Impact of environmental variables on abundance, growth and condition factor of Gymnarchus niloticus (Curvier, 1829) from Umueze-Ossissa lake system, Southern Nigeria

The impact of environmental variables on abundance, growth and condition factor of Gymnarchus niloticus from two lakes in Umueze-Ossissa, Southern Nigeria was investigated to provide baseline information for effective management of the lakes. G. niloticus were collected fortnightly for a period of 12 months from fishermen who used set nets and percentage abundance determined. Length-weight relationship was used to determine growth pattern, condition factor and relative condition factor. Environmental variables such as water quality parameters affecting fish abundance were predicted with multiple regression analysis. G. niloticus was more abundant in Otu Lake than in Abiandu Lake. Male G. niloticus were significantly (P<0.05) more than female with female to male ratio of 1:1.8 and 1:1.7 in Otu and Abiandu lakes respectively. Gymnarchus niloticus showed negative allometric growth in the two lakes studied meaning that fish get slimmer as it increases in size. Condition factor of G. niloticus ranged from 0.24 to 4.09 and 0.59 to 3.73 and relative condition factor from 0.82 to 1.08 and from 0.92 to 1.12 in Otu and Abiandu Lakes respectively. High air temperature observed corresponded with high number of fish, while high amount of rainfall corresponded with low number of G. niloticus. Water quality parameters showed that depth, transparency, DO and BOD were significantly higher (P<0.05). The model used successfully predicted G. niloticus abundance. These findings are useful during planning for effective management of the lakes for sustainable economic and livelihood development of the area.