Stock assessment of fishery target species in Lake Koka, Ethiopia

Abstract

Effective management is essential for small-scale fisheries to continue providing food and livelihoods for households, particularly in developing countries where other options are often limited. Studies on the population dynamics and stock assessment on fishery target species are thus imperative to sustain their fisheries and the benefits for the society. In Lake Koka (Ethiopia), very little is known about the vital population parameters and exploitation status of the fishery target species: tilapia Oreochromis niloticus, common carp Cyprinus carpio and catfish Clarias gariepinus. Our study, therefore, aimed at determining the vital population parameters and assessing the status of these target species in Lake Koka using length frequency data collected quarterly from commercial catches from 2007-2012. A total of 20 097 fish specimens (distributed as 7 933 tilapia, 6 025 catfish and 6 139 common carp) were measured for the analysis. Von Bertalanffy growth parameters and their confidence intervals were determined from modal progression analysis using ELEFAN I and applying the jackknife technique. Mortality parameters were determined from length-converted catch curves and empirical models. The exploitation status of these target species were then assessed by computing exploitation rates (E) from mortality parameters as well as from size indicators i.e., assessing the size distribution of fish catches relative to the size at maturity (L m ), the size that provides maximum cohort biomass (L opt ) and the abundance of mega-spawners. The mean value of growth parameters L ∞ , K and the growth performance index ø' were 44.5 cm, 0.41/year and 2.90 for O. niloticus, 74.1 cm, 0.28/year and 3.19 for C. carpio and 121.9 cm, 0.16/year and 3.36 for C. gariepinus, respectively. The 95 % confidence intervals of the estimates were also computed. Total mortality (Z) estimates were 1.47, 0.83 and 0.72/year for O. niloticus, C. carpio and C. gariepinus, respectively. Our study suggest that O. niloticus is in a healthy state, while C. gariepinus show signs of growth overfishing (when both exploitation rate (E) and size indicators were considered). In case of C. carpio, the low exploitation rate encountered would point to underfishing, while the size indicators of the catches would suggest that too small fish are harvested leading to growth overfishing. We concluded that fisheries production in Lake Koka could be enhanced by increasing E toward optimum level of exploitation (E opt ) for the underexploited C. carpio and by increasing the size at first capture (Lc) toward the L opt range for all target species. Rev. Biol. Trop. 63 (3): 755-770. Epub 2015 September 01.

Assessment of catfish (Clarias gariepinus) stock in Lake Baringo, Kenya

“…Thus, the length–weight relationship for C. gariepinus indicates isometric growth. The fish asymptotic length and growth coefficient were low, however, compared to a total length of 121 cm and a growth constant of 0.16 year −1 , as reported by Tesfaye and Wolff (). The low asymptotic length and growth coefficient were probably attributable to reported adverse environmental factors in the lake.…”

Population Dynamic Parameters of the Comber Serranus Cabrilla (Teleostei, Serranidae) in Western Mediterranean (Eastern Coast of Algeria)

“…The difference between total and natural mortality rates (Z and M respectively) gave an estimation of fishing mortality rate (F) as F = Z -M, and the exploitation rate as E = F / Z (Pauly, 1984). The stock is optimally exploited when E = 0.5 (F = M), lightly or overexploited (E ˂ 0.5 and E ˃ 0.5 respectively) (Tesfaye and Wolff, 2015). The length at first capture (L c or L c50 ) was computed from the length-converted catch curve out put by FiSAT II, it is an important tool for fishery managers, to determine the minimum size of the target species of fishery.…”

Length-based stock assessment of Oreochromis mossambicus and Oreochromis niloticus (Actinopterygii: Perciformes: Cichlidae) in multi-mesh gillnet fisheries in reservoirs of Sri Lanka

“…Montaño and Morales (2013) also recommended mesh size limits for Cynoscion acoupa (Lacepède, 1801) in Lake Maracaibo, Venezuela using Y′ × R -1 analysis. Exploitation status of major fishery target species in Lake Koka, Ethiopia, assessed by FiSAT II software, was found to be different among species and as such, the optimal levels of exploitation determined from the analysis were in conflict (Tesfaye and Wolff 2015). Novaes and Carvalho (2012) analysed length frequency data of Oreochromis niloticus population in Barra Bonita reservoir, Brazil using FiSAT software and recommended that although there was no indication of overfishing, no restrictions would need to be imposed to its fishing activities due to exotic condition of the species.…”