30 Aug 2020
Gambusia is a species of fish that has become a major pest in fresh and estuarine waters of NSW since its introduction to Australia in the 1920s. They were introduced in an attempt to reduce mosquito numbers but ended up becoming a pest in their own right. At Sydney Olympic Park our challenge is to reduce the impact their voracious appetite has on the endangered Green and Golden Bell Frog.
Gambusia (Gambusia holbrooki) is also known as ‘Plague Minnow’ as they are prolific breeders, with females producing 50-300 young per brood, and having up to 9 broods per year; they give birth to live young that can mature in less than 3 months. They are relatively small fish, with mature females reaching 6cm in length and mature males reaching 3.5cm in length.
Although tiny in size, they are a considerable threat! The presence of Gambusia threatens many native frog species as they prey upon frog eggs and tadpoles. The endangered Green and Golden Bell Frog are particularly susceptible to this predation which is listed as a ‘Key Threatening Process’ under NSW legislation.
Gambusia is present in some precincts within Sydney Olympic Park, such as Narawang Wetland but not in others, such as the Brickpit. The Authority implements strict hygiene protocols to stop the introduction of this fish into new waterbodies, as well as implementing strategic programs to limit existing infestations.
One such program is the annual Cyclic Draining Program implemented at Narawang Wetland since 2003 where a selection of ponds throughout the wetland are slowly drained when frog activity is low. After the ponds have remained dry for a period of time sufficient to remove the pest, the ponds are refilled with fish-free water before the frog breeding season commences. This program provides the Green and Golden Bell Frog with a window of opportunity to breed in the absence of fish which should increase recruitment of young frogs into the population. Research on the program has shown that the Green and Golden Bell Frog responds positively to the draining with males more likely to call from fish-free ponds.
This program follows a cyclic schedule where different ponds are targeted each year and all habitat ponds within the precinct receive treatment over a 3-year period. As Narawang Wetland functions as a floodplain, Gambusia is reintroduced during flooding events so the ongoing program is essential in maintaining Green and Golden Bell Frog habitat. This year marks the seventeenth year of the program’s implementation!
The 2020 Cyclic Draining Program has been challenging so far, combating approximately 150mm of rain before and during works. The success of this program is attributed to the diligence of staff involved. Bring on the Green and Golden Bell Frog breeding season!
Gambusia or ‘Plague Minnow’, a predatory threat to native frog eggs and tadpoles
A drained pond gets a little extra assistance to get the last of the water out
Green and Golden Bell Frog tadpoles enjoying a pest fish-free season