large‐scale fish culling experiment. Initially, an overcrowded Arctic charr population dominated the study system, with brown trout being nearly absent. The culling experiment reduced the littoral Arctic charr density by 80%, whereupon brown trout gradually increased its density in the system. Thus, over the study period, the Arctic charr population went from high to low intraspecific competition, followed by increasing interspecific competition with brown trout. As hypothesized, the relaxed intraspecific competition following the experimental culling reduced individual diet specialization and compressed population niche width of Arctic charr. During the initial increase of the brown trout population, there was a large dietary overlap between the two species. Over the subsequent intensified interspecific competition from the population build‐up of brown trout, their trophic niche overlap chiefly declined due to a dietary shift of Arctic charr towards enhanced zooplankton consumption. Contrary to theoretical expectations, the individual and population niche widths of Arctic charr increased with intensified interspecific competition. In contrast, the diet and niche width of brown trout remained stable over time, confirming its competitive superiority. The large‐scale culling experiment and associated long‐term research revealed pronounced temporal dynamics in trophic niche and resource use of the inferior competitor, substantiating that intra‐ and interspecific competition have large and contrasting impacts on individual and population niches.

Free‐living parasite life stages may contribute substantially to ecosystem biomass and thus represent a significant source of energy flow when consumed by non‐host organisms. However, ambient temperature and the predator's own infection status may modulate consumption rates towards parasite prey. We investigated the combined effects of temperature and predator infection status on the consumer functional response of three‐spined sticklebacks towards the free‐living cercariae stages of two common freshwater trematode parasites (Plagiorchisspp.,Trichobilharzia franki). Our results revealed genera‐specific functional responses and consumption rates towards each parasite prey: Type II forPlagiorchisspp. and Type III forT.franki, with an overall higher consumption rate onT. franki. Elevated temperature (13°C) increased the consumption rate onPlagiorchisspp. prey for sticklebacks with mild cestode infections (<5% fish body weight) only. High consumption of cercarial prey by sticklebacks may impact parasite population dynamics by severely reducing or even functionally eliminating free‐living parasite life stages from the environment. This supports the potential role of fish as biocontrol agents for cercariae with similar dispersion strategies, in instances where functional response relationships have been established. Our study demonstrates how parasite consumption by non‐host organisms may be shaped by traits inherent to parasite transmission and dispersal, and emphasises the need to consider free‐living parasite life stages as integral energy resources in aquatic food webs.

Amphipods are often key species in aquatic food webs due to their functional roles in the ecosystem and as intermediate hosts for trophically transmitted parasites. Amphipods can also host many parasite species, yet few studies address the entire parasite community of a gammarid population, precluding a more dynamic understanding of the food web. We set out to identify and quantify the parasite community ofGammarus lacustristo understand the contributions of the amphipod and its parasites to the Takvatn food web. We identified seven parasite taxa: a direct life cycle gregarine,Rotundulasp., and larval stages of two digenean trematode genera, two cestodes, one nematode, and one acanthocephalan. The larval parasites use either birds or fishes as final hosts. Bird parasites predominated, with trematodePlagiorchissp. having the highest prevalence (69%) and mean abundance (2.7). Fish parasites were also common, including trematodesCrepidostomumspp., nematodeCystidicola farionis, and cestodeCyathocephalus truncatus(prevalences 13, 6, and 3%, respectively). Five parasites depend entirely onG. lacustristo complete their life cycle. At least 11.4% of the overall parasite diversity in the lake was dependent onG. lacustris, and 16% of the helminth diversity required or used the amphipod in their life cycles. These dependencies reveal that in addition to being a key prey item in subarctic lakes,G. lacustrisis also an important host for maintaining parasite diversity in such ecosystems.

Changes in abiotic and biotic factors between seasons in subarctic lake systems are often profound, potentially affecting the community structure and population dynamics of parasites over the annual cycle. However, few winter studies exist and interactions between fish hosts and their parasites are typically confined to snapshot studies restricted to the summer season whereas host‐parasite dynamics during the ice‐covered period rarely have been explored. The present study addresses seasonal patterns in the infections of intestinal parasites and their association with the diet of sympatric living Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) in Lake Takvatn, a subarctic lake in northern Norway. In total, 354 Arctic charr and 203 brown trout were sampled from the littoral habitat between June 2017 and May 2018. Six trophically transmitted intestinal parasite taxa were identified and quantified, and their seasonal variations were contrasted with dietary information from both stomachs and intestines of the fish. The winter period proved to be an important transmission window for parasites, with increased prevalence and intensity of amphipod‐transmitted parasites in Arctic charr and parasites transmitted through fish prey in brown trout. In Arctic charr, seasonal patterns in parasite infections resulted mainly from temporal changes in diet toward amphipods, whereas host body size and the utilization of fish prey were the main drivers in brown trout. The overall dynamics in the community structure of parasites chiefly mirrored the seasonal dietary shifts of their fish hosts.