Morgan & Beatty: Feral goldfish in Western Australia 
all larger fish and the majority of the apparent juvenile 
(0+) fish were removed, immersed in methyl salicylate 
and viewed though a dissecting microscope using 
reflected light. The number of translucent zones was 
counted and it was assumed that these corresponded to 
year classes (while not specifically validated, this 
technique has been validated for the majority of native 
freshwater fishes in south-western Australia and for an 
introduced fish in the region (e.g. Morgan et al. 1995, 
2000, 2002)). A length-frequency distribution was 
produced separately for those Goldfish captured on each 
sampling occasion. 
Tlie length of each individual Goldfish was plotted 
against its age and a growth curve fitted using a von 
Bertalanffy growth equation with October 1 as an 
estimated birth date. This estimate was made from the 
small size of individuals captured in December and from 
the capture of larval (newly-hatched) Goldfish in other 
parts of south-western Australia during early spring. The 
von Bertalanffy growth curve is where 
L, is the length at age t (years), L_ is the asymptotic 
length of the population, K is the growth coefficient and 
tg is the hypothetical age at which the fish would have 
zero length. 
Results 
Distribution, population structure and growth of 
Goldfish in the Vasse River 
Goldfish were only captured in the lower sections of 
the Vasse River and none were captured in the Diversion 
Drain or upper catchment. Large numbers of juveniles 
were captured around the Old Butter Factory slot-boards 
and other structures such as bridges, while the larger 
individuals were usually found in the close vicinity 
(usually just upstream) of structures such as bridges or 
around snags (Fig. 1). 
Analysis of length-frequency histograms and the 
number of translucent zones on the otoliths as an 
estimate of age (e.g. one zone = 1 year old, two zones = 
2 year old etc.) revealed that the population of Goldfish 
in 2003 and 2004 in the Vasse River at the time of 
sampling was dominated by the cohort born in the 
spring of 2003 (Fig. 2). Horyever, two older and larger 
fish, over 400 mm TL, were captured. During March 
2005, a total of 105 Goldfish were captured. 
Approximately 76% of these belonged to the 0+ age 
class, i.e. they were in their first year of life. The 
remaining fish varied in age with the oldest being in 
their sixth year of life (i.e. 5+). During sampling in May 
2006, 55 Goldfish were captured, with 84% belonging to 
the 0+ age class, one fish of 11 years, and another two 
older than 7 years. The majority of fish were captured 
at the Strelley Street Bridge (31, all 0+), while 15 were 
captured at the Vasse Bridge, six near the Bussell 
Highway Bridge and two at the Butter Factory slot- 
boards. During September 2006 no Goldfish were 
captured in the study area main channel. This, together 
with large reductions in our captures at the Bussell 
Highway Bridge and at Butter Factory, suggests that 
the control programme may have been successful in 
reducing the population; however, four Goldfish were 
captured in a wetland (New River Wetland) adjacent 
December 2003 
[ 
1 , 
0 50 too 150 200 250 300 350 400 450 
0 50 100 150 200 250 300 350 400 450 
Total length (mm) 
Figure 2. Length-frequency histograms of Goldfish captured in 
the Vasse River during December 2003, March 2004, March 2005 
and May 2006. 
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