Abstract
The aim of the present study was to evaluate the seasonal pattern of the gonadosomatic index (GSI), condition factor (K), and sex ratio in the catfish Auchenipterichthys longimanus (Siluriformes: Auchenipteridae) as an approach to identify its reproductive period. A total of 589 A. longimanus specimens (251 males and 338 females) were captured in the rivers of the Caxiuanã National Forest, in the Brazilian state of Pará, between July, 2008 and July, 2009. Among the male specimens, 171 were classified as adults and 80 as juveniles, while there were 249 adults and 89 juvenile females. Using a sinusoidal equation, analysis of the GSI revealed a reproductive asynchrony between the genders, with males attaining their highest GSI values in January, while females peaked in March. For males, the sinusoidal regression for GSI values was significant only when used the complete data set (P=0.001), wears no trend was identified for bimonthly means (P=0.136). For females, by contrast, significant values were obtained for both the complete data set (P=0.012) and bimonthly GSI means (P=0.026). For the condition factor, the sinusoidal equation returned significant seasonal variation in both raw data (P=0.02) and with mean values (P=0.00) for males, but only with raw data for females (P=0.04), which appears to reflect variation in the energy budget between genders. With regard to the sex ratio, more reproductive females were captured than males in January and March, 2009, which suggests a pattern of segregation related to the reproductive process. These parameters are fundamental to the assessment, protection, and management of natural fish stocks, as well as providing guidelines for the development of conservation strategies.
Highlights
The reproductive cycle of fish inhabiting rivers with inundated riparian areas are strongly influenced by the seasonal flood dynamics
Based on the sinusoidal equation adjusted for the gonadosomatic index (GSI), males presented the highest GSI values in January, 2009 (f .1; filling period), while the females peaked in March, 2009 (f .3; flood period)
Considering the number of captured animals, there was an overall bias towards females (χ2=12.85; P=0.000; Table 3), and the number of females exceeded that of males in March (P=0.003) and May (P=0.002), months of increased GSI values for females (Fig. 2a-2b). By analyzing both the frequency distributions of gonadal maturation stages and the seasonal variation of the GSI, we suggest an extended spawning period for A. longimanus population in rivers of the Caxiuanã National Forest, a pattern similar to the already described for other auchenipterids (Araújo et al, 1999; Bailly et al, 2008)
Summary
The reproductive cycle of fish inhabiting rivers with inundated riparian areas (várzea or igapó) are strongly influenced by the seasonal flood dynamics. According to the flood pulse concept, proposed by Junk et al (1989), the annual floodplain inundation triggers blooms of primary and secondary production, and the fish species in these complex systems have reproductive ecologies adapted to exploit this pulse of production (Bailly et al, 2008). The understanding of these seasonal patterns provides an important baseline for the description of the fish biology and their role in aquatic ecosystems (Vazzoler, 1996). The condition factor depends on the coefficient of allometry (b) of the weight-length equation, which reflects the growth pattern of a species (Le Cren, 1951)
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