Abstract
The present study is a diagnosis of the level of degradation of Cavally River in Cote d'Ivoire. Several abiotic indicators such as physico-chemical parameters were appreciated. These abiotic indicators were supported by biotic indicators sensitive to some variations in quality of water. That is the use of benthic macro-invertebrates as some species of freshwater shrimps of the genus Macrobrachium. Sampling was made between September 2015 and August 2016 on the upper course of Cavally River and the area on either side of the mining zone Ity. Measures of physico-chemical parameters such as dissolved oxygen, temperature of water, hydrogen potential, electrical conductivity, mercury rate and water transparency were made by season using multi parameters. Shrimps were caught monthly using dip nets and keep nets. The extreme value of physicochemical parameters is for dissolved oxygen value were 4.90 mg/l, temperature of water (25.55°C), hydrogen potential (8.48), electrical conductivity (53.14 μS/cm), mercury rate (5.75.10-3 mg/kg) and water transparency (27.06 cm). In general, the results showed that the extreme values of physico-chemical parameters were recorded in areas with high concentrations of anthropogenic activities. In the mining areas, high levels of mercury have been recorded above those of the WHO guideline for surface water. In total, M. vollenhovenii was the most abundant species with 383 individuals out of 879 specimens caught (43.57%). It is followed by M. dux with 321 individuals (36.51%) and M. macrobrachion with an abundance of 175 individuals (19.90%). Spatially, these species have been more abundant in the upstream and downstream areas which are more or less conserved environments. However, they were less abundant in mining areas. These results show that the upper course of the Cavally River is deteriorating. That constitutes a real threat to aquatic organisms and biodiversity.
Highlights
Shrimps are among the fishery resources that undeniably have a socio-economic importance for the populations [13, 5, 12, 25, 7, 4, 40, 26]
The annual overall median of the temperatures in the upper course of Cavally is superior to that recorded by N’Zi [32] in the basin of Comoe River (24.93°C). It is lower than those recorded by N’Zi [32], Aboua [1] and Boguhé [3] in the basin of Bandama River (28.35°C, 27.60°C and 28.07°C) and N’Zi [32] in the basin of Sassandra River (27.08°C) in Côte d'Ivoire. This difference in temperature observed in the upper course of Cavally River compared with other major rivers of Côte d'Ivoire may be due to the particular climatic conditions of this area
PH The annual overall median of pH values measured in the upper course of the Cavally River is weakly basic that is to say, it is slightly above neutral. These results indicate that the aquatic environment of the Upper course of Cavally River is generally in good condition
Summary
Shrimps are among the fishery resources that undeniably have a socio-economic importance for the populations [13, 5, 12, 25, 7, 4, 40, 26] They are exploited all over the world in order to address protein supply needs. Since the military-political crisis of 2002 [20], this hydrosystem is likely under the threat of unorganized and anarchic mining [34, 11] that could impact, like all other anthropogenic activities, the diversity and distribution of aquatic organisms [17, 39, 8] These three species of shrimps are well known and insufficiently harversted in Côte d'Ivoire [3, 17] and in Cavally.
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