Hypoxia effects on fish in west Norwegian fjords: harnessing the power of multidisciplinary studies

Loss of oxygen linked to ocean warming has become a large concern worldwide. Oxygen is a key element in metabolic processes in animals, and oxygen loss can lead to changed vertical migration behaviors and reduce habitat volumes, affect fish physiology, challenge the immune system, increase the fish’s susceptibility to pathogens, change trophic interactions and growth, and have consequences for recruitment, population dynamics and communities in deep sea ecosystems. This project (HypOnFjordFish) uses deep West Norwegian fjords as natural infrastructures to generate new knowledge on effects of oxygen loss on mesopelagic and demersal fish communities. Such information is needed for sustainable management of marine resources, and for determining optimal coastal and fjord sites for fish farming, representing one of Norway’s largest, most successful industries. The open ocean as well as coastal waters, along continental margins, enclosed seas and fjords have lost oxygen over the past 50 years. This includes many west Norwegian fjords, such as Masfjorden where warming of the Atlantic waters has led to increased time between basin water renewals. Little is known on how oxygen loss affects marine ecosystems. Studies are difficult in open oceanic ecosystems but is more readily conducted in enclosed systems. The semi-enclosed West Norwegian fjords are unique in this respect. Own data from an annual field course at BIO UiB (BIO325) and this project show that the basin water in Masfjorden lost >60% of its oxygen in less than 8 years, a change that is more pronounced than reported from marine ecosystems elsewhere in the world. This suggests that west Norwegian fjords can be mor vulnerable to climate change than previously thought. Since the project started in 2020, we have been on seven field campaigns (4 annual field courses for BIO325 & 3 project campaigns) where we have tracked distribution patterns of mesopelagic fish using acoustics, collected biological materials, water samples and measured light, temperature, oxygen and salinity in Masfjorden and 3 additional fjords. For Masfjorden these collections cover a time period prior to and during hypoxic conditions, but also just after basin water renewal which occurred late spring/early summer of 2021. All samples for all HypOnFjordFish work packages have been collected and data from 2011 – 2023 are synthesized into master files to allow for analyses of the changes in the communities and populations with time and response to oxygen status. Most biological materials have been or are under processing and are being explored by two PhD-students, one Postdoc and many MSci students (4 completed) and Bachelor students research projects (BIO299). We do not yet know how the populations respond to oxygen loss. Responses can be marginal in populations with prior experience from hypoxia and that are adapted to low oxygen, but in fish populations experiencing acute hypoxia for the first time, life history challenges are likely. The HypOnFjordFish- project took the initiative to organise the 1st Multidisciplinary Fjord Workshop held together with the CLIFORD-Bjerknes and FJO2RD projects at Solstrand in April 2023. 50 scientists from Norway and the Nordic countries participated.

This project will use West Norwegian fjords as natural infrastructure to generate new knowledge on effects of hypoxia on mesopelagic and demersal fish communities. Such information is needed for sustainable management of marine resources, and for determining optimal coastal and fjord sites for fish farms, representing one of Norway’s largest, most successful industries. Deoxygenation linked to ocean warming is a large concern worldwide. Loss of oxygen is pronounced in the open ocean, occurring in coastal waters, enclosed seas and fjords. This includes Masfjord, often visited by our research group. This fjord turned hypoxic in 2016. Little is known on how hypoxia affects marine ecosystems. Study is difficult in open oceanic ecosystems, but is more readily conducted in enclosed systems. West Norwegian fjords are unique for this. Their gradients of hypoxia are tractable for study and provide natural parallels with open ocean gradients. Each fjord contains an ecosystem with many physical and biological processes similar and relevant to those of large oceanic ecosystems. Our objective is to study effects of oxygen loss on vertical migration behaviors, fish physiology, behavioral responses to changes, growth, and consequences for populations and communities. We will also explore if patterns of trace-element deposition in otoliths from a long-lived fish could reflect previous hypoxic events. We will compare across fjords that differ in basin water oxygen levels, will use data and samples collected from two fjords (Masfjord and Lustrafjord) during field courses for MSc students, and supplement these with new field campaigns to additional fjords (Lysefjord and Osterfjord). We will also attempt to establish lab populations for two mesopelagic species, but we are aware of that this could be a challenge. For Masfjord, however, we have unique data from before and after the fjord turned hypoxic, enabling study of deoxygenation effects on a well-documented ecosystem.