Many aspects concerning the association of riverine fish with in-channel habitat remain
poorly understood, greatly hindering the ability of researchers and managers to address
declines in fish assemblages. Recent insights gained from landscape ecology suggest
that small, uni-scalar approaches are unlikely to effectively determine those factors that
influence riverine structure and function and mediate fish-habitat associations. There
appears to be merit in using multiple-scale designs built upon a geomorphologically-derived
hierarchy to bridge small, intermediate and large spatial scales in large rivers.
This thesis employs a hierarchical design encompassing functional process zones
(referred to hereafter as zones), reaches and mesohabitats to investigate fish-habitat
associations as well as explore patterns of in-channel habitat structure in one of
Australia's largest dryland river systems; the Barwon-Darling River.
In this thesis, empirical evidence is presented showing that large dryland rivers are
inherently complex in structure and different facets of existing conceptual models of
landscape ecology must be refined when applied to these systems. In-channel habitat
and fish exist within a hierarchical arrangement of spatial scales in the riverscape,
displaying properties of discontinuities, longitudinal patterns and patch mosaics. During
low flows that predominate for the majority of time in the Barwon-Darling River there
is a significant difference in fish assemblage composition among mesohabitats. There is
a strong association between large wood and golden perch, Murray cod and carp, but
only a weak association with bony herring. Golden perch and Murray cod are large
wood specialists, whereas carp are more general in there use of mesohabitats. Bony
herring are strongly associated with smooth and irregular banks but are ubiquitous in
most mesohabitats. Open water (mid-channel and deep pool) mesohabitats are
characterised by relatively low abundances of all species and a particularly weak
association with golden perch, Murray cod and carp. Murray cod are weakly associated
with matted bank, whereas carp and bony herring associate with this mesohabitat patch
in low abundance.
Nocturnal sampling provided useful information on size-related use of habitat that was
not evident from day sampling. Both bony herring and carp exhibited a variety of
habitat use patterns throughout the die1 period and throughout their lifetime, with
temporal partitioning of habitat use by juvenile bony herring and carp evident. Much of
the strong association between bony herring and smooth and irregular banks was due to
the abundance of juveniles (<100mm in length) in these mesohabitats. Adult bony
herring (>100mm length) occupied large wood more than smooth and irregular banks.
At night, juvenile bony herring were not captured, suggesting the use of deeper water
habitats. Adult bony herring were captured at night and occupjed large wood, smooth
bank and irregular bank. Juvenile carp (<200mm length) were more abundant at night
and aggregated in smooth and irregular banks more than any other mesohabitat patch.
Adult carp (>200mm length) occupied large wood during both day and night.
There is a downstream pattern of change in the fish assemblage among river zones, with
reaches in Zone 2 containing a larger proportion of introduced species (carp and
goldfish) because of a significantly lower abundance of native species (bony herring,
golden perch and Murray cod) than all other zones. In comparison, the fish assemblage
of Zone 3 was characterised by a comparatively higher abundance of the native species
bony herring, golden perch and Murray cod. A significant proportion of the amongreach
variability in fish assemblage composition was explained at the zone scale,
suggesting that geomorphological influences may impose some degree of top-down
constraint over fish assemblage distribution. Although mesohabitat composition among
reaches in the Barwon-Darling River also changed throughout the study area, this
pattern explained very little of the large-scale distribution of the fish assemblage, with
most of the variability in assemblage distribution remaining unexplained. Therefore,
although mesohabitat patches strongly influence the distribution of species within
reaches, they explain very little of assemblage composition at intermediate zone and
larger river scales. These findings suggest that small scale mesohabitat rehabilitation
projects within reaches are unlikely to produce measurable benefits for the fish
assemblage over intermediate and large spatial scales in the Barwon-Darling River. This
indicates the importance taking a holistic approach to river rehabilitation that correctly
identifies and targets limiting processes at the correct scales.
The variable nature of flow-pulse dynamics in the Barwon-Darling River creates a
shifting habitat mosaic that serves to maintain an ever-changing arrangement of habitat
patches. The inundation dynamics of large wood habitat described in this thesis
highlights the fragmented nature of mesohabitat patches, with the largest proportion of
total in-channel large wood remaining unavailable to fish for the majority of the time.
At low flows there is a mosaic of large wood habitat and with increasing discharge more
potential large wood habitat becomes available and does so in a complex spatial
manner. What results in this dryland river is a dynamic pattern of spatio-temporal
patchiness in large wood habitat availability that is seen both longitudinally among
different river zones and vertically among different heights in the river channel. Water
resource development impacts on this shifting habitat mosaic.
Projects undertaking both fish habitat assessment and rehabilitation need to carefully
consider spatial scale since the drivers of fish assemblage structure can occur at scales
well beyond that of the reach. Fish-habitat associations occurring at small spatial scales
can become decoupled by process occurring across large spatial scales, making
responses in the fish assemblage hard to predict. As rivers become increasingly
channelised, there is an urgent need to apply research such as that conducted in this
thesis to better understand the role that in-channel habitats play in supporting fish and
other ecosystem processes. Habitat rehabilitation projects need to be refined to consider
the appropriate scales at which fish assemblages associate with habitat. Failure to do so
risks wasting resources and forgoes valuable opportunities for addressing declines in
native fish populations. Adopting multi-scalar approaches to understanding ecological
processes in aquatic ecosystems, as developed in this thesis, should be a priority of
research and management. To do so will enable more effective determination of those
factors that influence riverine structure and function at the approariate scale.
| Identifer | oai:union.ndltd.org:ADTP/219577 |
| Date | January 2007 |
| Creators | Boys, Craig Ashley, n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Craig Ashley Boys |
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