Early life stages and interannual variations of fish from Canoas reservoirs, lower Paranapanema River, Brazil

Aim: To verify variations of early fish stages through density, species composition and ecological attributes in the lower Paranapanema River. Methods: The Canoas I and Canoas II reservoirs were sampled during three reproductive periods (RP´s) between 2012 and 2015. Eggs and larvae were collected using conical-cylindrical plankton nets and juveniles by seines and sieves. After euthanasia and fixation, the samples were identified at the lowest taxonomic level and grouped according to their reproductive strategy and life history. Variations in the eggs, larvae, and juvenile’s densities were tested by Kruskal-Wallis with post-hoc Dunn's test from RP´s, while Student T-test detected differences between reservoirs. Non-metric multidimensional scaling ordination (NMDS) with post-hoc analysis of similarities (ANOSIM) were employed to elucidate differences in species composition. Major results: We sampled 5,513 eggs, 3,137 larvae, and 187 juveniles in Canoas I Reservoir, and 2,709 eggs, 237 larvae, and 13 juveniles in Canoas II Reservoir. Fishes were identified in seven orders, 15 families, and 31 species, with Pimelodus maculatus, Anostomidae, and small-sized characids being the most abundant. There was a predominance of non-migratory reproductive strategy (46%) and periodic life history (60%), most of them opportunistic species. Differences in densities were observed between RP´s and reservoirs. A clustering of three groups were performed to larvae and juveniles, where the divergences were statistically confirmed (p < 0.05). Conclusions: Our findings demonstrated the decrease of early fish stages density overtime and the dominance of opportunistic species, suggesting that these species complete their life cycle in the reservoirs. Thus, P. maculatus can be an important species for the health of ecosystems, in addition to serving as a fishing resource in reservoirs. However, the absence of critical sites for reproduction and the climatic events, along with human responses to this, may have impaired the conclusion of life cycle from many species.