|Denmark River Fish Assemblages
by Paul Close & Dave Tunbridge (CENRM Fish Research)
The University of Western Australia's Centre of Excellence in Natural
Resource Management (CENRM) recently undertook surveys of fish
populations in the Denmark River Catchment. Craig Carter (WICC
Project Officer) joined Paul Close and David Tunbridge for two days of
electrofishing surveys at three sites on the Denmark River and one site
on Scotsdale Brook. These surveys were undertaken as a component
of a Department of Water project (funded through South Coast NRM)
addressing the environmental water requirements (EWRs) for the Denmark
River. EWRs are the water regimes required to maintain important
ecological values and processes at a low level of risk. Examples
of the water dependant ecological values identified in the Denmark
River include fish spawning, and the maintenance of in-stream habitat,
channel form and water quality. The surveys aimed to gain important
knowledge on the species of fish in the Denmark River catchment, and
more particularly their distribution and relative abundance. State of the
art electrofishing equipment was used to collect fish from representative
reaches (60m in length). This highly effective sampling technique uses pulses of electrical current to attract fish toward a handheld net, and momentarily stun them, allowing for their collection.
All fish were identified and counted before being released into the habitats from where they were collected.
Seven fish species were collected from the Denmark River catchment including Balston's pygmy perch (Nannatherina balstoni).
The collection of Balston's pygmy perch is an important finding as they have not been previously reported from the Denmark
River. Other species collected included nightfish (Bostockia porosa), western pygmy perch (Edelia vittata), western minnow
(Galaxias occidentalis) and pouched lamprey (Geotria australis). Two exotic species were also collected, namely rainbow trout
(Oncorhynchus mykiss) and mosquito fish (Gambusia holbrooki). These data show that native species dominated assemblages
both in terms of species richness and relative abundance. Interestingly, the exotic species were only collected from the lower reaches of the Denmark River in relatively low abundances. No exotic species were collected
from Scottsdale brook or the upper reaches of the Denmark River.
This newly acquired knowledge will be used in combination with existing knowledge of the habitat
requirements and ecology of individual species, to critically assess the environmental water
requirements of the Denmark River and Scottsdale Brook. These assessments will form one
component of the Denmark Water Management Plan which, in consultation with the community,
seeks to share water between environmental, socio-cultural and economic interests within the
catchment. For more information on the Denmark Water Management Plan or the fish surveys,
contact Sharon Stratico (Department of Water: 9842 5760) or Paul Close (UWA: 98420833).
|WILSON INLET GROUNDWATER NUTRIENT STUDY
The need to monitor nutrients from urban groundwater sources, in particular the
unsewered areas of Weedon Hill, Little River and Minsterly Road, is identified in the
WINRAP as a priority action. Subsequently, a monitoring program was conducted
from May 2006 to April 2007 to investigate nutrient concentrations in groundwater
collected from a series of bores along the north-western foreshore of Wilson Inlet,
from Poddyshot boat ramp to Denmark Earthmoving on Inlet Drive. The following is a
summary of the data collected during this program.
Of the monthly nutrient data collected, just under 80% of the samples registered
moderate to very high total nitrogen (TN) concentrations in accordance with the
Australian and New Zealand Environment and Conservation Council (ANZECC)
Guidelines for Fresh and Marine Water Quality. A relatively large proportion
(approximately 50%) registered very high TN (>2 mg/L).
Samples collected from monitoring bores along Inlet Drive & Minsterly Road
consistently measured exceedingly high nitrogen concentrations, peaking at 12 mg/L.
To put this into context, background nitrogen concentrations measured in
groundwater samples collected from 'The Cove', a pristine native bush block on
Payne Road, were very low (between 0.19 and 0.38 mg/L).
Generally results revealed that for most monitoring locations across the project area,
phosphorous is less significant with only about 20% of samples registering high to
very high concentrations. Findings of this project will hopefully help provide the impetus for these areas to be considered for the
Water Corporation's Sewer Infill Program.