Abstract We examined how the sampling technique can affect the evaluation of Minuca vocator (Herbst, 1804) population structure. We used two sampling procedures: catch-per-unit-effort (CPUE) and quadrat technique (QT). Body size, size-frequency distribution, population density, juvenile recruitment rates, proportion of ovigerous females and sex ratio were compared between the sampling procedures. QT allowed us to estimate and compare crab and burrow densities. CPUE sampled both adult crabs and the largest crabs more frequently, whereas QT recorded a greater proportion of smaller crabs. CPUE underestimated the size of M. vocator populations, while density estimates obtained with QT were accurate. The proportion of juveniles was higher with QT than with CPUE, suggesting that recruitment rates estimated by QT were more suitable. The sampling effort provided by CPUE was more efficient for obtaining ovigerous-dependent information than QT. Both sampling techniques showed a predominance of males in all three M. vocator populations. The population density estimation based on burrows overestimated the natural density of M. vocator in all mangroves. Our results suggest that neither CPUE nor QT individually were accurate sampling techniques, but together provided reliable assessments of fiddler crab populations.

Os macroinvertebrados bentônicos têm sido amplamente utilizados como bioindicadores de qualidade ambiental. O objetivo deste estudo foi comparar a qualidade da água que sai de uma área com cultivo de arroz irrigado (água de drenagem) com a água de irrigação, oriunda do Rio Gravataí, em um ponto na grande Porto Alegre. Este estudo de monitoramento foi conduzido na Estação Experimental do Arroz, pertencente ao Instituto Rio Grandense do Arroz (IRGA), em Cachoeirinha, Rio Grande do Sul (RS), ao longo da safra de 2006/2007. Uma camada de 5cm de solo contendo macroinvertebrados bentônicos foi amostrada, utilizando um Corer (78,54cm²), na fonte de irrigação da lavoura (Rio Gravataí e Canal de Irrigação) e no receptor de seus efluentes (Canal de Drenagem). Além disso, o oxigênio dissolvido (O2) na água foi registrado em cada ponto. O material foi filtrado em uma peneira com malha de 0,5mm, fixado, triado e identificado. A abundância, riqueza, biodiversidade e equitabilidade foram avaliadas e comparadas entre os pontos amostrados. Tais parâmetros ecológicos demonstraram uma comunidade mais estruturada e complexa no canal de drenagem que no canal de irrigação e no Rio Gravataí, indicando que a qualidade ambiental é melhor no ponto que recebe a água que passa pela lavoura que nos pontos de entrada da água. Portanto, a comunidade de macroinvertebrados bentônicos neste estudo sugere que os efluentes produzidos pelas cidades e indústrias, como os presentes na bacia hidrográfica do Rio Gravataí, em especial da Grande Porto Alegre, são mais agressivos aos ambientes aquáticos que os efluentes drenados de uma lavoura manejada de acordo com as recomendações técnicas para a cultura.

The macroinvertebrates have been used as indicators of environmental quality. The objective of this study was to compare the quality of water which drains from the rice flooded crop (drainage water) with the irrigation water, sourced from Gravataí River, near the Porto Alegre City, using benthonic macroinvertebrate community as bioindicator. This study was carryout at the Rice Experiment Station of IRGA, in Cachoeirinha (RS), during the 2006/2007 harvest. A layer of 5cm soil, containing benthonic macroinvertebrate was sampled, using a Corer (78.54cm²), at the irrigation source (Gravatai River and Irrigation Channel) and at the effluents receptor (Drainage Channel). Oxygen dissolved (O2) in water also was registered in each point. The sampled material was filtered in a mesh sieve with f 0.5mm and fixed. The macroinvertebrates were identified and the ecologic parameters as abundance, richness, diversity and evenness were evaluated and compared among the sample points. Such ecologic parameters showed a more complex and structured community at the drainage channel, than at irrigation channel, and at Gravatai River, indicating that the environmental features is better in the point which received the water that pass through the crop, than at the irrigation points. Therefore, the benthic macroinvertebrate community in this study suggests that effects generated by cities and industries of Porto Alegre region are more aggressive to aquatic environments than that promoted by rice crop managed according to the technical guidelines.