Functional and Comparative Insights into the Vertical Movement Ecology of Fish with Special Reference to Zooplanktivores

 2025

Dr. Calvin Steven Beale

Abstract

“Vertical movement behaviours are a fundamental aspect of the ecology of marine megafauna, enabling resource acquisition, predator avoidance, and navigation in a dynamic three-dimensional environment. These behaviours are shaped by a complex interplay of ecological pressures, physiological constraints, and evolutionary history. This thesis examines the vertical movement ecology of marine fishes, with a particular focus on zooplanktivores, including oceanic manta rays Mobula birostris, reef manta rays Mobula alfredi, and whale sharks Rhincodon typus. By integrating high sampling-frequency depth time-series (DTS) data collected from satellite telemetry with advanced analytical methods, this research offers novel insights into the functional and comparative aspects of vertical movement behaviours.

The overarching aim of this thesis was to develop and apply quantitative frameworks to leverage the wealth of historical and new archival DTS data, facilitating both inter- and intra-specific comparisons of vertical movement behaviours. Advanced techniques such as wavelet analysis, machine learning, and hierarchical clustering were applied to explore how species partition vertical space, adapt to environmental conditions, and balance the energetic costs and benefits of different movement behaviours. This approach enabled detailed examination of behavioural plasticity, ecological specialisation, and evolutionary divergence among marine megaplanktivores.

First, I investigate deep diving in oceanic manta rays – a rare and poorly understood vertical movement behaviour. Using high sampling frequency DTS data, I characterise deep (≥200 m) and extreme dives (≥500 m), identifying distinct dive characteristics, including rapid descent velocities, horizontal steps, and the absence of bottom-phase foraging. I examine the potential role of horizontal steps in relation to dissolved oxygen and environmental gradients. I find that deep diving occurs primarily in offshore waters far from land, and is associated with subsequent long-distance movements, suggesting a navigational role rather than prey capture. These findings challenge previous assumptions that deep dives are foraging-related and highlight their potential function in environmental sensing and movement decision-making.

Second, I introduce FishDiveR, a quantitative framework for classifying vertical movement behaviours using DTS data. I demonstrate the power of continuous wavelet transformation for detecting dominant periodicities in movement patterns and integrate this transformation with principal component analysis and clustering methods to objectively classify behaviour. Using both simulated and empirical datasets, I validate the accuracy of this method in identifying behavioural states across multiple species. FishDiveR is broadly applicable across taxa and represents a significant advancement in processing large DTS datasets, offering new avenues for quantifying behavioural complexity in marine species.

Third, I apply FishDiveR to examine species-specific and habitat-driven patterns in vertical movement among sympatric planktivores, the oceanic manta ray, reef manta ray, and whale sharks. As expected, vertical movements were most similar between manta ray species due to their recent evolutionary divergence. I found that morphology and the phylogenetic history of the species influence vertical movement behaviours, while ecological pressures further modulate movement patterns. Although reef and oceanic manta rays demonstrated the most similar vertical movement behaviours, they appear to prefer different environments, with reef manta rays almost solely present in nearshore environments, and oceanic manta rays primarily in offshore environments. Whale sharks exhibited the greatest diversity in vertical movement patterns, with relatively slower, more prolonged, deep dives offshore, likely influenced by their large body size and thermal inertia. These findings suggest that coexistence among these planktivores is maintained by a balance between evolutionary constraints and ecological adaptability, allowing species to fine-tune their vertical movement niches within a shared ecosystem with overlapping resource use.

Finally, I discuss the broader implications of my findings, particularly the opportunities that FishDiveR presents for the behavioural ecology community. My thesis demonstrates how vertical movement behaviours mediate resource use, reduce interspecific competition, and contribute to the coexistence of sympatric species in complex marine ecosystems. It underscores the value of high sampling-frequency DTS data in uncovering subtle behavioural differences and the importance of integrating quantitative tools for processing and interpreting large datasets. By providing a unified framework for analysing vertical movements, my work advances our understanding of the ecological and evolutionary processes shaping marine megafauna.

My findings have broad implications for movement ecology, offering new perspectives on the adaptive behaviours of marine fishes and the environmental drivers of vertical habitat use. These insights are critical for informing conservation strategies in the face of anthropogenic pressures and environmental change. Collectively, this thesis contributes to the growing body of research bridging behavioural ecology and evolutionary biology, providing a foundation for future investigations into the vertical movement ecology of aquatic species.”