The effects climate-related stressors on sleep and associated behavioural and physiological consequences in fish

The occurrence of sleep is widespread among animals, occurring in taxa as diverse as cnidarians, insects, reptiles, amphibians, fish, birds, and mammals (Rattenborg and Ungurean, 2023). Despite this taxonomic diversity, the neurophysiological properties of sleep are largely conserved across these groups (Allada and Siegel, 2008). Still, most of our knowledge of the sleep comes from work with endotherms, for which it is known that altered sleep patterns can have important sublethal effects on physiology and behaviour, including altered cognitive abilities, social behaviour, and metabolism (Rattenborg et al., 2022).

In contrast, we still know relatively little about the causes and consequences of sleep in ectotherms. Ectotherms are highly sensitive to perturbations to their external environment, especially shifts in temperature, and so anthropogenic sources of environmental stressors could affect their sleep differently than in endotherms. Fishes, in particular, are the most diverse vertebrate taxa, occupying key ecological roles in virtually every aquatic ecosystem on earth. While there is a large body of evidence for the occurrence of sleep in fish, we know virtually nothing about the factors that affect sleep in fish nor the consequences of altered sleep for their ecological interactions.

Numerous biotic and abiotic factors may affect sleep in fish either independently or synergistically. Temperature, for example, strongly affects nearly all aspects of physiology and behaviour in fish (Pilkouta et al. 2020, 2023a, 2023b) and yet its effects on sleep are unknown. For example, higher temperatures could decrease sleep duration or quality due to a lowered arousal threshold for activity, which could in turn affect behaviour the following day. Preliminary data from the host laboratory has also shown that larger social groups can have more waking episodes per night due to waking and activity in one individual causing waking in groupmates, an effect that could be exacerbated at higher temperatures. Alternatively, elevated temperature could increase spontaneous activity during the waking period, possibly leading to increased sleep later to recover from periods of high activity. This could have impacts on predation risk during sleep if fish are less responsive for longer periods. In addition, increased sleep brought on by increased temperature may reduce the time available for foraging, which could be important considering that warming will generally increase energy demand in fish.

Dissolved oxygen may also affect sleep in fish in ways that differ from terrestrial animals. Increasing temperatures are exacerbating the frequency and intensity of hypoxia in aquatic ecosystems, with numerous effects on fish physiology and behaviour. Reduced aerobic scope and spontaneous activity brought on by hypoxia could lessen sleep requirements, although increased environmental hypoxia during the night, caused by microbial respiration, could adversely affect sleep quality in fish, as has been observed in endothermic taxa in hypobaric chambers. Hypoxia also alters social behaviour in fish (Domenici et al. 2017), often by decreasing social cohesion, possibly causing indirect effects on sleep that are modulated by factors such as social group size and associated risk-perception.

Any reductions in sleep quality or duration caused by temperature or hypoxia may feedback to alter fish behaviour and physiology in ways that have not been examined. Changes in aerobic metabolism and hormonal signalling caused by reduced sleep could further affect tolerance to warming and reduced oxygen, or environmental preferences (Christensen et al. 2022), but there could also be important direct effects on behaviour. Decreased cognitive ability or learning could affect foraging activity or predator avoidance, while increased aggression (as has been noted to be caused by sleep reduction in mammals) could affect social behaviour and its associated costs and benefits (Ward and Webster 2016).

This project will use an experimental approach to test for the first time how the changes in temperature and oxygenation brought on by climate change will affect sleep in fish. Further, the project will examine how altered sleep quality affects fish physiology and behaviours associated with foraging, predator-avoidance, and sociality. Specifically, increased temperature and decreased oxygen availability may decrease sleep, causing negative effects on these aspects of behaviour that are yet to have been considered. This interdisciplinary project will thus combine behavioural ecology with ecophysiology, and will be the first to examine sleep in the context of ongoing environmental change in ectotherms.


To address these issues, the proposed studentship will initially involve experiments using social species of fish to:

(1) Examine how temperature and hypoxia affect sleep quality and duration, and whether these affects are correlated with spontaneous activity during the day and during waking periods during the night.
(2) Investigate how the effects of temperature and hypoxia on sleep interact with perceived environmental risk or the immediate social environment
(3) Test whether altered sleep duration or quality further affects metabolic rates, environmental preferences or tolerances, aerobic capacity, foraging activity, learning, risk-taking, and sociability/aggression.

Methods will be multidisciplinary, and will include the measurement of metabolic rates using intermittent-flow respirometry, and numerous behavioural assays including foraging tasks, cognitive assays (e.g. maze learning trials), sociability tests, and assessments or risk-taking behaviour and predator avoidance.

Project Timeline

Year 1

Literature review, training in animal handling and application for Home Office personal licence. Training in experimental techniques (intermittent flow respirometry, temperature choice chambers, behavioural assays). Experiments designed to test Q1.

Year 2

Further experimental work to test Q2-3; write up initial experiments for publication. Presentation of work at conferences.

Year 3

Work will depend on the outcome of these experiments and the interests of the student. Further write up of experiments for publication and conference presentations.

Year 3.5

Completion of writing up of experimental work and submission of thesis; further submission of papers for publication.

& Skills

This project will provide a broad training in behavioural, ecological, and physiological techniques. In addition to learning about experimental design and statistical analysis, the student will learn how to quantify behaviour (e.g. sociability, temperature preference through use of dynamic choice chambers), automated tracking of animal movements, and measure whole animal performance (e.g. minimal and maximum metabolic rate). Additional transferable skills wll be gained via postgrad training courses in statistics, communication, teaching and mentoring, and consideration of EDI. Training and supervision will also be provided in grant writing for additional funds, such as student awards from the Fisheries Society of the British Isles, ASAB, the Company of Biologists, and the Society for Experimental Biology.

References & further reading

Christensen, EAF, Berggsson, H, Andersen, LEJ, Steffensen, JF, Killen, SS. 2021. Shuttle-box systems for determining environmental preference and avoidance in aquatic animals. Conservation Physiology. 1:coab028.
Domenici, P., Steffensen, J. F., & Marras, S. (2017). The effect of hypoxia on fish schooling. Philosophical Transactions of the Royal Society B: Biological Sciences, 372(1727), 20160236.
Killen, S.S., Cortese, D., Cotgrove, L., Jolles, J.W., Munson, A., Ioannou, C. 2021. The potential for physiological performance curves to shape environmental effects on social behaviour. Frontiers in Physiology. 2:754719.
Pilakouta, N, O’Donnell, P, Crespel, A, Levet, M, Claireaux, M, Kristjánsson, BK, Skúlason, S, Lindström, J, Metcalfe, N, Killen, SS, Parsons, K. 2023a. Warmer environments can reduce sociability in an ectotherm. Global Change Biology. 29: 206-214.
Pilakouta, N, Humble, JL, Hill, IDC, Arthur, J, Costa, APB, Smith, BA, Kristjánsson, BK, Skúlason, S, Killen, SS, Lindström, J, Metcalfe, NB, Parsons, KJ. 2023b. Testing the predictability of morphological evolution in contrasting thermal environments. Evolution. 77:239–253.
Pilakouta, N., Killen, S.S., Kristjánsson, B., Skulason, S., Lindstrom, J., Metcalfe, N.B., Parsons, K. 2020. Multigenerational exposure to elevated temperatures leads to a reduction in standard metabolic rate in the wild. Functional Ecology. 34:1205–1214.
Rattenborg, N. C., & Ungurean, G. (2023). The evolution and diversification of sleep. Trends in Ecology & Evolution.
Rattenborg, N. C., Lesku, J. A., & Libourel, P. A. (2022). Sleep in nonmammalian vertebrates. Principles and practice of sleep medicine, 1, 106-120.
Ward, A., & Webster, M. (2016). Sociality: the behaviour of group-living animals (Vol. 407). Berlin, Germany: Springer.

Apply Now