Variações do conteúdo energético de loricariídeos ao longo do ciclo reprodutivo na Bacia do Rio Macaé, Rio de Janeiro, RJ

Abstract

The caloric density is one of the most appropriate energy units for bioenergetic studies, and has recently been applied to measurements of energy flow in aquatic ecosystems. The present study investigated changes in caloric density of four fish species of the family Loricariidae (order Siluriformes). These species can be classified into two different groups regarding reproductive patterns: (i) Hypostomus punctatus and Neoplecostomus microps breed in the rainy season; (ii) Rineloricaria sp.1 and Rineloricaria sp.2 exhibit reproductive activity during most the year. Samples taken from March/2009 to November/2010 in seven sites distributed in the upper, middle and lower stretches of the Macaé River (RJ), captured a total of 279 individuals. Caloric density in the muscles were determined in a calorimetric bomb (IKA-C200) and the values obtained were related to reproductive stage, sex, sampling site and ontogenetic stage within the same species and in relation to abiotic variables (water temperature, conductivity and flow) and biological indices (gonadosomatic index and condition factor). The caloric density differed significantly among reproductive stages of N. microps males and in both sexes of R. sp2.but not of R. sp1. Patterns of energy variation along the reproductive cycle could be detected: females had higher values in maturation stage, low values when mature and showed a substantial recovery during the semi-spawned (R. sp1 and R. sp2) and spawned stages (N. microps), whereas males had low values of caloric density in semi-spent and spent stages (N. microps and R. sp2). Juveniles showed lower caloric density than adults. Differences in caloric density related to sex were significant only for R. sp.2, although, for all species, males exhibited lower means. Energy variations among localities can be attributed to trophic plasticity (N. microps), unequal sampling of reproductive stages among the sampling sites (R. sp1) and variations in diet (R. sp2). The caloric density of R. sp1 showed significant positive correlation with temperature and negative correlation with conductivity in both sampling sites, whereas for R. sp.2 the opposite pattern was observed, but only in one site. The condition factor was significantly correlated with caloric density only for N. microps in one of the sampling sites. The interspecific comparison revealed that the Rineloricaria species differ between each other, and R. sp1 differs from N. microps, indicating that loricariid species may show different caloric densities within a same trophic level and even within a group that exhibits a same reproductive pattern. Intraespecific energetic variations in loricariids were shown to be related to sex, ontogenetic stage, reproductive stage and subtle differences in trophic position. The variations in the caloric density of loricariids herein studied demonstrate that this parameter is dynamic even in species occupying the same trophic level and also within the same species, hence such variations should be considered in bioenergetic models in tropical lotic ecosystems.