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|
|Supervisor||Richard Norris (Supervisor), Brendan Ebner (Supervisor) & Mark Lintermans (Supervisor)|