Macquarie perch under pressure: predation risk to an endangered fish population

Abstract

The extinction of freshwater fish species poses a threat to the health of freshwater ecosystems and must be prevented. Habitat loss is largely responsible for the decline of many of Australia’s freshwater fish species, resulting in small and often isolated populations that are vulnerable to stochastic processes. Macquarie perch, Macquaria australasica (Percichthyidae), is one such species, and is now listed nationally as endangered. Cotter Reservoir in the Australian Capital Territory (ACT) is home to one of the last self-sustaining populations of Macquarie perch. The success of the Macquarie perch population at Cotter Reservoir has been largely attributed to the abundance of structural habitat around the perimeter of the reservoir, which established primarily in response to long-term stable water levels. This habitat is thought to mitigate predation by cormorants which are known to prey on adult Macquarie perch, a vulnerable component of the population. However, habitat availability in the reservoir will soon change when it is enlarged from 4 to 78 GL and regularly drawn down for water supply. It was the aim of this thesis to guide effective conservation measures to protect Macquarie perch from cormorant predation in the enlarged Cotter Reservoir by: 1. Determining spatial and temporal trends in cormorant predation risk to Macquarie perch in Cotter Reservoir. 2. Investigate trends in the spatial ecology and habitat use of Macquarie perch over the course of a year to develop an understanding of refuge and spatial requirements. Predator abundance, predator-prey overlap, prey behaviour and habitat characteristics, were considered key parameters for achieving the objectives of this thesis. Adult Macquarie perch were radio-tracked at Cotter Reservoir every two weeks from April 2008 to June 2009. During this period a 2 m drawdown occurred for one month in each season to mimic structural habitat availability in the enlarged reservoir. Cormorants were counted from February 2008 to June 2009 and their activity and location within the reservoir were recorded. Habitat was mapped using aerial photographs taken during a 5 m drawdown. Macquarie perch, cormorant and habitat data was analysed in ArcGIS to investigate spatial and temporal relationships. Predation risk to Macquarie perch was greatest in the shallow upstream section of Cotter Reservoir during spring and summer, when predator-prey overlap and predator abundance were highest. This corresponded with a shift by the Macquarie perch population into the upstream section of the reservoir during its spawning period. A decline in predator-prey overlap at the fish microhabitat scale suggests that the use of structural habitat by adult Macquarie perch in the current reservoir mitigates predation. A 2 m drawdown resulted in the loss of all emergent macrophytes and 55% of structural woody habitat. During drawdown fish did not aggregate in remaining structures or increase their use of depth as refuge, resulting in increased vulnerability to predation. The addition of constructed habitats in the upstream section of the enlarged reservoir is essential to ensure that Macquarie perch are able to make a spawning run into the river and access upstream foraging habitat without increased risk of cormorant predation. Adult Macquarie perch established larger home-ranges than previously reported for percichthyids in rivers and demonstrated a spatial awareness of Cotter Reservoir. The population distribution shifted temporally, with upstream shifts in spring and summer and downstream shifts in response to drawdown. Smaller fish inhabited deeper habitats (>15 m) while larger fish inhabited shallower habitats (<15 m), potentially in response to a trade-off between avian predation risk and food availability. This information contributes significantly to understanding the resource requirements of Macquarie perch and provides valuable baseline data for investigating the effects of reservoir enlargement. The present thesis provides a valuable approach for developing an understanding of predator-prey interactions, particularly when dealing with small populations in the field. Investigation of predator-prey overlap allowed the identification of when and where predation risk is greatest, which cannot be obtained from conventional consumption estimates. Information about spatial and temporal trends in predation risk is critical for implementing effective conservation measures to protect small prey populations threatened by predation.

Date of Award	2010
Original language	English
Supervisor	Richard Norris (Supervisor), Brendan Ebner (Supervisor) & Mark Lintermans (Supervisor)