Lucena and O'Brien: Effects of gear selectivity and different calculation methods on growth parameters of Pomatomus saltctrix 
441 
to 430 mm TL in Senegal (Conand, 1975, Champagnat, 
1983). In southern Brazil, length at first maturity for blue- 
fish is between 350 to 400 mm TL (Haimovici and Krug, 
1992) and this stock is considered to be relatively fast 
growing. 
It is difficult to obtain samples representative of the 
population’s age structure when using a single gear be- 
cause of the size selectivity of most gears (Bagenal and 
Tesch, 1978). Also, the ranges of fish length and age rep- 
resented in a study may affect the estimation of popu- 
lation parameters if they are not representative of the 
stock (Goodyear, 1995). Our study suggests that the data 
used for the estimation of the growth parameters for a fish 
stock should allow for the possible effects of gear selec- 
tivity. Data from different gears should be analyzed and 
should cover as wide an area of the stock distribution as 
possible. Moreover, the technique applied for estimation 
of growth parameters should be carefully assessed and 
the one chosen, strongly just ified. Verification of the valid- 
ity of the back-calculation method and validation of the 
annual nature of increment formation are also strongly 
recommended. 
Acknowledgments 
The authors are grateful to Gladimir Barenho for tech- 
nical field assistance in Brazil and to Jim Ellis, Richard 
Millner, Simon Jennings, Paul Kinas, and Verena Tren- 
kel for their comments on earlier drafts of the paper. The 
authors are also grateful to Manoel Haimovici for provid- 
ing additional sample data for our analysis. This study 
was partially financed by CAPES, Brazil, through a grant 
to the first author (FML) and by the Ministry of Agri- 
culture, Fisheries and Food, UK, with funding support 
(contracts MF0310 and MF0316) provided to the second 
author (CMO’B). 
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