Ichthyoplankton and limnological factors in the Cinzas River - an alternative spawning site for fishes in the middle Paranapanema River basin, Brazil Ictioplâncton e fatores limnológicos no Rio das Cinzas -um local alternativo de reprodução para os peixes

Abstract

Abstract: From early spring to late summer 5 samplings were performed to characterize the ichthyoplankton distribution along the Cinzas River, a tributary of Capivara Reservoir. Eggs and larvae were sampled at dusk, midnight and dawn using stationary plankton nets. Adults and juveniles were collected with gill nets and their reproductive activity evaluated through macroscopic gonads analysis. Limnological data were simultaneously obtained. A total of 2572 eggs and 457 larvae were captured, mostly at midnight. Higher absolute number of eggs was found 61.5 km far from the mouth, and the maximum density at the river mouth. Relatively high amount of eggs was also collected in the upper sampling station (120 km from mouth) showing that the entire studied river stretch is used for fish spawning. Among the larvae Siluriformes were dominant, followed by Characiformes and Gymnotiformes. The most representative families were Pimelodidae and Anostomidae. Most larvae were found 31.5 km downstream the station with a maximum number of eggs. This indicates that the embryonic development is completed during the eggs derive. Temporally, the distribution was significantly different for eggs and larvae, and also spatially for eggs. Considering adults and juveniles, 25 taxa of Characiformes, 24 of Siluriformes, 5 of Gymnotiformes and 3 of Perciformes were identified. Correlation between eggs and percentage of sexually mature individuals was positive for most sampling stations. In general, the correlation analyses with the limnological variables demonstrated the coupling of fish reproduction with more productive and high water conditions, determined by intensive rain precipitation. Eggs were positively correlated with pH, nutrients, inorganic suspended solids, velocity, flow rate, turbidity and chlorophyll-a and negatively with conductivity and transparency. Larvae were negatively correlated with transparency and positively with inorganic suspended solids, velocity, flow rate and turbidity. A longitudinal degradation gradient (cumulative processes) in the river water quality was identified. This pattern was indicated by downstream increase in electric conductivity, nitrite and silicate and decrease in dissolved oxygen. Keywords: fish eggs, fish larvae, ichthyofauna, water quality degradation. Resumo: Desde o início da primavera até o final do verão, foram realizadas 5 amostragens para caracterizar a distribuição do ictioplâncton ao longo do rio das Cinzas, um tributário do reservatório de Capivara. Ovos e larvas foram amostrados ao entardecer, meia-noite e amanhecer, usando redes de plâncton estacionárias. Os adultos e juvenis foram coletados com rede de espera e sua atividade reprodutiva avaliada através da análise macroscópica das gônadas. Dados limnológicos foram simultaneamente obtidos. O total de ovos e larvas capturados foi de 2572 e 457, respectivamente, principalmente a meia-noite. O maior número absoluto de ovos ocorreu a 61,5 km da montante da foz e a densidade máxima na foz do rio. Quantidades de ovos relativamente elevadas também foram coletadas na estação de amostragem mais a montante (120 km da desembocadura), mostrando que todo o trecho estudado é utilizado para desova. Os Siluriformes predominaram entre as larvas, seguidos pelos Characiformes e Gymnotiformes. As famílias mais representativas foram Pimelodidae e Anostomidae. A maioria das larvas foi encontrada 31,5 km abaixo da estação com o maior registro de ovos, indicando que o desenvolvimento embrionário completa-se durante o processo de deriva. Temporalmente a distribuição foi significativamente diferente para ovos e larvas, e também espacialmente para ovos. Considerando-se os adultos e juvenis, 25 táxons de Characiformes, 24 de Siluriformes, 5 de Gymnotiformes e 3 de Perciformes foram identificados. A correlação entre ovos e indivíduos sexualmente maduros foi positiva para a maioria das estações de amostragem. Em geral, as análises de correlação com as variáveis limnológicas demonstraram o acoplamento entre a reprodução dos peixes e condições mais produtivas e de águas altas, determinadas pelo incremento 140 Vianna, NC. and Nogueira, MG