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Fish kills in the lower Nowendoc River
in February 2017:
Nature of the kill and the most likely cause
(currently self funded)
Where in the Nowendoc River system?
As of 17th February 2017,
fish kills were being reported in many pools along this section
(also in the upper Manning River
and the Manning estuary)
The pools specifically examined
on 17th February 2017 (1600-1830 hrs):
​Lower pool
​Upper pool
Overall nature of kill in the pools:
Clearly the lower pool was the exclusive area for the kill!
Detail of the kill: fish species and their sizes
Key findings re species and sizes:
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Seven fish species were recorded in the kill.
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Larger individuals of larger-growing species dominated (whilst small growing species were still actively swimming in surface waters).
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The recreationally-valuable Australian bass dominated the kill (at least 300 individual were killed in the lower pool).
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The commercially-important Sea mullet and Freshwater mullet were next most abundant in the kill.
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The fish in the kill appeared to have been dead for 7-10 days. At that time flow in the river was 17-20 ML/d (DPI Water data for Rocks Crossing). Flow on the day of observation, 17th February 2017, had fallen to 6.2 ML/d.
Dissolved oxygen (DO) concentrations in the pools:
A number of factors indicated that it was of key importance to obtain
data on dissolved oxygen concentrations in the two pools examined:
​
-
a key 'signature' of fish kills caused by low DO is that the fish killed are mostly large individuals of large species (small species survive because they can obtain DO by skimming waters just under the surface that are enriched by atmospheric oxygen)
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Given a lack of major flushing flows for many years, it was known that the river system was heavily laden with aquatic plants, including abundant adherent algae. When this highly vegetated 'state' is combined with low river flows, and very hot conditions, there is a strong likelihood that DO concentrations will plummet as dissolved organic matter 'demands' large quantities of DO.
Dissolved oxygen was measured at the following locations:
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water flowing into the lower pool (i.e. flowing out from the upper pool)
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water flowing out of the lower pool
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two depth profiles in the lower pool
-
one depth profile in the upper profile
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The results are shown below. DO conditions which would indicate the commencement of stress to fish are shaded red (follows National guidelines). DO conditions indicative of 'environmental hypoxia', which would be fatal to many fish species, are shaded yellow (follows McNeil and Gloss 2007).
Key findings re dissolved oxygen
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Severe dissolved oxygen depletion was apparent within both pools, although most severe in the lower pool where the fish kill was focused.
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'Environmental hypoxia' characterised waters entering and leaving the lower pool; the waters entering had higher DO concentrations and this would reflect the better conditions in the upper pool (see below).
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In the lower pool 'environmental hypoxia' characterised the entire water column at Profile 1, and ~88% of the column at Profile 2.
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In the upper pool 'environmental hypoxia' characterised ~69% of the column at Profile 1.
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The upper pool had a markedly lesser load of adherent algae growing on aquatic plants (predominately Hydrilla verticillata) and this is likely to have resulted in a lower DO demand in that pool.
Overall summary
​
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The fish kill had a clear 'signature' of being caused by dissolved oxygen depletion (i.e. small fish species still living while large individuals of large species primarily killed)
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This indication was strongly supported by the recorded dissolved oxygen data which showed severe oxygen depletion.
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Severe oxygen depletion was undoubtedly a result of: i) a very heavy load of aquatic plants including adherent algae (both a result of a lack of major flushing flows in recent years), ii) low river flows, and iii) very high water temperatures.
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This depletion appeared to be strongest in the lower pool where the fish kill was focused. This pool-to-pool difference provided additional evidence that the kill was primarily caused by dissolved oxygen depletion.
An obvious implication arising from this investigation is that flow rules used to protect environmental values of rivers from significant water extraction (e.g. irrigation), need to be adjusted according to the state of the system. For example:
​
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When in a well-flushed state with a small load of aquatic plants, the flow rules could be set lower.
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Conversely, when in a poorly-flushed state with a heavy load of aquatic plants, the flow rules should be higher - for example, it appears for the lower Nowendoc River in a poorly flushed state a minimum flow of at least 20 ML/d may be appropriate given that major environmental degradation - i.e. fish kills - is likely to commence below this flow level.
Thanks to:
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Kristina Strat from Cundle Flat Farm Riverside Camping for alerting me to the developing fish kills.
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Michiyo Nikaido for field assistance and valuable discussions.
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MidCoast Water (Graeme Watkins) for strategic information and loaning a Hach HQ40d portable dissolved oxygen meter when my meter developed battery problems.
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Luke Everingham (and caretaker Kevin) for providing strategic information regarding the Nowendoc River pools examined in detail.
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