The dourada (Brachyplatystoma rousseauxii) and the piramutaba (B.
vaillantii), two migratory freshwater catfishes, are exploited by both traditional and
industrial fishery in the Amazonian. They are very important for the commercial fishery
in this region and their stocks are currently sobrexploted. However there is a lack of
information concerning the biology and ecology of juvenile stages of these catfishes in
the scientific literature. In population dynamics studies, the correct age estimate
influences the growth parameters, which are important to evaluate stocks. The studies of
age determination are usually made using sub-adults and adults specimens. The
juveniles are not analyzed and their age is back-calculated. Which could result in an
error estimate. The aims of the present research were: (1) to describe and estimate the
relation between the lapilli otoliths morfometric measurements and the body
measurements of dourada and piramutaba, and (2) to examine the occurrence of the first
growth rings in the juvenile specimens. The morfometric and descriptive studies were
carried out using otoliths from dourada and piramutaba juveniles, sub-adults and adults
collected in the municipalities of Colares (PA), Almeirim (PA), Santarém (PA), and
Manaus (AM), in the District of Mosqueiro (PA), and in the Amazon estuary. The
period of collection was different for each local, to September of 1996 to January of
2006. The following relations were estimated: furcal length from specimens (LF) x total
weight from specimens (WT); LF x maximum otolith length (MOL); LF x maximum
otolith width (MOW); LF x otolith total weight (OWT); WT x MOL; WT x MOW and
WT x OWT. The estimated equations were: Dourada: WT = 9.5 x 10-6 x LF 3.0279, LF =
25.259 * MOL1.1571, LF = 40.626 * MOW1.2127, LF = 840.419 * OWT0.4077, WT = 0.125
* MOL3.6547, WT = 0.568 * MOW3.8294, WT = 7549.98 * OWT1.2651; Piramutaba: WT =
xxx
5.4 x 10-6 x LF 3.1441, LF = 31.871 * MOL1.3381, LF = 55.852 * MOW1.3722, LF =
1641.064 * OWT0.4686, WT = 0.3127 * MOL4.1857, WT = 1.823 * MOW4.2788, WT =
66675.977 * OWT1.4508. In the descriptive study, the lapilli of dourada and piramutaba
were classified in the following categories according to their development and
specimens LF: dourada I (60 a 80 mm), dourada II (80 a 110 mm), dourada III (110 a
210 mm), piramutaba I (65 a 80 mm), piramutaba II (80 a 110 mm), and piramutaba III
(110 a 190 mm). In the lapillus ring reading study, only the juveniles specimens £ 200
mm LF were used. In the Marajó bay, douradas and piramutabas with 175.03 mm and
171.03 mm mean LF, respectively, showed the first growth ring. Both species were
between nine and twelve months old when they showed the first growth ring. The mean
rays of the first hyaline ring in the transversal cut of dourada’s and piramutaba’s lapillus
were 0.36 mm (± 0.03) and 0.33 mm (± 0.01), respectively. The mean total ray of the
lapillus transversal cut was 0.64 mm (± 0.14) in dourada and 0.53 mm (± 0.09) in
piramutaba. The estimated equation relating the total ray of the lapillus transversal cut
of douradas juveniles with its LF was LF = 248.589 * Rt1.1901. The estimated equation
for piramutaba was LF = 400.494 * Rt1.7178. Douradas and piramutabas 60-70 mm LF
were three to six months old, 70-110 mm LF were six to nine months old, and 110-200
mm LF were nine to twelve months old.
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