Background

As owner and operator of Awoonga Dam, GAWB monitors spillway discharges and understands the consequences that they have upon fish that traverse the spillway to successfully migrate seaward - as the spill eases, it is possible that some fish can become landlocked in downstream ponds. 

GAWB is required to stock Lake Awoonga with fish of various species approved by the Department of Primary Industries. These species do not breed in fresh water but attempt to migrate out to sea each year to spawn.  Due to its operation of a fish hatchery, GAWB has staff that are both qualified and experienced in the care of barramundi and the other fish species stocked in Lake Awoonga.

Fish Migration During Spill from Lake Awoonga

In December 2010, Queensland encountered historic rainfall, and flooding throughout much of the state.  This included the Gladstone region, with Lake Awoonga spilling for the first time since the raising of the dam spillway to 40 metres AHD. As a consequence of the amount of water entering the system and the timing of the event, barramundi and other catadromous species¹  exited the lake via the spillway.

GAWB estimates (based on soundings/fish surveys, observations and sample counts) that:

• there are approximately 500,000 fish still within Lake Awoonga;

• approximately 20,000 mature-sized barramundi exited Lake Awoonga via the spillway during the course of the 2010/11 spill event; and

• of these, approximately 1,200 fish perished, principally due to physical trauma during the initial stages of the spill event.

Substantially all the barramundi which exited the lake did so by late January 2011, while sea mullet and other fish exited lake Awoonga (in smaller numbers) principally in late March/April.  This is consistent with the species' natural spawning migration periods.

During the first four weeks of the spill (when water flows exceeded 200m³/s), dead fish beyond Pikes Crossing (3km below the spillway) were managed by local authorities as necessary, while GAWB removed dead fish adjacent to the spillway and outlet valves.   Fish removed by GAWB were buried well above the river approximately 100m downstream from the spillway.

Fish which had exited Lake Awoonga were not returned to it, to ensure that any pathogens naturally present in the Lower Boyne and marine waters were not introduced to the lake.  GAWB collected sample fish from the Lower Boyne from commencement of the spill to monitor their health and condition.

Mature fish captured in Lake Awoonga by GAWB staff in 2011 were in full health and condition, consistent with the fish encountered during surveys conducted in 2007and 2010.

Landlocked Fish

In the period from late January to June 2011 GAWB staff undertook a series of actions to prevent the unnecessary death of barramundi that had been landlocked after exiting Lake Awoonga.  These actions were undertaken in consultation with relevant government agencies and, where necessary, with the assistance of specialist external contractors (such as veterinarians) to ensure the best possible outcomes.

Approximately 2,500 fish were trapped behind a rock wall below the spillway from late January.  These fish were monitored and healthy individuals were released into the Lower Boyne via channels through the rock wall and, from mid-May, by line or net capture and release.  The remaining fish in this landlocked area perished in early July 2011 as a result of reduced temperatures, water depth and dissolved oxygen levels.  These fish were also removed by GAWB and buried well above the river downstream from the spillway.

Barrier Systems

GAWB has reviewed possible barrier systems which might restrict or limit the number of fish exiting Lake Awoonga during a spill event.  GAWB is of the view that such devices would not achieve a satisfactory result due to the design of the dam and spillway with regards to water flow, and to likely safety and operational risks.

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¹Fish which live in fresh water but must migrate down rivers to the sea to breed