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Fishery Bulletin 109(1) 
gain in growth, particularly when measuring a robust, 
lively shark. The impact of this error would therefore be 
reduced when sampling intervals are years apart and 
more growth has occurred. Where growth is modest and 
measuring error occurs, the scope for inaccurate size 
estimates is greater. For this reason, size estimates of 
sharks from aquaria may be less reliable because of the 
relatively short time between measurements. However, 
this study would best be repeated in aquaria without 
unseasonably high summer temperatures. 
The results from the demographic model indicate 
that T. megalopterus can sustain very limited fishing 
pressure, and therefore these results reinforce the bio- 
logical interpretation of its life history parameters. It 
has been shown that species that are long lived, have a 
low rate of natural mortality, and produce few offspring 
per year cannot sustain high levels of fishing pressure 
(Holden, 1974; Cailliet, 1992; Simpfendorfer, 2005; Dul- 
vey and Forrest, 2010). Even small increases in fishing- 
induced mortality, particularly at the current size of 
selection will negatively impact the population. At 11 
years, sharks are harvested 4 years before the onset 
of sexual maturity. This age is of concern because the 
model predicts that an average age of a female shark is 
around 19 years and it will produce only a single female 
offspring over her lifetime. Any additional increase in 
fishing mortality would further decrease the number of 
adults and possibly contribute to recruitment overfish- 
ing. There are numerous examples in the literature of 
overexploitation and even extirpation of populations of 
chondrichthyans because their life history parameters 
are not understood and/or taken into consideration in 
management scenarios (Dulvey and Forrest, 20101. 
The life history parameters of Triakis megalopterus 
were similar to those of its congener, T. semifasciata. 
Both sharks live to about 25 years of age, have similar 
natural mortality rates, and produce similar numbers 
of embryos per annum. As a result, it is not surprising 
that the demographic model applied to both species 
shows similar trends. Cailliet (1992) recommended that 
fishing mortality be reduced to 0.5 M, and that the size 
at capture be increased to prevent a decline in abun- 
dance. We determined in this study that, at current 
selection levels, the fishing mortality rate required for 
a stable population size would be 0.02 M. Even if the 
age at 50% capture were increased significantly to 20 
years, T. megalopterus would not be able to sustain 
even moderate levels of fishing mortality. Any possi- 
ble increases in fishing mortality should, therefore, be 
closely monitored. 
Triakis megalopterus is legislated as a noncommercial 
species with zero commercial harvest. This species is, 
however, targeted by recreational anglers. From per- 
sonal observations, there has been a steady increase in 
recreational anglers targeting elasmobranchs because 
of a reduction in the availability of other favoured tel- 
eost species. Increased targeted fishing of this shark 
species could possibly result in higher levels of postre- 
lease mortality from hooking and handling. Despite its 
noncommercial status, T. megalopterus is unfortunately 
mistaken as Mustelus mustelus in a small, yet develop- 
ing, inshore shark longline fishery. Even if bycatch 
rates were to remain constant, the increased catches in 
this developing fishery would result in obvious increases 
in mortality from commercial fishing. 
Of the triakids that have been demographically mod- 
eled (Table 5), the Mustelus species appear to be the 
most resilient to harvesting pressure and have been 
shown to support sustainable fisheries (Walker, 1992; 
Chiaramonte, 1998; Francis and Shallard, 1999). The 
two reef-associated Triakis species have less habitat 
available and correspondingly smaller population sizes. 
Given their reduced habitat and life history character- 
istics, it is not surprising that their populations will 
decrease with commercial harvesting. 
Acknowledgments 
This study would not have been possible without the 
assistance of numerous anglers, and in particular M. 
Spies, who assisted with tagging with A. Goosen, who 
initiated the growth study with MJS. C. Attwood and 
his tagging team are acknowledged for their assistance 
at De Hoop, and for supplying data and material used 
in this study. Work at Bird Island was initiated under 
research permits from the Eastern Cape Department 
of Economic Affairs, Environment and Tourism, and 
South African National Parks. Financial support from 
the National Research Foundation and administration 
by Bayworld Centre for Research and Education are 
gratefully acknowledged. K. Goldman and two anony- 
mous reviewers are thanked for their helpful comments. 
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