Zoological Sciences

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Browsing Zoological Sciences by Author "Abera, Lemma"

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    Current status and trends of fishes and fishery of a shallow rift valley lake, Lake Ziway, Ethiopia
    (Addis Ababa University, 2016-06) Abera, Lemma; Getahun, Abebe (PhD)
    Lake Ziway is an economically important lake in the country. However, the physico-chemical parameters of the lake seem to be threatened by anthropogenic and climatic factors, which in turn affect biotic factors as reflected in fish catch. A study was made to assess the status and trends of fishes and fishery during October 2012 to September 2014 in Lake Ziway. Conductivity and pH showed more or less no change during the last two decades as the mean values ranged from 361.5±9.7 to 484.51±15.3 μS/cm and 8.03±0.2 to 8.37±0.1, respectively. Dissolved oxygen content has higher values around the northern part of the lake at the inlet of Meki River (5.71±0.6) and Ketar River (6.01±0.5). Ten fish species were identified with low species diversity for the lake (H′ value of 1.67). The composition of the fishes has undergone some changes as compared to the last few decades. For instance, Carassius auratus and Labeobarbus microterolepis were not cought in this study. Inaddition, Cyprinus carpio, which was never reported inearlier catch earlier, attained the highest relative frequency (25.19 %) in this study, next to Oreochromis niloticus (27.88 %), and then followed by Carassius carassius (20.71%) and Clarias gariepinus (20.51%). Canonical Correspondence Analysis (CCA) showed that the average abundance of C. carpio, ii C. carassius and C. gariepinus were positively correlated with nutrients, whereas the abundance of Labeobarbus ethiopicus and Labeobarbus intermedius had negative correlation with most of the physico-chemical variables. Some biometric measurements (length-weight relationship, condition factor, and sex ratio) were examined for the fishes. The length-weight relationship in Barbus paludinosus, C. carassius, C. carpio, L. intermedius, C. gariepinus, Tilapia zillii and O. niloticus were curvilinear and statistically highly significant (P < 0.05). Fulton Condition Factor values (mean ± SD) of the fishes were 1.26±0.19 (B. paludinosus), 1.52±0.14 (C. carpio), 2.13±0.57 (C. carassius), 1.73±0.38 (L. intermedius), 0.76±0.21 (C. gariepinus), 1.9±0.35 (T. zillii) and 1.83±0.21 (O. niloticus) for both sexes. Females were more numerous than males for all commercially important fish species except C. gariepinus, and the ratio was significantly different from the hypothetical distribution of 1:1 (X2 > 4). The frequency of ripe C.carpio as well as GSI values were generally high during February through to June and peaking between March and May sugesting that fish resources were intensivelly in breeding condition. Fecundity, for C. carpio of fork length from 32 cm to 46 cm, ranged from 75,645 and 356,743 eggs with a mean of 210,538. Fecundity was significantly (P < 0.05) related to fork length to total weight as well as to gonad weight. The relationship between fecundity and fork length was curvilinear whereas that between fecundity and total weight and that between fecundity and gonad weight were linear. The fish catch of Lake Ziway currently declined from 3180 tons in 1997 to 1157.14 tons in 2014. Most of the threats resulted from the anthropogenic impacts on the lake. Increased pressure in fishing was a problem in the lake. Currently, plenty of pumps are abstracting fresh iii water from the lake by state and private commercial farms throughout the year that are critically impacting the water level of Lake Ziway. As a result, the lake ecosystem is being affected by catchment degradation and siltation. The study suggested that if nutrient levels continue to increase and water levels continue to decline, further changes in fish composition can be expected in the lake, especially with a shift towards fish that are mainly turbiditytolerant species such as C. carpio. The study showed that the fishery sector has been of critical importance to the economy and to the social well-being of the fishermen in the study area. However, current harvest trends and fishery conditions put these attributes of the production at risk. It is threatened with problems of open access to the resources, pollutions, marketing, and lack of technology. Hence, appropriate management is an urgent requirement that could assist in sustainable exploitation of the resources, so that the resource could contribute to food security in the study area in particular and in the country in general