Effects of climate-related stressors and pollutants on social behaviour in gregarious fishes

Tiddy, Izzy Coire (2026) Effects of climate-related stressors and pollutants on social behaviour in gregarious fishes. PhD thesis, University of Glasgow.

Abstract

Anthropogenic activities have far-reaching impacts on natural systems and have documented effects on physiology and behaviour for aquatic animals. In fish species, gregarious behaviour in the form of shoaling or schooling is a key adaptive trait, with benefits including foraging and predator evasion. As anthropogenic impacts become increasingly ubiquitous, the effects of these stressors on adaptive traits such as social behaviour are of urgent research interest. Climate-related stressors, linked directly or indirectly to anthropogenic climate change, include high temperatures, hypoxia, and acidification. These may affect shoaling behaviour through effects on metabolic rate or neurological functioning. Alongside climate-related stressors, the incidence of anthropogenic pollutants in aquatic systems has increased markedly in recent years. Chemical pollutants include organic or inorganic substances and mixtures of these, which may have metabolic, endocrine, or neurological effects. Non-chemical pollutants such as light, noise, and turbidity can cause stress-related changes in behaviour, and interfere with detection of conspecifics and thus shoaling efficiency. As the occurrence of individual stressors becomes more frequent, stressor combinations are also increasingly observed and the potential effects of these on ecosystems are of emerging research interest. For gregarious species, the social context is an essential consideration for determining the ecological effects of anthropogenic stressors. While the primary physiological and behavioural impacts of stressors act at the level of the individual, their effects on ecological interactions often manifest at the level of the group. Understanding how stressor effects on individuals translate to impacts on shoaling dynamics is therefore essential to predicting ecological impacts in gregarious species. Consolidating the current level of knowledge in this field is a key first step in determining research gaps and opportunities for further study. In addition, despite the importance of social context, effects of anthropogenic stressors on fish social behaviour at the individual and group levels have rarely been empirically linked. The degree of prior research is also highly variable among stressors, with further study urgently needed on artificial light at night (ALAN) and stressor combinations.

In this thesis, I aimed to identify gaps in the literature surrounding the effects of anthropogenic stressors on fish social behaviour by first conducting systematic reviews of published studies. I then use lab-based observations to address the need for studies linking individual- and group-level behaviours in gregarious fish in the context of two climate-related stressors, temperature and hypoxia. Finally, I examine the effects of an understudied stressor, ALAN, on sociability and risk-taking behaviour in a gregarious reef fish.

In chapter 1, I introduce the impacts of different anthropogenic stressors on fish species. I also describe the relevance of social behaviour, and the importance of social context in gregarious species. I highlight key knowledge gaps, and challenges associated with studying the effects of anthropogenic stressors on social behaviour.

Chapter 2 consists of a meta-analysis conducted on studies of the effects of climate-related stressors on fish social behaviour. A systematic search revealed 45 studies of temperature, acidification, and/or hypoxia. My findings show that group cohesion is negatively correlated with oxygen concentration in fish species but did not reveal effects of temperature or acidification on social behaviour. Temperature was better-studied than other stressors, and studies of multiple stressors were few.

In chapter 3, I expand my review of the literature to studies of the effects of anthropogenic pollutants on fish social behaviour. A total of 165 studies were identified, with a large number of studies of organic pollutants (n = 92). Both organic and inorganic pollutants were associated with reduced sociability, while exposure to mixed pollutants led to increased sociability but reduced group cohesion. Among non-chemical pollutants, no effect of turbidity was found, but sociability decreased with light exposure. The effect of noise pollution was dependent on duration, as group cohesion decreased with longer exposure time.

In chapter 4, I sought to address a knowledge gap identified in chapter 2 by linking changes in individual behaviour across conditions of temperature with behaviour of those same individuals in a shoal. Using a model species, rummy nose tetra Hemigrammus rhodostomus, I carried out assays of sociability and risk-taking behaviour at three temperatures (24°C, 27°C, and 30°C) for each individual. Groups comprised of the same individuals used in sociability and risk-taking behaviour assays were then observed at the same three temperatures for shoaling behaviour. Both individuals and groups became more active at higher temperatures. Response to temperature in groups was also influenced by behaviour displayed in individual trials, however, with social, less-active fish becoming less active at high temperatures.

In chapter 5, I carried out a similar experiment to that described in chapter 4, to investigate the effects of hypoxia on links between individual and group behaviour in H. rhodostomus. Fish were assayed alone and in groups at three levels of oxygen saturation (100%, 70%, and 30%). Under hypoxia, fish generally became less active and inter-individual distances in shoals decreased. Individuals that displayed high risk-taking behaviour were more active in group behaviour trials under hypoxia, however, indicating that individual behavioural traits may modulate shoaling responses to abiotic stressors.

Based on the low number of studies of ALAN found in my review of the literature, chapter 6 describes an experiment investigating the effects of chronic exposure to ALAN in the wild on a gregarious reef fish, Dascyllus trimaculatus. Fish were subjected to a sociability test, and an emergence test from a shelter following a chase stimulus. No effect on sociability was found, but ALAN-exposed fish were faster to emerge from a shelter following a chase stimulus. This increase in risk-taking behaviour may indicate a response to greater metabolic needs, high stress levels, or selective mortality in ALAN-exposed fish.

In chapter 7, I review my key results and summarise the role of this thesis in advancing the study of fish social behaviour. I then discuss the implications of my findings and contextualise the results of my experimental chapters with the trends identified from reviewing the literature. Finally, I reflect on the limitations of my thesis and the opportunities for future work.

As a whole, my thesis provides a summary of work conducted to date and begins to explore new avenues and fill knowledge gaps in the study of anthropogenic impacts on fish social behaviour. Using a combination of literature-based and experimental methods, my work shows general trends in stressor effects and highlights the importance of social context and intra- and interindividual variation in determining outcomes. Overall, this thesis both consolidates existing understanding of anthropogenic effects on a key adaptive behaviour in fishes and paves the way for future work addressing gaps in this understanding.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Supported by funding from an IAPETUS2 Doctoral Training Partnership.
• Subjects: S Agriculture > SH Aquaculture. Fisheries. Angling
• Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Biodiversity, One Health & Veterinary Medicine
• Funder's Name: provided by the IAPETUS2 Doctoral Training Partnership
• Supervisor's Name: Killen, Professor Shaun, Torney, Professor Colin, Webster, Dr. Michael M. and Ioannou, Professor Christos C.
• Date of Award: 2026
• Depositing User: Theses Team
• Unique ID: glathesis:2026-86268
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 23 Oct 2026 10:15
• Last Modified: 26 Oct 2026 15:00
• Thesis DOI: 10.5525/gla.thesis.86268
• URI: https://theses.gla.ac.uk/id/eprint/86268

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