The importance of complex habitats for the predator-prey interaction between a threatened galaxiid fish and introduced salmonid
Creators: name: Stuart-Smith, Richard
Publisher: name: University Of Tasmania
Year: 2023

description: Theoretically, there are a number of ways in which predator and prey can co-exist, including temporal and spatial partitioning and the presence and use of refuge habitats by prey. Whilst these factors have been shown to promote co-existence in well-studied, co-evolved predator and prey pairs, there has been a lack of applied research into how predator-prey theory fits when predators are introduced species. Understanding the mechanisms behind the success, or lack of, by native species when predators have been introduced, thus requires knowledge of whether the same factors promote co-existence, and contexts in which predation pressure is likely to be greatest on native species. This thesis examines the interactions of Galaxias auratus, a threatened galaxiid species that has shown unusual resilience to the predation pressure imposed by the introduced brown trout, in order to gain insight into the mechanisms that have assisted it in co-existing with an introduced piscivore that has adversely impacted other galaxiid populations. There has been a substantial reduction in the availability of complex habitats in the entire natural range of this species, prompting an investigation into the likely importance of these habitats in facilitating the co-existence of this unnatural predator-prey pair. An initial study on the feeding of G. auratus in the wild revealed it to be an efficient forager that feeds constantly, both day and night. This occurred whilst it appeared to undertake a diel shift in habitat use, and its daily ration was still comparable to other well-studied freshwater fishes, despite it using complex habitats for some of this time. A laboratory experiment was conducted to examine this apparent diel switch between habitats, and test whether G. auratus behaviourally altered this pattern when predation risk was high. This confirmed that G. auratus uses complex habitats (macrophytes and rocks) during the day and open water during the night, but reduces its use of open water significantly when brown trout are near and predation risk is high. In order to determine whether this habitat use pattern and response to the predation risk imposed by brown trout was likely to promote co-existence of these species, two experiments were run to assess the potential foraging costs usually associated with using complex habitats, and the benefits to using such habitats in terms of reducing predation risk. The foraging of G. auratus was found not to be significantly reduced in complex habitats, but the risk of predation was substantially reduced. These results imply that the behaviour of G. auratus is \adaptive\" and is likely to have contributed to its success with brown trout. These findings also suggest however that if the availability of macrophytes was severely reduced for this species as is likely to continue in the future the importance of predation pressure in determining the outcome of its interaction with trout is likely to substantially increase. The implications for other native species are that the importance of predation by introduced species is context-dependent and that conservation must focus on maintaining contexts in which predation pressure is likely to be lessened. This includes conservation of important habitats that may mediate interactions between introduced predators and native prey."