Fish Kills in the Mary River (DBIRD_NT)

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                                                                        No. 26

                                                                        March 1998

                                                                        ISSN No: 1035-008X

Fish Kills in the Mary River
R. Griffin and P. de Lestang, Fisheries Research, Darwin

Every year Territorians are astounded by the number of dead fish that appear in billabongs
during the build-up. Theories as to what causes these fish deaths range from man-made dams
to poison leaking out of the soil. However in almost every case, fish kills within the Mary River
wetlands have been natural occurrences.


In October 1997, a large fish kill occurred just east of the Mary River involving an estimated
5,000 fish, of which 90% were barramundi. Extremely large kills such as this one are rare, but
smaller fish kills are not uncommon. Over the past 10 years, at least 23 fish kills have been
reported on the Mary River, ranging from just under 100 to over 1,000 dead fish, and have
included such species as barramundi, catfish, bony bream, saratoga and rainbow fish, as well
as freshwater shrimps and crayfish (Table 1). It has been reported that in one fish kill alone, fish
from up to 18 species were found dead. There have undoubtedly been many more fish kills that
have gone unreported.


Fish kills can occur throughout the year but are most common during the beginning of the wet
because of:

•   shrinking billabongs;
•   large numbers of fish concentrated in small areas;
•   warmer temperatures; and
•   runoff from the first rains.

During the wet, barramundi, tarpon, bony bream and a number of other fish species spread out
over the wetlands, inhabiting areas that were dry only days earlier.

These areas are rich in food and provide a sheltered habitat from the many predators. In March-
April, as the wet subsides and the water slowly drains out to sea, more and more fish slowly
converge into the deeper permanent channels, greatly increasing demand on oxygen and food
sources, especially in the smaller creeks. This demand does not increase dramatically until
October-November when the temperature starts to rise again, leading to an increase in fish
activity. The increase in temperature also has the effect of increasing microbiological activity

(algae and bacteria) and lowering the amount of oxygen the water can hold. Evaporation and
leaching during the dry turns the once deep channels into shallow, warm bodies of water. It is at
this time that fishing greatly improves!

The combination of large concentrations of fish in relatively shallow, warm billabongs is a recipe
for disaster.


There are two basic causes of natural fish kills:

•   deficiency of oxygen; and
•   lethal levels of naturally occurring toxins.

These may occur together or separately but the end result is fish death.


Mass fish death can occur when oxygen is rapidly removed from the water body, which in the
Territory is commonly caused by:

1. Mixing of surface and bottom layers of water;
2. Breakdown of organic matter; and
3. Inflow of low oxygenated water

1) Water within the creeks at this time of the year can become layered (or stratified). Generally
there is a relatively thin surface layer containing high amounts of oxygen and a thicker bottom
layer of water depleted of oxygen (Figure 1). This layering may be turned over or mixed by
strong winds or a sudden rush of water entering the system, which occurs during the first rains.
The end result is low oxygen throughout the water column, killing both fish and crustaceans.

Figure 1. Dissolved oxygen levels (mg/L) taken at Corroboree Billabong on 9 December 1997,
showing the relationship between the concentration of dissolved oxygen and depth.

2) Oxygen can be removed from the water by the action of bacteria and other microscopic
organisms, breaking down algae and decaying leaves, wood etc. A combination of warmer
temperatures and large amounts of rotting plant material built up from the dry greatly increases
bacterial activity, removing oxygen from the water very rapidly. This bacterial action is further
increased by the build-up of fish carcasses and will continue until the first major rains flush out
the system.

3) Water flowing into the billabongs can be very low in oxygen and contain high amounts of
organic matter. When this water mixes with billabong water, the concentration of oxygen in the
mixed water falls, leading to fish deaths.

Build-up of Toxins
Naturally occurring toxins are derived from:

•   the soil;
•   local flora; and
•   fish populations.

Sulphate soils occur naturally throughout the wetlands and in some conditions runoff from those
soils can be quite acidic. The acidic conditions cause naturally occurring aluminium in the water
to interfere with the gill membranes of fish, preventing them from absorbing oxygen.

Some flora commonly found in the wetlands release toxic compounds when decomposing.
These compounds are found in such low concentrations that a fish kill occurring due to these
toxins is highly unlikely.

In very concentrated fish populations, waste products such as ammonia are released from the
fish, which can build up to toxic levels. Except in extreme cases this is usually not a problem in
natural situations.


During the past 20-30 years within the Mary River floodplains, salt water has intruded inland
causing the destruction of large areas of paperbark swamp and floodplain grass habitats. There
are many theories on the cause of this intrusion, from rises in the sea level to erosion (largely
attributed to damage by feral buffaloes) and even dynamiting of river bars! Whatever the cause,
the end result is the same: a large loss of freshwater habitat for juvenile fish, including

To prevent salt water from entering freshwater habitats, earth walls (barrages) have been built
across some of the smaller creeks. Where a build-up of water occurs behind the wall during the
wet, spillways are progressively being put into place, to allow fish passage both up and down
stream. This helps ensure that fish do not become stranded and die as a result of the barrage.
The major effect of the walls is therefore to protect and possibly increase fresh water habitats.

Such a situation can be seen at Shady Camp, where in November 1988, a barrage
incorporating a spillway was constructed to prevent salt water from encroaching further into the
main channel of the Mary River. A permanent fresh water billabong has now been re-
established above the barrage and during the wet fresh water flows strongly over the spillway to
the ocean. Fish kills have been noted in this area both before and after construction, suggesting
that the barrage does not cause fish kills (Table 1).


Fish kills are a common occurrence within the Mary River system and in the tropics generally.
Apart from ensuring that the passage of fish is not restricted by barrages, it is not feasible at the
moment to prevent these kills. It is almost impossible to say how many fish are stranded by

falling water levels and die every year within the drying wetlands but obviously a much larger
number utilise these conditions and survive to spawn and carry on to the next generation.

The Department of Business, Industry and Resource Development and the Department of
Infrastructure, Planning and Environment monitor the Mary River wetlands. This monitoring will
enable us to have a better understanding of fish kills and perhaps in the future, a method of
both predicting and even preventing these kills.


Fish kills are most commonly found and reported by recreational fishermen and tourists. If you
witness any fish kills, it would be appreciated if you notify one of the departments listed below
as soon as possible. This will enable a quick response and investigation into the cause of the

          Department of Business,                        Department of Infrastructure,

    Industry and Resource Development                     Planning and Environment

            Phone: (08) 8999 2144                           Phone: (08) 8924 4139

            Fax:    (08) 8999 2065                           Fax:   (08) 8924 4053

                                POLLUTION RESPONSE LINE

                                        24hrs Freecall

                                     Phone: 1800 064 567


Advice from S. Townsend (DIPE) on environmental factors affecting water quality within the
Mary River system is gratefully acknowledged. J. Lloyd and G. White (DBIRD) are thanked for
their advice and proof reading of this article.

   Table 1. Fish kill reports in the Mary River from January 1987 to January 1998

Date                      Location                               Species                Approximate number of
Jan-87                  Shady Camp                    barramundi/saratoga/bony                       400
Mar-87               Lower Corroboree                      barramundi/catfish                         40
Apr-87                   Corroboree                        barramundi/catfish                        120
Dec-89                   Corroboree                           not recorded                           500
Feb-90        Y - Shaped billabong (Between                barramundi/catfish                        280
                Rockhole and Corroboree)
Dec-90               Lower Corroboree                barramundi/5 other species                      150
Dec-91                   Corroboree                        barramundi/catfish                         20
Dec-91                  Shady Camp                                catfish                              0
Dec-91                   Corroboree                   barramundi/catfish/3 other                       9
Dec-91               Lower Corroboree                      barramundi/catfish                         85
Jan-92                    Rockhole                         barramundi/catfish                       >880
Jan-92                   Corroboree                    barramundi/bony bream/                        >55
Feb-92                  Shady Camp                        barramundi/catfish/                    500-1,000
                                                         gudgeon/rainbow fish
Jan-93          Corroboree (Palm Lagoon)                    barramundi/bony                           45
Jan/93                   Corroboree                               catfish                              0
Dec-94          Floodplains between Shady                  barramundi/others                        Many
                   Camp and Rockhole
Jan-95                  Shady Camp                         barramundi/others                         200
Dec-95                  Shady Camp                       barramundi/saratoga/                        250
                                                          catfish/bony bream
Apr-96                   Corroboree                       barramundi/catfish/                          2
Nov-96                   Corroboree                            barramundi                          Several
Nov-96                   Corroboree                            barramundi                             20
Oct-97                 Clarks Crossing                     barramundi/catfish                        980
Oct-97            5 km South of Rockhole                   barramundi/catfish                        450

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   Published: Monday 2 March 1998.

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