Belgium

Emily Lechner1* , Mafalda Tomas1 , Leon Pfeufer1 , and Fredrik Jutfelt1 
1  Department of Biological and Environmental Sciences, Faculty of Science, University of Gothenburg, 41390, Gothenburg, Sweden
* Author for correspondence: emily.lechner@bioenv.gu.se
A warming climate can have consequences on the performance of ectotherms through increases in metabolic demand. Understanding how variation in water oxygen saturation and supra-optimal temperatures affect the vulnerability of fishes to heatwaves and hypoxic zones is imperative to predicting anthropogenic impacts on wild organisms. However, we do not yet have a full understanding of how water oxygen saturation influences aerobic capacity, and how this may be temperature dependent.
This study investigates the constraints imposed by warming and hypoxia on aerobic performance in two model species with distinct thermal niches: the cold-adapted rainbow trout (Oncorhynchus mykiss) and the warm-adapted zebrafish (Danio rerio). Here, we present swim performance and aerobic scope profiles of these two species in response to variable oxygen saturation and temperature regimes. We used racetrack swim-tunnel respirometry to measure swim performance and maximum metabolic rates along a gradient of air saturation (from 25% to 200%). Fish will be acclimated to three temperatures that are ecologically relevant to their species (sub-optimal, optimal, supra-optimal). The results will be discussed in relation to the mechanisms limiting aerobic performance in fish and will address gaps in knowledge that relate to oxygen dynamics and thermal resilience of aquatic ectotherms. This work is essential for improving projections of aquatic biodiversity under future climatic scenarios.

SEB Conference 2025

Exploring Fish Aerobic Scope in a Changing Climate 

swim tunnel

sub-optimal

supra-optimal

hyperoxia

hypoxia

zebrafish

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poster

Marine heatwaves (MHWs) are becoming more frequent, intense, and prolonged with ongoing global changes. This study aimed to evaluate the impacts of sex-specific exposure to MHWs on the reproduction and energy metabolism of the amphipod Gammarus locusta. Females (F) and males (M) were sampled at a coastal lagoon and kept individually during a 7-day period under a control (20 °C) or a MHW (25 °C) scenario. After 7 days, females and males were sampled for mitochondrial and digestive enzymes activities, while the remaining specimens were used to form breeding pairs in different combinations (FControl /MControl , FControl /MMHW , FMHW /MControl , and FMHW /MMHW ). These breeding pairs were kept at control temperature (20 °C) until either reproducing or dying. Survival, reproductive success, fecundity and offspring body-sizes were subsequently determined. Reproductive success and fecundity were extremely reduced when both adults were exposed to MHWs, when compared to breeding pairs with both adults coming from the control treatment. Significant differences were also observed in the sizes of neonates, with offspring from control couples displaying smaller sizes, when compared to those from breeding pairs in which one of the adults was exposed to MHWs. We predict that MHWs will induce sex-specific enzymatic responses, such as differential energy allocation via changes in mitochondrial enzyme activity and the disruption of the endocrine system-related enzymes. MHWs imposed significant metabolic challenges on G. locusta, leading to sex-specific responses. Our findings highlight the need to consider sex as a biological variable in global change research to better predict species resilience and population stability.

When the Heat Rises: Sex-Specific Effects of Marine Heatwaves on Reproductive Success and Energy Metabolism in the Amphipod Gammarus locusta

In a changing climate, organisms are experiencing increasing thermal stochasticity. However, the role of the unpredictability of conditions on the biological consequences of thermal variability remains poorly explored. In particular, the sensitivity of metabolic performance to temperature could have large consequences from individual fitness to ecological communities via its influence of resource intake. In this context, we investigated the effect of ecologically relevant patterns of stochastic and cyclic thermal variations on the energetic budget of a pelagic freshwater fish, the European chub (Squalius cephalus). Fish (n=131) were exposed to constant temperatures regimes (13°C, 19°C or 25°C) or experienced profiles with variable temperatures sharing the same mean (19°C) and variance. Variable groups underwent either stochastic variations randomly oscillating between 13°C and 25°C, or cyclic variations following a circadian pattern. Both energy expenditure through whole-organism oxygen consumption and energy intake with predation rate were assessed on the same fish after one and six weeks of exposure to thermal treatments. After one week of exposure, preliminary results show that metabolic responses are driven by the mean temperature experienced, conversely to predation rates that are even sensitive to the pattern of variations. These results point out the necessity of accounting for thermal variability in experimental contexts and shed the light on our upcoming results investigating the plasticity of responses after a prolonged exposure. Additionally, such findings raise questions about a potential shift of thermal optima across biological traits and therefore highlight the relevance of assessing multiscale effects of environmental variability in studying climate change.

Does predictability matter? Effect of stochastic and cyclic thermal variations on energetic budget in a freshwater fish

Coastal habitats are among the most productive ecosystems on Earth, acting as biodiversity hotspots and essential nursery grounds for marine life. In Scandinavia, seagrass Zostera marina habitats provide food and shelter for many fishes. However, these meadows are typically confined to shallow areas due to water turbidity and eutrophication. As global warming intensifies the severity, frequency, and duration of heat waves, eelgrass-associated fishes are increasingly at risk of thermal stress. Yet, the extent of species-specific thermal limits and their plasticity for acclimation remains poorly understood. To address this gap, we investigated the effects of warming on wild eelgrass-associated fish during summer, when their thermal history is highest and temperature extremes are most impactful. We exposed twelve species from six families to a control temperature (19°C) or a simulated heatwave (23°C) for five days. We then studied their thermal tolerance using the Critical Thermal Maximum (CTmax) test and measured the temperature at loss of equilibrium. Our results showed differences in upper thermal limits across species, partly influenced by phylogenetic history. Most species exhibited an acclimation response to warming, reflected by an increase in CTmax, though some species acclimated faster than others. Our findings emphasize the importance of considering species-specific thermal responses when assessing the ecological impacts of heatwaves on coastal fish communities.

Capacity for rapid thermal acclimation differs across eelgrass-associated fishes

Pacific salmonids are anadromous and generally migrate to the Bering Sea in summer to forage. Chum salmon (Oncorhynchus keta), one of the most abundant Pacific salmonids in the summer Bering Sea, spends a significant portion of its life at sea. However, due to its migratory nature, little is known about its foraging migration ecology. This study investigated the open-ocean migratory behavior of chum salmon using satellite pop-up archival tags (PSATs). A chum salmon, captured in the central Bering Sea (56.0°N, 175.0°W) on 30th July 2024, was tagged with a PSAT (miniPAT-348) and released. The PSAT surfaced as scheduled 75 days later in the North Pacific Ocean near the waters off Amchitka Island, Aleutians (50.8°N, 179.3°E). Over the next seven days, the PSAT transmitted depth, water temperature, and activity data, with 50.4% of the recorded data successfully retrieved. From 60 days after release until three days before surfacing, the PSAT recorded sea surface temperatures above 20 °C, suggesting predation by an endothermic fish. The estimated migration path, excluding the predation period, indicated a generally southward movement from the release site but remained in the Bering Sea. The salmon primarily used the surface (above 20 m) but occasionally dived below the thermocline (below 40 m). The fish did not exhibit diel vertical movement; however, it tended to increased activity around sunrise and sunset. This study provides insights for the open-ocean migratory behaviour of chum salmon but further study is required to explore this in more detail.

Ocean Migration Behaviour of Chum Salmon in the Summer Bering Sea Using Pop-up Satellite Archival Tag　

Environmental temperature is a key determinant of fitness in insects, influencing survival, reproduction, and behaviour. With climate change increasing the frequency and intensity of extreme thermal events, it is crucial to understand the adaptational capacity of insects to this new thermal reality. While the impacts of acute thermal stress on physiological traits are well-documented, less is known about the developmental effects of such extreme events, particularly regarding environmental buffering mechanisms like parental care behaviours. In this study, we investigated how exposure to a heatwave during the larval stage ofNicrophorus vespilloidesinfluenced reproductive success and parental care in adulthood. Larvae were exposed to either a heatwave (26°C for 72 hours) or kept at control conditions (20°C) and assessed for reproductive and behavioural outcomes in the adult focal individuals and their offspring. We found that heatwave exposure significantly reduced survival to eclosion in the focal larvae. However, there was no significant impact of the heatwave on either the parental behaviour or the duration of care exhibited. Similarly, no developmental effects of thermal stress were observed on adult reproductive traits or for offspring fitness. These results suggest that extreme thermal events early in life do not necessarily have long-term detrimental effects, contrary to recent findings on ontogenetic sensitivity in insects. This highlights the complex interactions between temperature and insects’ life history and underscores the need for further research to more accurately assess resilience in a rapidly changing world.

Exploring developmental effects of acute thermal stress on reproductive and parental care abilities in burying beetles

Light pollution is the most common and fastest-growing form of urban pollution worldwide. While urbanization is recognized as one of the primary drivers of the global decline of amphibians, little attention has been paid to the impact of artificial light at night (ALAN) on this particularly threatened group of vertebrates. In this study, we investigate the direct effects of two types of LED lights (5 lux warm-2700K and 5 lux cold-6000K) on the metabolic rate of larvae of the crested newt species,Triturus ivanburschi. We also examined the delayed effects of ALAN. Metabolic activity was investigated in juvenile individuals three weeks after their development under ALAN. The oxygen uptake of individuals at rest was measured as a proxy for the standard metabolic rate at both life stages. Our findings showed that larvae developed under both cold and warm LED lights had higher metabolic rates compared to the control group. However, in juveniles, alterations were observed only in individuals that have been exposed to cold LED lights; these individuals exhibited lower metabolic activity than both the control and warm LED groups. Overall, the results suggest that ALAN affects the metabolic activity of crested newt larvae, with possible delayed effects in subsequent life stages. These changes may have long-term negative impacts on the fitness of newt populations, which is crucial for biodiversity conservation in a world plagued by light pollution.

How exposure to artificial light at night affects the metabolic rate of crested newts during early life stages

Artificial light at night (ALAN) is a widespread pollutant that impacts wildlife at various levels. However, our understanding of the effects of ALAN on amphibians, particularly tailed amphibians, is limited. In this study, we examined how ALAN affects the development, growth rate, and activity of larvae from two species of crested newts:Triturus macedonicusandT. ivanbureschi, as well as their hybrids. Given that responses to ALAN can vary based on light intensity, spectral composition, and duration of exposure, we compared the effects of warm (2700K) LED light and cold (6000K) LED light. Larvae were exposed to two light intensities (5 lux and 30 lux) from hatching until they reached developmental stage 62. Our results indicated that while both types of light can alter development, body size, and movement activity, cold LED light at higher intensity resulted in more significant effects. Comparing the species, we found thatT. ivanbureschiand theT. macedonicus-mothered hybrids showed a greater response to ALAN. Specifically, these larvae experienced prolonged development, lower growth rates, and reduced activity when exposed to 30 lux of cold light. This type of light is generally considered to be more harmful to organisms. Our research highlights the complex effects of ALAN on amphibians and emphasizes the difficulty in generalizing its impact across different species.

Effects of two types of LED lights at night on the development, body growth, and behavior of larvae of two crested newts species and their hybrids

As ectothermic animals, fish are particularly vulnerable to the impacts of rising temperatures, with many populations approaching their thermal limits. A holistic understanding of the vulnerability of fish populations is thus vital to mitigate environmental and societal impacts under climate warming. While early life stages have been suggested to be more vulnerable to thermal stress than juveniles and adults, existing methods for measuring heat tolerance are inconsistent between fish life stages. These methodological inconsistencies have fuelled debates about the increased sensitivity of embryos to thermal stress and have limited our ability to predict the impacts of climate change on fish populations. Fortunately, alternative approaches to measuring thermal tolerance now allows us to standardise assessments across life stages. I propose to conduct a distributed experiment—uniting researchers across the world to collect data on embryonic thermal tolerance using standardised methods. These experiments will be simple, replicated across various species and regions, and all contributors will be co-authors on the resulting publication. By identifying the developmental stage most sensitive to heat stress, this project will yield crucial knowledge to predict the impacts of climate change on fish populations.

A call for distributed experiments on fish embryonic thermal tolerance

Koala populations in Australia have declined by 54% in the last three decades. Millions of dollars are spent annually on protecting koalas, focussing on safeguarding habitat and treating disease. Yet, two thirds of all koala deaths occur when they are moving along the ground, due to vehicle strikes and dog attacks. Despite decades of research, little is known about how koalas move on the ground. Traditional GPS tracking provides limited insight into these movements, as they generally record locations only once or twice a day. To address this gap, we use accelerometers—bio-loggers that record three-dimensional movement at hundreds of points per second—to determine the orientation and movement of koalas during specific behaviours. We collared 10 koalas in the wild with accelerometers and high-resolution GPS devices for an average of 8 days. From these data, we quantified (1) the frequency and duration of ground visits, (2) the speed and distance moved, and (3) the locomotion style used on the ground (e.g., walking, bounding). We found that koalas visited the ground three times per day on average, totalling only 45 minutes. Koalas travelled on average 260 metres per ground visit, mainly walking at 1.7 km/h with occasional bounding at 10.4 km/h. Combining accelerometer and GPS data, we identified the specific trees koalas moved between. This is the first comprehensive analysis of koala movement in the wild, revealing new insights into their ground behaviour. Understanding these movement patterns is crucial for protecting koalas where they are most vulnerable.

Monitoring the 'Mundane': What Do Koalas Do When They're NOT Sleeping?

The king penguin is one of the champion avian divers, surpassed only by its larger relative, the emperor penguin. The foraging ecology of king penguins is typically studied during the austral summer, when foraging trips are relatively short (weeks). Consequently, little is known about the fall/winter period, when birds undertake long foraging trips (months) and the descent of their preferred prey to greater depth might challenge their dive capacity. Investigating the dive behaviour of 7 king penguins from the Crozet Islands during fall/winter, we found that birds conducted deeper and longer dives during this period. Most birds exceeded the previous depth and duration records but the deepest (424.5 m) and longest (10 min) dives were conducted by the same individual. These dives are the deepest and longest yet reported for king penguins and illustrate their ability to respond to seasonal changes by enhancing their dive capacity, likely through gradual acclimatization.

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When the Heat Rises: Sex-Specific Effects of Marine Heatwaves on Reproductive Success and Energy Metabolism in the Amphipod Gammarus locusta

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