Ictioplâncton da bacia hidrográfica do Rio Jacuí : modelagem, distribuição e ocorrência de espécies migradoras e invasora

Abstract

The Jacuí river basin is the second largest hydrographic basin in Rio Grande do Sul and, despite its relevance for biodiversity and conservation, it is not known which stretches currently maintain reproductive fish populations. In addition, it suffers numerous anthropogenic impacts, aggravated by climate change, which promotes habitat degradation and declines in fish populations. The study aimed to analyze the distribution of larvae of migratory fish species (Salminus brasiliensis, Megaleporinus obtusidens, Prochilodus lineatus and Pimelodus pintado), in the Jacuí river basin (RS), proposing statistical distribution models based on geomorphological descriptors and description of the ontogenetic stages of development of S. brasiliensis (dourado), P. lineatus (grumatã) and M. obtusidens (piava). The collections were carried out in the reproductive period (September to February), for three years (2017 to 2020), in 110 sampling points. For the collections, a cylindrical-conical net for ichthyoplankton (60cm in diameter, 3m in length and 500μm mesh) was used, during 30 minutes at each sampling point. The probability of presence of eggs and larvae was estimated through logistic regression, calibrated in the SPSS statistical software (20.0). Image processing and map creation took place in IDRISI and QuantumGis software, with coefficients calibrated in SPSS. Fish ontogenetic development was monitored with specimens from fish farming (from the egg to the juvenile stage). 660 samples of ichthyoplankton were analyzed. Among the target species, only the presence of M. obtusidens (piava) was detected in two sampling points, which made the modeling unfeasible. In turn, a species considered as short-range migrator, Pimelodus pintado (pintado), had its presence detected in 15 sampling points (13.6% prevalence) and, thus, a modeling of its distribution was carried out, which demonstrated a model precision between 66% and 75%. The estimated parameters of the equation for P. pintado are: b0= 1.863 ± 1.762; b1 (basin area, ln(km²)) = 0.128 ± 0.233; b2 (altitude, ln(m)) = -0.681 ± 0.279. The highest probabilities of occurrence of P. pintado larvae were estimated for areas of lower altitude and larger basin area. Regarding the ontogenetic development of long-distance migratory species (S. brasiliensis – dourado, P. lineatus – grumatã and M. obtusidens – piava), it was possible to observe the rapid change during ontogenetic development in a pattern of growth, appearance of characters and body color similar to those already described in other studies. The low capture of long-distance migratory species demonstrated a collapse of reproductive populations, which may be consequence of human activities and/or climate changes that promote changes in the water regime. Secondarily, the occurrence of Cyprinus carpio (carpa-comum) larvae, an exotic species, was recorded at two collection points, indicating that the species is completing its reproductive cycle in a natural environment and that adult specimens captured by amateur fishing and artisanal activities may not be the exclusive result of escapes from fish farming activities.