Investigation of coral trout (Plectropomus spp.) movement patterns and resource use: a multidisciplinary approach using acoustic telemetry and dietary indicators

Abstract

Understanding how co-occurring species within comparable trophic guilds (sympatry) partition resources provides fundamental information about their ecological roles within an ecosystem. Despite morphological and biological similarities, resources may be selected and exploited independently, leading to alternative interactions and influences within the ecosystem. Studying movement and dietary patterns directly relates to an animal's resource use, and is a valuable approach to characterise preferred prey and habitat within and between sympatric species. Expanding knowledge of resource use is essential to address how animals are affected by, and how they might respond to, an increasingly variable environment, and is necessary to implement ecosystem-based management practices. Coral trout or coralgrouper (Plectropomus spp.) are iconic and economically significant mesopredatory reef fishes within Australia's Great Barrier Reef (GBR) and throughout the Indo-Pacific region. Despite the importance of Plectropomus spp. in the Queensland Coral Reef Fin Fish Fishery, investigations focussed on their ecology are surprisingly limited. Much of the behavioural-based research has been conducted in scenarios of captivity, is biased by confounding sampling limitations, or only provides short-term, data-poor perspectives. Consequently, interpretations of findings are often applicable only to certain periods or locations, or are only based on patterns from a small number of individuals. This hinders the ability of managers to evaluate how fishing pressure, protection initiatives, and environmental fluctuations or disturbances might impact populations. Furthermore, research is overwhelmingly directed at P. leopardus (or grouped as Plectropomus spp.) which forms the majority of commercial catches on the GBR. Nevertheless, other species such as P. maculatus and P. laevis are readily captured by both recreational and commercial sectors, but their resource selection patterns and interactions with P. leopardus are unknown. The research in this thesis employed two methodological approaches – passive acoustic telemetry and stable isotope analysis, to study movement and dietary patterns, respectively, in three exploited species of coral trout – P. leopardus, P. maculatus, and P. laevis. The research was conducted at three primary locations – Orpheus Island, four mid-shelf/offshore reefs in the Townsville region (Townsville reefs), and the Marine and Aquaculture Research Facilities Unit (MARFU) at James Cook University. Samples and data were collected over the course of three years (2013-2015) providing extensive ecological and behavioural information from more than 300 individual Plectropomus. The overall aim of this research was to quantify, qualify, and compare long-term movement and dietary patterns of sympatric Plectropomus spp. By using multiple approaches, this thesis showed that broad resource selection trends differ between sympatric species, but interestingly, the way they differ is unique to each species pairing. At the Townsville reefs, P. laevis moved greater distances and had increased variability in depth use compared to P. leopardus. Movement patterns were correlated with distinct dietary niches between species, particularly when colour phases of P. laevis (footballer and blue-spot) were separated. The limited isotopic niche overlap between species was not correlated with fish size, indicating alternate prey selection, feeding styles, or energetic requirements engrained at a species level. Based on results from an aquarium-based stable isotope feeding trial, the trophic position of P. leopardus in the wild varied little between sampling locations and time periods. Similarly, the isotopic niches between species remained constant for several tissues (a proxy to feeding timeline) and at several reefs, suggesting feeding pressures exerted by each species is consistent within the region. Consequently, it is hypothesised that both P. laevis and P. leopardus will respond to environmental or humaninduced disturbances in similar ways within and across compatible reefs. At Orpheus Island, P. maculatus shared the same home range size as P. leopardus, however P. maculatus remained deeper in the water column throughout daily and monthly periods. These spatial patterns were correlated to overlapping isotopic niches - or similar prey selection. These trends indicate a high potential for competition that may be mediated by spatial or habitat partitioning. Overall, this research highlights the need for greater species-specific consideration relative to conservation and management initiatives since Plectropomus spp. readily demonstrate distinct behavioural patterns, and will likely respond to disturbances differently. Without fundamental knowledge of how co-occurring species select and partition resources, their interactions and impacts throughout the reef ecosystem remain unknown. Not only did this thesis provide new information about each species, it produced preliminary evidence that interactions between species may shape how resources are utilised on coral reefs.