Polar cod (Boreogadus saida) is considered a key species in the Arctic marine ecosystems. Yet detailed or even basic knowledge regarding its biology and adaptations, especially during the polar night, is in many cases poor. Data are presently unavailable in Western literature on the gonad development of polar cod and its reproductive biology in wild specimens. Accordingly, gonad

Climate change and pollution resulting from human activities in the Arctic require reliable monitoring systems and these are usually based on biomarkers in sentinel species.Mytilus spp. are used as sentinel species all around the world. The use of Mytilus spp. in environmental monitoring requires knowledge of their biology and natural variations in biomarkers. Seasonal variations in baseline levels of pollution biomarkers were studied over a year in the mussels from upper and lower littoral zone in Rakkfjorden, Norway, as they underwent their annual reproductive cycle. Spatial variations of these baseline biomarker levels were measured in five mussel populations within a 60-km radius from Rakkfjorden to investigate universality of the results from the specific population of Rakkfjorden at a regional scale. Seasonal variations in biomarker levels were revealed and seemed to be related to the reproductive state of the mussels and the tidal zone. The mussels appeared to be more sensitive to oxidative stress during gametogenesis in autumn and winter, when having lower lysosome membrane stability and lower baseline levels of antioxidant biomarkers. An increase in baseline levels of these biomarkers was reported during spawning in spring, however, it was not possible to reveal whether these changes were due to spawning, or to a higher metabolic activity in mussels in response to elevated water temperature and food abundance. The differences between the tidal zones reflected in reduced size of the mussels from the upper littoral zone, their late spawning in the season and increased baseline levels of antioxidant biomarkers during the coldest month, indicating a more challenging environment in the upper littoral zone. The spatial study indicated that the baseline biomarker levels measured in Rakkfjorden were no different from the levels measured in the mussels from five other sites and thus, are representative for all mussels on a regional scale.

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