November 14, 2016

This study had the same basis as the upper estuary vegetation study in the Hastings River.  Nevertheless, details of the basis are repeated here for ease of reference.

Beds of freshwater vegetation in the upper estuary are very important places for fishes as they use these areas variously for sheltering, feeding, breeding and raising their young. Their importance parallels crayweed beds in the marine environment - "the biological engine of the southern reef" - which are now subject to recuperative efforts since their demise in the 1980s  (see crayweed project). 

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When river flows reduce during droughts, or importantly, as a result of water extraction, these upper estuary beds are threatened by the toxic effects of saltwater intruding up from the ocean.  From early 2006 to late 2014, the abundance and health of these beds was regularly surveyed along an thirteen kilometer length of the upper estuary.

The objective of this work was to determine if  there are critical river flows  which, if passed (to lower levels), will result in a rapid increase in salt-induced damage to the beds. Having this knowledge is very important in developing effective environmental flows to protect the Manning River from damage caused by water extraction. The word 'effective' is used as many environmental flow specifications have dubious effectiveness as they are based on 'informed' guesswork, rather than hard data such as that gathered in this study.

November 17, 2016

This intensive (weekly) fish surveying was initiated in 2007 to identify sensitive periods for fish passage, and develop a means for obtaining evidence of passage blocks in the lower section of the system where water extraction impacts are largest. The end game of this work was to verify hydraulic-based predictions of fish passage availability. The technique used for survey is identified as 'Night-count transects' which is direct-observation, non-destructive approach that was initially develop within Kakadu National Park by the Federal Government in the 1980s.

Pulses in the flux of migrants were assumed to be represented by noticeable peaks in abundance in association with peaks in an index of schooling behaviour. Blocks to the upstream passage of different fish species were assumed to be indicated by peaks in abundance at the downstream site (Neap tidal limit) which were not transferred to the upstream site (Bungay pool).

November 17, 2016

At least twenty-five  species of fish have been recorded in the freshwaters of the Manning River. These species are valued in various ways: recreational angling (e.g. Australian bass, yellow-finned bream and dusky flathead), conservation (e.g. Bellinger River catfish or east coast catfish), or from a commercial point of view (e.g. sea mullet and freshwater mullet). The abundance of these species varies greatly between years and there is very little scientific understanding of the factors which drive these changes. Without such an understanding it is impossible to distinguish between  changes caused by mans' activities and those simply caused by natural variation.

Since 2007, the abundance of the fishes has been  surveyed on a three-times-year basis at twelve sites in the lower-most 58 kilometers of the river.  The technique used for survey is identified as 'Night-count transects' which is direct-observation, non-destructive approach that was initially develop within Kakadu National Park by the Federal Government in the 1980s. This long-term, landscape scale data has no equals when it comes to Australian freshwater fish ecological studies.

There are two key objectives of this work.  The first is to provide a linkage with the finer-scale fish surveying study in lower river,  thus enabling a broader-system interpretation of the findings from that study. The second objective is to determine if changes in the abundance of the fishes can be related to changing river flows. If this is the case, particularly in relation to the duration of lower river flows, the study will provide very important knowledge for the development of effective environmental flows to protect the Manning River from damage caused by water extraction. As before, the word 'effective' is used as many environmental flow specifications have dubious effectiveness because they are based on 'informed' guesswork, rather than hard data such as that gathered in this study.

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