Size Frequency, Length–Weight Relationships, and Condition Factors of the Four Introduced Fish Species in Geray Reservoir, Northwest Ethiopia

Studying size frequency, size relationships, and the conditions of fish is vital for managing the resources properly. Some biological aspects of the fishes in Geray Reservoir were studied from October 2017 to August 2018. Fish were sampled monthly by overnight setting of gillnets. Fishers’ catch was also used. Fish were dissected and sexed. Biological parameters were measured following standard procedures. Four fish species (Oreochromis niloticus, Tilapia rendalli, Carassius auratus, and Cyprinus carpio) belonging to the Cichlidae and Cyprinidae families were identified. From the total 570 fish specimens collected, 426 (74.7%), 66 (11.6%), 44 (7.7%), and 34 (6.0%) were O. niloticus, T. rendalli, C. auratus, and C. carpio, respectively. In Geray Reservoir, most of the sampled fish specimens were found in length intervals of 24–27, 18–24, 24–32, and 41–49 cm for O. niloticus, T. rendalli, C. auratus, and C. carpio, respectively. Length–weight relationships of O. niloticus, T. rendalli, C. auratus, and C. carpio were curvilinear and statistically significant (p < 0.05). For all introduced fish species, b values were less than 3, negative allometry growth. The mean Fulton condition factor of O. niloticus, T. rendalli, C. auratus, and C. carpio was > 1 and showed significant variation (p > 0.05) among months. In Geray Reservoir, all four economically important fish species are fully established and contribute significantly to local livelihoods. However, in this sampling scheme, the number of catches of T. rendalli, C. auratus, and C. carpio deserve due attention for the management of the exotic fish species. The data presented in this work could be vital for further biological investigations, helping researchers to assess the status of the introduced fishes in the reservoir and for local fish managers to select appropriate management strategies.