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Population dynamics and exploitation patterns of Oreochromis niloticus in Lake Tana, northwest Ethiopia
Author(s) -
Assefa Workiyie Worie,
Wondie Ayalew,
Enyew Belachew Getnet
Publication year - 2019
Publication title -
lakes and reservoirs: research and management
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.296
H-Index - 39
eISSN - 1440-1770
pISSN - 1320-5331
DOI - 10.1111/lre.12290
Subject(s) - overfishing , maximum sustainable yield , fishing , oreochromis , fishery , stock assessment , population , stock (firearms) , overexploitation , fisheries management , biology , mortality rate , geography , ecology , fish <actinopterygii> , demography , archaeology , sociology
Oreochromis niloticus is the dominant commercial fish in the Lake Tana region. However, its fishery is progressively declining over time. Little or no updated information exists on the population dynamics and exploitation patterns of the species, which is crucial to guide its sustainable management. Accordingly, the purpose of the present study was to generate essential biological parameters on the growth, mortality and stock status of O. niloticus, using length‐frequency data collected monthly from the commercial fish catches of 1 year (2014–2015). The total mortality coefficient ( Z ) was derived from the length‐converted catch curve. Biological reference points were predicted from relative yield‐ and biomass‐per‐recruit analyses. The estimated values of the von Bertalanffy growth parameters were L ∞ = 44.1 cm, K = 0.44/year, and t 0 = −0.34/year, and the growth performance index (Φ′) was 2.93. The total mortality ( Z ), natural ( M ) and fishing mortality ( F ) rates were 2.37, 0.98 and 1.39 per year, respectively. The current fishery exploitation rate of 0.59 exceeds the estimated biological reference points of E max (0.52), confirming the stock of O. niloticus in the lake is being overexploited above optimum levels. Size indicators of the catches further illustrate 31% of the landed fish are harvested before reaching sexual maturity, with mega‐spawners comprising only 15%. This indicates the stock is suffering from both growth and recruitment overfishing. The logistic selection model indicated 50% of the fish vulnerable to capture was at 18.14 cm TL. The fish exhibited a year‐round recruitment pattern, with a major peak during May and June. Sustainably managing the fishery, therefore, requires increasing the fish size at first capture ( L c ) towards L opt .