Fish Kills in the Mary River

Fishnote No. 26, March 1998, ISSN No: 1035-008X
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.

 Download: Fish Kill Investigation Manual

History

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.

Why do fish die at this time of the year?

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 2 (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.

How do the fish die?

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.

Oxygen Depletion

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.  
   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.
   
   
2. Breakdown of organic matter.
   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. Inflow of low oxygenated water.  
   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.

Barrages and Fish Kills

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 barramundi.

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 reestablished 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.

Monitoring

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 4 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 Regional Development, Primary Industry, Fisheries and Resources 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.

What can you do?

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 the Department of Regional Development, Primary Industry, Fisheries and Resources (contact details listed below) as soon as possible. This will enable a quick response and investigation into the cause of the kill.

Department of Regional Development, Primary Industry, Fisheries and Resources:
T: (08) 8999 2144; F: (08) 8999 2065
Pollution Response Line:
24hrs Freecall Phone: 1800 064 567

Fish Kill Reports in the Mary River from 1987 - January 1998