CAPÍTULO II:MARCO TEÓRICO
2.5 Los titulares de las portadas del diario Líbero
The currently employed monitoring of population size is a simple method to detect large
scale changes in demographics as a function of natural and anthropogenic disturbance.
However, such approaches may often fail to capture impact at the individual level, which
is the underlying cause for changes in the population. Our new protocol of weighing
individual sawfish and identifying suitable proxies for body condition are a further step
towards more efficient monitoring, but still only represent a proximate response to
underlying changes in the physiology and ecology of individuals. The behavioural data
collected by accelerometers permit a detailed comparison of time and energy budgets of
individuals exposed to broadly different conditions, by comparing incidence of
ecologically important activities, such as daily number of prey strikes, time spent
swimming or escape responses. Together with conventional monitoring described
elsewhere, such data can provide unparalleled insight into the underlying mechanisms
behind changes in population health and provide an invaluable management tool.
Our preliminary analysis shows that sawfish most likely have the ability to ascend a
vertical‐slot fishway, due to their ability to swim in short bursts, which would allow them
to clear slots between chambers with high water flow. However, the specifics of optimal
design will require further work of this nature, which is on‐going. The implementation of
a large‐scale project using such technology would provide a valuable tool to quantify
population health beyond standard monitoring and allow the efficacy of a constructed
fishway to be tested.
General conclusions
Much information is yet to be gathered comparing sawfish recruits upstream and
downstream of the barrage. Plans are in place to conduct an experiment comparing
sawfish recruits that have been captured from below the barrage, tagged with
accelerometers and then translocated into the pool immediately upstream of the barrage
with animals from the same cohort that remain downstream of the barrage. It is
anticipated that this will provide data to ascertain if any behavioural differences exist
between animals below and above the barrage which might provide evidence of a
competitive advantage for animals above. Such data will help to objectively inform the
decision as to whether a vertical‐slot fishway is needed. Further data, using acoustic tags
(see Whitty et al. 2009a) could be generated on the actual stage heights that sawfish move
over the barrage, and this could be achieved by placing a ‘gate’ of acoustic receivers
above and below the barrage.
Negative impacts upon recruitment in species, not only have short term consequences,
but also have an insidious trickle‐down effect throughout successional generations. For a
species like Freshwater Sawfish which appears to experience infrequent successful
recruitment events (for example, as occurred in 2011), perhaps occurring only once or
twice per decade in response to ideal wet season flow conditions, it seems almost certain
that the restriction of access to critical habitat upstream of the barrage has counteracted a
potential boost in sawfish numbers.
We would argue strongly that any effort to alter existing habitat downstream of the
Barrage to offset a loss of species fitness due to restriction of access to upstream habitat
would be ineffective. The main difficulties for sawfish recruits as the situation stands are
at least twofold. Firstly, stress on recruits and mortality due to predation by apex level
predators such as sharks and crocodiles, and illegal poaching by humans is of concern
when large numbers congregate below the Barrage. Secondly, those recruits that avoid
predation are exposed to increased competition amongst themselves for food resources in
the limited amount of deep‐water pool habitat downstream of the Barrage. To mitigate
these pressures, one would have to either reduce the numbers of apex predators by
means of an annual cull, and/or to increase the availability of prey in the downstream
habitats by means of annual stocking. Both of these options are not only ecologically (and
perhaps also ethically) unsound, but would also be much less cost effective in the long
term compared to constructing and maintaining the proposed vertical‐slot fishway.
The data gathered during our long‐term monitoring program thus far points strongly
in the Fitzroy River by the construction of a vertical‐slot fishway at the Camballin
Barrage. However, a shortfall in our knowledge remains on crucial aspects of the design
and utilisation of the proposed fishway. Highly targeted additional research (mentioned
throughout this report) will be extremely beneficial in order to maximise the effectiveness
of a proposed fishway as part of a strategy to ensure the sustainability of the critically
endangered Freshwater Sawfish in the Fitzroy River.
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