Morgan & Beatty: Feral goldfish in Western Australia 
• I 
Age (years) 
Figure 4. Total length at age of Goldfish captured during the study in the Vasse River including the von Bertalanffy growth curve. N.B. 
approximate age classes were based on the number of translucent zones on the otoliths and October 1 was assigned as the birth date. K 
= 0.651, t„ = 0.0163, Loo = 374.26. 
growth study on wild Goldfish populations in Australia 
was by Mitchell (1979) who used scales to age fish from 
South Australia. The growth rates here substantially 
exceed those in Mitchell's study and are similar but 
higher to those published by Izci (2001) for a wild 
population of Goldfish in Turkey. Mitchell (1979) found 
one fish living for over 10 years that weighed over 2 kg. 
From the length-weight relationship in the Vasse River it 
is predicted that Goldfish would attain 2 kg at 447 mm 
TL. 
Kolmakov & Gladyshev (2003) demonstrated that 
significant growth of the cyanobacteria Microcystis 
aeruginosa is stimulated by the passage through Goldfish 
intestines, while other cyanobacteria such as Anabaena 
flos-aquae and Planktolhrix agardhii that were passed 
through Goldfish guts exhibited greater growth 
compared to the controls. Tire process of cyanobacteria 
stimulation is not known, however the authors 
considered that the passing through the Goldfish gut 
may give nutrient enrichment or that mechanical re¬ 
agglutination of cells from colonies may occur. 
Microcystis sp. and Anabaena sp. are known to cause 
algal blooms in the Vasse River. The above findings of 
the diets of the Goldfish in the Vasse River, together with 
the fact that they are known to stimulate significant 
growth in blue-green algae, is cause for concern in a 
system that is currently exposed to severe algal blooms 
during spring, summer and autumn (see Paice 2001). 
Thus, a substantial increase in Goldfish biomass could 
become a major factor attributing to algal blooms in the 
Vasse River and indeed within nutrient enriched 
environments elsewhere. Furthermore, the vigorous 
bottom feeding methods of Goldfish resuspends nutrients 
making them available to algae. Within the Vasse River 
algal blooms have lead to a number of fish kills since 
1997 (Paice 2001), a period after the initial introduction of 
Goldfish, noting that during 2006 we captured a 10 year 
old fish. An increase in algae provides this feral species 
with an abundance of a food source that they can utilise 
from a very young age. The high growth levels found in 
Goldfish within the Vasse system may be a combination 
of the high degree of food availability and warmer 
conditions provided by the diversion of flows around 
this system creating a lentic rather that lotic environment. 
Feral Goldfish have the potential to prey on the eggs, 
larvae and adults of native fishes and have been known 
to cause declines in native fish populations in the U.S. 
(e.g. Deacon et al. 1964). Goldfish compete with native 
fishes for food and space and by growing to a much 
larger size than all but one of the native freshwater fishes 
in the region, they would escape predation from a young 
age (probably by a few months old they would attain 
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