Estimates of the size at first maturity from Gonadossomatic Index (IGS) data: <i>Oligosarcus jenynsii, Oligosarcus robustus, Hoplias malabaricus, Cyphocharax voga, Astyanax fasciatus </i>(Characiformes), <i>Parapimelodus nigribarbis, Pimelodus maculatus

Abstract

Estimates of the size at first maturity from Gonadossomatic Index (IGS) data: Oligosarcus jenynsii, Oligosarcus robustus, Hoplias malabaricus, Cyphocharax voga, Astyanax fasciatus (Characiformes), Parapimelodus nigribarbis, Pimelodus maculatus, Trachelyopterus lucenai, Hoplosternum littorale, Loricariichthys anus (Siluriformes) and Pachyurus bonariensis (Perciformes) in Lago Guaíba and Laguna dos Patos, RS, Brazil.

Correct estimates of the size at first maturity (L50) are necessary for fish stock management and different methods have been proposed. In this study we apply a modified logistic model to estimate the size at first maturity from Gonadossomatic Index (GSI) data. This research results from a monthly sample program carried out in Lagoa do Casamento (November 2002 to April 2004) and Lago Guaíba (June 2005 to May 2006), Rio Grande do Sul, Brazil. The specimens were sampled using gillnets of different mesh sizes (30 meters long; 15, 20, 25, 30, 35, 40, 50, 60 e 70 mm between knots, 36 hours/month). The length at first maturity (LMAT) of each fish specie was estimated trough a modified logistic equation, adjusted from GSI data through Levenberg-Marquardt algorithm (SPSS 11.5): IGSMED=IGSMIN+(IGSMAX-IGSMIN)/ (1+exp(-Tax*(LTOT-LMAT))); where IGSMED is the average GSI for females in each size class; IGSMIN is the average GSI for juvenile females; IGSMAX is the average GSI for adult females; Tax is the rate of changing from juvenile to adult; LTOT is the average total length of each size class; and LMAT is the length at first maturity. For species with sample size too small to adjust the model, the first maturation (LMIN) were estimated as the total length of the smaller female with GSI equal or larger than 30% of the maximum registered GSI for the specie. The length at first maturity of Oligosarcus jenynsii was estimated as 16.2cm (LMAT; SE=0.167). For Oligosarcus robustus the reproductive activity starts at 21.4cm (LMAT; SE=0.463) of total length. The first maturity was determined as 13.9 cm (LMIN) for Hoplias malabaricus females. Cyphocharax voga starts to reproduce with 18.5cm (LMAT; SE=0.738) of total length. The length of first maturity for Astyanax fasciatus was estimated as 12.2cm (LMAT; SE=0.661). For Pimelodus maculatus females the reproductive activity starts with 18.0cm (LMIN). The first maturity was estimated at 8.0cm (LMIN) for Parapimelodus nigribarbis. Hoplosternum littorale starts to reproduce with 14.1cm (LMAT; SE=0.199). For Loricariichthys anus females, first maturity was estimated at 27.0cm (LMAT; SE=1.151). The length at which Pachyurus bonariensis starts reproduction is 17.2cm (LMAT; SE=2.024). Finally, the first maturity of Trachelyopterus lucenai was estimated as 13.7cm (LMAT; SE=0.537). Keywords: Fish, first maturity, Guaíba, Laguna dos Patos, Lagoa do Casamento, GSI.