No abstract

This study examined otolith geochemistry as a natural marker of natal origins in young‐of‐the‐year (YOY) Cichla temensis in the Negro River Basin of Brazil. We analysed trace element and isotopic composition of otoliths of YOY collected off spawning nests from the main stem and major tributaries. These were compared with regional bedrock geologic composition to explore underlying mechanisms of differences in otolith geochemistry. Our results suggest that spatial differences in otolith geochemistry can be used to distinguish natal origins based on 87Sr/86Sr, Sr/Ca and Ba/Ca ratios. This approach allowed us to correctly classify 99% of juvenile fish to their natal streams using cross‐validation in a linear discriminant function analysis (LDFA). Patterns of otolith isotopic composition correspond with patterns in regional geology as expected based on previously demonstrated correlations, although some fine‐scale spatial differences cannot be accounted for by available geologic information. These results demonstrate that otolith chemistry is valuable as a natural marker of natal origins in this system and suggest that inferences from geologic maps may be useful for interpreting movements based on otolith geochemical signatures. This information provides the basis for future work to investigate the early life history and spatial ecology of this important cichlid.

The functioning of large river systems with adjacent floodplains is strongly influenced by the flood pulse. This phenomenon is the main structuring force for the biota, including fish communities that use floodplain environments for spawning, feeding, nursery and refuge. In floodplains and in the entire basin, the volume of water controls internal flows. During rising water, the high discharge of the river acts as a natural barrier to the canals that connect floodplain lakes and the Solimões River, because the water flows from river to lake. During the dry period, there is a reduction of discharge and the water flow is reversed or stationary. These canals are environments with distinct ecological characteristics such as differentiated limnology and water level variation intensely affected by the hydrological cycle. Therefore, we surveyed the influence of the flood pulse on fish communities that inhabit two canals that connect floodplain lakes to the Middle Solimões River. Particularly, we evaluated the hypothesis that the Solimões River flow direction is not perfectly parallel to its banks, which creates peripheral flows that direct water from the rivers to the floodplain lake canals. Our analysis indicated that the seasonal pattern is stronger than the spatial. Beside this, we observed that the positions of the canals in relation to the main river flow somehow affect the fish assemblages. Finally, we conclude that the flood pulse is the main structuring force acting on these fish communities.A dinâmica dos sistemas de grandes rios com áreas inundadas adjacentes é fortemente influenciada pelo pulso de inundação, sendo este fenômeno o principal agente estruturador da biota, inclusive dos peixes, que utilizam este ambiente para a desova, alimentação, berçário e refúgio. Nesta região e em toda a bacia, a intensidade da vazão controla os fluxos internos. Na época da cheia, a forte vazão atua como uma barreira natural nos canais de ligação entre os lagos de várzea e o rio Solimões, com um fluxo no sentido rio-lago. Enquanto na seca, com a redução da intensidade da vazão, o fluxo se inverte. Estes canais são ambientes com características ecológicas especiais, profundamente afetadas pelo ciclo hidrológico. Neste trabalho, estudamos a influência do pulso de inundação sobre as comunidades de peixes existentes nos canais entre dois lagos de várzea e o Médio rio Solimões, avaliando a hipótese de que a direção do fluxo do rio Solimões não é perfeitamente paralelo às margens criando, desta forma, fluxos preferenciais que direcionam o sentido da correnteza para os canais dos lagos de várzea. Nossas análises indicaram que padrões sazonais predominam sobre padrões espaciais. Além disso, observamos que a posição do canal em relação ao fluxo da corrente do rio principal afeta de alguma maneira as assembléias de peixes. Finalmente, concluímos que o pulso de inundação é a principal força responsável pela estruturação destas comunidades de peixes.

Geological heterogeneity in the Negro River basin creates differences in isotopic and elemental composition of rivers. Recent studies indicate that 87Sr/86Sr, Sr/Ca and Ba/Ca can be used to determine the natal origins of peacock bass Cichla temensis (Humboldt) and reconstruct individual movement histories. This method was used to analyse otoliths from adult C. temensis collected from two locations on the mainstem Negro River and six tributaries around these core areas. Applying discriminant functions derived from young‐of‐year reference otolith data to determine natal origins of adults indicated that 42% of adults sampled were collected from the same area in which they were spawned. However, variation in 87Sr/86Sr isotope values along the axis of otoliths indicated that C. temensis movements are much more dynamic than previously concluded. This new information on the spatial distribution of C. temensis can be used to support effective strategies for fishery management in the Negro River basin.

Colossoma macropomum is the second largest scaled fish in the Amazon basin. Reduced harvests in recent decades suggest that populations of this species are already overexploited in some areas. In this study, the population dynamics of C. macropomum was investigated using length and weight data for fish caught in the Manacapuru Lake system, a large ria lake located along the lower reach of the Solimões River. A total of 1270 individuals of C. macropomum with standard lengths varying from 17 to 62 cm and mean length of 31.0 ± 6.7 cm were sampled. Weights varied from 165 to 8195 g, and mean weight was 1148.3 ± 883.8 g. Maximum sustainable yield was estimated at 285.8 g recruit−1, corresponding to a fishing mortality of 0.51 year−1. A scenario analysis based on changes in fishing mortality and age at first capture, two parameters potentially addressed by fishing management strategies, suggested that any improvement in fishing yield depends on increases in age at first capture.