Master's opportunities at Marine Developmental Biology

Professor: Sigurd Stefansson

Associate Professor: Tom Ole Nilsen

Associate Professor: Tom Ole Nilsen

Postdoctoral Fellow: Thi Hong Gam Le

Professor: Jon Vidar Helvik

Researcher: Mariann Eilertsen

We offer Master’s thesis projects within the research area of appetite control, digestion, and growth physiology in teleost fish, with a focus on Atlantic salmon and rainbow trout. Our research explores the complex regulation of feeding behavior, energy balance, and growth. We investigate physiological and molecular responses associated with growth under varying environmental conditions, such as temperature, dissolved oxygen levels, stress, and different feeding strategies—including fasting and compensatory growth. Some theses may focus more specifically on central and peripheral appetite regulation, particularly neuroendocrine control mechanisms in the brain, as well as the gut-brain axis and its role in digestion, gut transit, and nutrient sensing.

Experimental work is carried out in controlled environments, including flow-through systems, recirculating aquaculture systems (RAS), and open-sea cages, depending on current activities. Many projects are conducted in collaboration with academic and industrial partners based in the Bergen region, elsewhere in Norway, and internationally.

These thesis opportunities are ideal for students interested in integrative fish physiology, aquaculture, and applied biological research with real-world commercial relevance.

Professor: Ivar Rønnestad

Researcher: Floriana Lai

Professor: Sigurd Handeland

Researcher: Pablo Balseiro Vigo

Researcher: Enrique Pino Martinez

Researcher: Floriana Lai

Sebastian Corneliussen

This study examined the effects of long-term hypoxia on feed intake, growth, and appetite regulation in Atlantic salmon, focusing on the role of ghrelin and hypothalamic neuropeptides. Despite the overall reduced feed intake and growth under low oxygen conditions, there were no changes in ghrelin or neuropeptide expression, suggesting other cues may drive long-term appetite regulation during hypoxia. After a meal there was an increased expression of ghrelin and orexigenic neuropeptides in the hypothalamus, indicating a rapid central response to appetite signals. Overall, the findings suggest that ghrelin may not directly influence central appetite pathways under chronic hypoxia, but its role may be more prominent in short-term regulation.

Supervisors: Ivar Rønnestad, Floriana Lai, Nina Liland

Emma Louise Jenkins Slutås

Nephrocalcinosis, NC, a condition characterized by mineral deposits in the kidney, is reportedly the number one concern among Rainbow trout (Oncorhynchus mykiss) smolt producers. This study investigated the physiological responses of Rainbow trout to repeated fluctuations in CO₂ (25mg/L) and O₂ (105%) levels, mimicking conditions commonly observed in commercial land-based facilities. The first trial aimed to simulate repeated drops in water CO₂ (and O2) after a prolonged exposure (12 days), and the second trial assessed fish diagnosed with moderate, severe or no degrees (control) of NC. Repeated CO₂ fluctuations disrupted acid-base balance, leading to elevated plasma HCO₃⁻ and altered ion concentrations. NKA activity increased significantly in the kidney and intestine, indicating compensatory upregulation, while gill NKA activity remained largely unchanged. Fish diagnosed with severe NC exhibited clear signs of kidney failure, including elevated urine pH, HCO₃⁻, and PO₄³⁻, as well as reduced NKA activity in both the kidney and gills. These findings suggest that chronic fluctuating CO₂ exposure may overwhelm physiological compensation and kidney function in NC inflicted fish do not recover.

Supervisors: Tom Ole Nilsen, Marius Takvam, Elsa Denker, Thi Hong Gam Le

Elen Marie Garshol

This thesis investigated the influence of five photoperiod protocols on the development of Nephrocalcinosis (NC), plasma parameters and overall health in large Atlantic salmon smolt in a commercial RAS facility. Long-term effect of these protocols was assessed 6 months after transfer to seawater cages. Smolts and post-smolts held under constant light exhibited higher incidences and severity of NC than fish experiencing photoperiod regimes. From a diagnostic point of view, macro scoring of NC was found to be less reliable than micro scoring by histopathology for mild cases of NC. Alterations in plasma Ca2+, Mg2+, and PO₄³⁻ levels showed strong associations with the observed kidney pathology. Liver enzyme levels and metabolic parameters were altered, suggesting a more extensive systemic effect of NC on fish health. Overall, this thesis contributes to a deeper understanding of NC pathogenesis and highlight the significance of well-managed photoperiod regimes in RAS to reduce the risk of NC development.

Supervisors: Tom Ole Nilsen, Marius Takvam, Trond Waldemar Trosten

Eivind Engan

This thesis investigated the physiological responses of Atlantic salmon exposed to two cyles of fluctuations in CO₂ (30mg/L) and O₂ (120%) levels, mimicking conditions commonly observed in commercial land-based facilities. By the termination of the project, both the CO2 group and CO2 + O2 group experienced a growth penalty compared to the controls. Blood and urine pH, bicarbonate, and TCO2 were elevated compared to the control group in both experimental groups during hypercapnia/hyperoxia and 3 hours after the drop in CO2 and O2. This effect can be attributed to respiratory acidosis and subsequent pH buffering by raising plasma bicarbonate levels. This increase in pH in the blood and urine creates favorable conditions for the precipitation of minerals (calcium, magnesium, inorganic phosphate) in the kidney tubules. Repeated CO₂ fluctuations disrupted acid-base balance, leading to elevated plasma HCO₃⁻, increased urine pH and altered ion concentrations in both plasma and urine. This may reflect increased precipitation in primary urine and prevalence of nephrocalcisnosis.

Supervisors: Tom Ole Nilsen, Thi Hong Gam Le, Marius Takvam

Hedda Wahl-Ovesen

This study aimed to evaluate gas bubbles in fins and angiotensin-converting enzyme (ACE) activity as diagnostic tools for detecting gas bubble disease (GBD) in Atlantic salmon under simulated well-boat conditions. Exposure to reduced pressure (0.4 ata) induced GBD symptoms, including abnormal behavior, intravascular gas bubbles, and in some cases, mortality, particularly after exposures longer than 60 minutes. Ultrasound and macroscopic scoring of gas bubbles in the left pectoral fin were effective for detecting GBD shortly after exposure, while elevated pressure (2.6 ata) did not induce symptoms under the tested conditions. ACE activity in the third gill arch showed high individual variability and did not correlate with GBD markers, making it unsuitable as a diagnostic tool in this context.

Supervisors: Ivar Rønnestad, Silje Stensy-Skjærvik

Helene Bjørnevik Bjelland

This study investigates the effect of two different fish size groups, small (S – 60 g) and large (B – 300 g) size at the initiation of a winter signal (WS) when applying a classic square-wave photoperiod protocol. Both size groups also had a continuous light (LL) control group. Two different size groups, small (S – 60 g) and large (B – 300 g), were maintained under LL until the desired size was reached. Then, half of each group was either kept on LL as a control group (S-LL and B-LL), while the remaining fish were exposed to a 6 weeks WS before returning to LL for 570-degree days (dd) (S-LD and B-LD). The groups maintained under LL exhibited better growth in FW than those exposed to a WS, and S groups showed better growth than B groups. Characteristic signs of smoltification in the S-LD group were observed, with a significant increase in gill NKA activity after return to LL. Concurrently, the SLL group displayed constant elevated gill NKA activity as expected for large juvenile salmon kept under LL. However, smoltification related changes in gill NKA-activity occurred in the B-LD and B-LL groups. These results suggest that large juvenile Atlantic salmon (~300 g) exposed to a 6-week WS does not exhibit a classic smoltification response following the onset of a spring signal.

Supervisors: Tom Ole Nilsen, Marius Takvam

Frida Aalerud

Earth’s orbit around the sun generates highly anticipated annual rhythms of sunlight, posing a selection pressure for organisms to interpret and use daylength for critical life history transitions. Atlantic salmon (Salmo salar) is a strictly photoperiodic species that for example undergoes an extensive transformation from freshwater to saltwater, smoltification, involving critical changes in morphology, physiology, and behaviour. However, the mechanisms behind photoperiodic regulation in Atlantic salmon are poorly understood, although it is known that the salmon brain is directly light-responsive due to deep brain photoreceptors. The processes by which photoperiodic signals are perceived, transmitted, and converted into endocrine signals that trigger smoltification remain unresolved, but the salmonid paralogue tshβb is believed to play a key role in smoltification. This study use in situ hybridization to characterize the expression pattern of clock genes and clock-controlled genes, in the search for specialized calendar cells in the brain of Atlantic salmon.

Supervisors: Jon Vidar Helvik, Mariann Eilertsen

Christina Maria Ingdal

This thesis explores the influence of temperature and salinity on the parr-smolt transformation (PST) and subsequent seawater (SW) performance in juvenile Atlantic salmon (Salmo salar L.) reared in Flow-Through (FTS) and Recirculating Aquaculture (RAS) Systems. The effect of various temperature regimes (8 °C, 12 °C, 14 °C, and 8 °C to 12 °C) during the freshwater (FW) phase, as well as two different salinity regimes after completion of PST: 1) direct transfer to 32 ppt for approximately 21 days and 2) a gradual increase to 32 ppt over the same period were investigated. The findings revealed that higher rearing temperatures (12 °C and 14 °C) enhanced growth, smolt index, and kidney Na+, K+ ATPase (NKA) enzyme activity. Notably, smolts raised in RAS displayed greater variability in NKA activity compared to the more stable development observed in RAS. Interestingly, a delayed response in kidney NKA activity compared to the gills indicated that preparatory changes in the kidney lags behind gill responses. Additionally, gradual salinity increases prior to seawater open net-pen transfer improved welfare outcomes and reduced mortality rates, particularly in RAS, where such adjustments appear important for mitigating stress and enhancing survival. Collectivly, these findings highlight critical differences between FTS and RAS, underscoring the need for tailored management strategies in different rearing systems to optimize smolt resilience and performance in SW.

Supervisors: Tom Ole Nilsen, Marius Takvam

Kasper Sætre Sollien

This thesis investigated the appetite response and growth performance of grow-out Atlantic halibut fed 12 different diets with varying macronutrient compositions over 60 weeks. Results showed that lower protein diets (down to 45%) combined with elevated lipid (up to 25%) and carbohydrate (up to 25%) levels supported good growth, although appetite declined at the highest lipid levels. A reduction in growth was observed when dietary lipid exceeded 25%, confirming an upper tolerable limit, while increased carbohydrates improved appetite, contrary to previous findings. These results suggest that optimized, lower-protein feeds can reduce costs and improve feed efficiency in halibut aquaculture, while also highlighting the need for further research into the appetite-modulating roles of lipids and carbohydrates

Supervisors: Ivar Rønnestad, Endre Lygre, Ana Gomes

Ida Kristine Hundven

This study investigated how temperature influences feed intake, gut transit, and ghrelin-mediated appetite regulation in Atlantic salmon post-smolts reared at 8°C, 12°C, or 15°C over two months. Warmer temperatures led to increased feed intake, faster gut transit, and higher growth, though unexpected early maturation in some 15°C males negatively affected growth performance. Despite temperature-dependent differences in ghrelin gene expression and plasma levels, no correlation was found between stomach fullness, ghrelin expression, or circulating ghrelin. These findings suggest that ghrelin may not serve a primary orexigenic role in Atlantic salmon under varying thermal conditions.

Supervisors: Ivar Rønnestad, Floriana Lai

Ragnhild Beck Hestness

Living organisms have daily rhythms in behavior, physiology and gene expression driven by the need to adapt to environmental changes. Correct timing of physiological processes is crucial for relative fitness and the circadian rhythms are generated by endogenous clocks that synchronize biological processes to the 24h light/dark cycle. In mammals, circadian rhythms are synchronized by a “master clock” localized in the suprachiasmatic nucleus in the hypothalamus. Fish lack this central circadian pacemaker and instead the circadian system is composed of a net of independent light-responsive circadian pacemakers (peripheral clock). The peripheral clocks are entrained by photoperiodic information from the retina and the pineal organ, in addition to possible deep-brain photoreceptors. The localization and regulation of these peripheral clocks in Atlantic salmon are not yet fully understood, and the role of nonvisual photoreception in the circadian and neuroendocrine systems in animals is an intriguing question of modern biology. This comparative study localized peripheral clocks in the Atlantic salmon brain, with special focus on the pituitary, by mapping and characterizing the expression of the circadian core clock genes.

Supervisors: Jon Vidar Helvik, Mariann Eilertsen

Hanna Fjørtoft Otterlei

The habenula is a brain region viewed as an important relay nucleus pivotal for controlling animal behaviour. Our understanding of the teleost habenula has increased over the past

decade, but the region holds many interesting characteristics that remain largely unexplored. This thesis aimed to bridge some of these knowledge gaps by mapping clock genes, deep brain photoreceptors, and serotonergic cells in the habenular nuclei of the Atlantic salmon (Salmo salar). Salmon is a suitable model organism due to its life cycle that is highly susceptible to seasonal variations, in addition to its exceptionally large component of gene duplicates due to whole genome duplication. Furthermore, contributing to the knowledge on salmon is of commercial value because of the species contribution to global economies through the aquaculture industry. Based on literature and a pilot study, the clock related genes of interest were per2a and per2b, and the opsins studied were opn7b, tmt3a, opn4x1b1/2, and VA opsin. Expression patterns of the opsins and clock genes were mapped using in situ hybridization, and serotonergic cells and neuronal projections were characterized by immunohistochemistry. For the first time, a detailed cellular expression pattern of per2a/b and tmt3a are described in Atlantic salmon. Interestingly, it was observed several brain regions where opsins and clock genes displayed similar expression patterns. Furthermore, a fascinating trait of the habenula is its asymmetric characteristics, a quality found in few brain regions. Collectively, this study presents a novel anatomical description of asymmetric traits of the habenula and the potential presence of lightsensitive peripheral clocks in the Atlantic salmon brain.

Supervisors: Jon Vidar Helvik, Mariann Eilertsen