Report of fish mass mortality from Lake Hashenge, Tigray, Northern Ethiopia and investigation of the possible causes of this event

This study was intended to report the disastrous mass mortality of Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio) in Lake Hashenge, Tigray occurred in June, 12 to 15th, 2014 and to investigate the possible causes of this phenomenon. The dissolved oxygen (DO) of the water of the lake was 2.20 mg/L in the north part and 2.09 mg/L in the south west part of the lake with an average of 2.15 mg/L. The temperature was not out of the permeable range (18.05 to 24.30°C) which was 22.19°C and pH value was 7.67. Mass kills as well as respiratory distressed fish were seen along the lake. The disaster zone was so serious in the south western part of the lake when compared with the other parts of the lake, which were estimated at several thousands of Nile tilapia and common carp were dead. Clinically, the affected fish showed asphyxia with mouth wide opened and flared gills. Respiratory stressed fishes were smaller in size, while most of the mortalities were limited to large sized fishes. Previous record indicates that similar cases of mass kills have occurred 14 years ago in 2000, but the losses were much less. Laboratory analysis of water quality indicated that the low level of DO was the primary cause for the mass mortality of the fishes. The physico-chemical characteristics of water revealed the presence of abnormal water color, low level of DO (2.39 mg/L), low secchi disk reading, and slightly alkaline pH. So it was concluded that the turnover of the lake due to the mixing of the thermally layered water was the reason for the low DO, which causes mass kills of the fishes. The mass mortality was more severe in Nile tilapia as compared to mortality of common carp.