432 
Effects of gear selectivity and different calculation 
methods on estimating growth parameters of 
bluefish, Pomatomus saltatrix 
(Pisces: Pomatomidae), from southern Brazil 
Flavia M. Lucena 
Carl M. O'Brien 
CEFAS Lowestoft Laboratory 
Pakefield Road 
Lowestoft, Suffolk NR33 OHT, United Kingdom 
Present address (for F. M. Lucena): Umversidade Federal Rural de Pernambuco 
Departamento de Pesca. Av. Dom Manoel s/n 
Dois Irmaos. Recife, PE. Cep: 52171-900, Brazil 
E-mail address (for F M. Lucena): flavialucena@hotmail.com 
Abstract— We examined the effects 
of gear selectivity and the con- 
sequences of a chosen method 
for estimating growth parameters 
of bluefish, Pomatomus saltatrix, 
from southern Brazil. Samples were 
obtained from commercial landings 
at Rio Grande ( 1992-97 ). Age deter- 
mination for 1159 fish indicated 
that gill-net and purse-seine fish- 
eries caught 1-5 year-old and 1-7 
year-old fish, respectively. Trawl- 
ers caught fish from ages 1 to 7. 
Because of their high degree of 
selectivity, the gill nets caught the 
larger individuals of ages 1 and 
2, as well as the smaller individu- 
als of ages 3-5. Faster-growing fish 
captured with gill nets had smaller 
scales than fish of a similar length 
caught with purse seines. By con- 
trast, the slower-growing fish cap- 
tured with gill nets had larger scale 
size than fish of a similar length 
caught with purse seines. For gill 
nets and purse seines, there were 
differences between the von Ber- 
talanffy growth estimates derived 
from both mean values of back- 
calculated length-at-age and indi- 
vidual back-calculated length-at- 
age. We also recorded differences 
in growth parameters obtained 
from back-calculated length-at-age 
derived from measurements to the 
last annulus only and from mea- 
surements of all annuli up to the 
sampling age. Selectivity-related 
bias was incorporated in the esti- 
mation of growth parameters, yield- 
ing unrealistic estimates of and 
k for the gill-net growth curve. 
Manuscript accepted 18 december 2000. 
Fish. Bull. 99:432-442 (2001). 
The bluefish, Pomatomus saltatrix (L.) 
(Pomatomidae), is a highly mobile 
pelagic predator (Haimovici and Krug, 
1996), widely distributed along the con- 
tinental shelf in the temperate and 
warm waters of the Atlantic, Pacific, 
and Indian oceans (Wilk, 1977). Abun- 
dant in Brazilian waters, this species is 
commercially exploited by purse-seine 
and gill-net fleets operating in the sub- 
tropical coastal waters of Rio Grande 
do Sul between Conceigao (31°42'S) and 
Chut (33°43'S), at depths between 8 and 
100 m (Lucena and Reis, 1998) (Fig. 1). 
Purse-seine boats are approximately 
23 m in length and have 220-330 HP 
engines, whereas gill-net boats are usu- 
ally 14-16 m in length and have 90-150 
HP engines (Boffo and Reis, 1997). Gill 
nets used in the bluefish fishery are 
drift gill nets 1800 m in length and that 
have a stretched mesh size of 90 mm. 
Commercial catches of bluefish were 
first monitored from 1976 to 1983 on the 
fishing grounds from Sarita (32°37'S) to 
Conceigao at depths up to 35 m (Krug 
and Haimovici, 1991). Bluefish were 
then primarily exploited by purse seine 
but were also taken as bycatch in a gill- 
net fishery targeting wreckfish (Poly- 
prion arnericanus ) and elasmobranchs. 
Commercial catches were dominated by 
fish between 1 and 3 years old for the 
purse-seine fleet and by fish aged 2-4 
years old for the gill-net fleet ( Krug and 
Haimovici, 1989). 
As catches from the estuarine Patos 
Lagoon declined, beginning in 1982, 
the artisanal fleet began exploiting new 
fishing grounds and a fishery developed 
in shallow coastal waters (Reis, 1992). 
Since 1990, bluefish have been caught 
primarily by gill net (up to 72% of land- 
ings by weight) and the fishing grounds 
now extend southward into waters up 
to 100 m (Lucena and Reis, 1998). For 
the period 1991-95, commercial catches 
have included individuals to age 10 (Lu- 
cena, 1997). 
Since the emergence of the gill-net 
fleet, the exploitation of the bluefish has 
been increasing. From 1991 to 1996, 
there has been an 87% increase in the 
length of netting deployed and a 166% 
increase in the soak time of the gear 
(Lucena and Reis, 1998). In addition, 
there has been a decrease in mean 
length of fish caught by purse seines 
and gill nets from 1992 to 1995 (Luce- 
na, 1997). Average annual landings of 
the bluefish decreased from 3100 (dur- 
ing the period 1990-95) to 1700 tonnes 
(during 1996-98) (IBAMA 1 ). 
The growth parameters of the south- 
ern Brazilian bluefish population were 
estimated by Krug and Haimovici 
(1989), using scales from fish landed 
commercially by several gears (gill nets, 
purse seines, trawlers, long lines, and 
beach netting) from 1976 to 1983. Their 
study excluded individuals larger than 
63 cm TL because the larger fish lived 
1 IBAMA ( Instituto Brasileiro do Meio Ambi- 
ente e dos Recursos Naturais Renovaveis). 
1990-1998. Unpubl. data. Centro de 
Pesquisa do Rio Grande (CEPERG)-RS, 
IBAMA, Brazil. Cep: 96201-900. 
