Elias Dadebo

Assessment of heavy metals in water samples and tissues of edible fish species from Awassa and Koka Rift Valley Lakes, Ethiopia

The Ethiopian Rift Valley Lakes host populations of edible fish species including Oreochromis niloticus, Labeobarbus intermedius and Clarias gariepinus, which are harvested also in other tropical countries. We investigated the occurrence of six heavy metals in tissues of these fish species as well as in the waters of Lake Koka and Lake Awassa. Both lakes are affected by industrial effluents in their catchments, making them ideal study sites. Mercury concentrations were very low in the water samples, but concentrations in the fish samples were relatively high, suggesting a particularly high bioaccumulation tendency as compared with the other investigated metals. Mercury was preferentially accumulated in the fish liver or muscle. It was the only metal with species-specific accumulation with highest levels found in the predatory species L. intermedius. Lower mercury concentrations in O. niloticus could be attributed to the lower trophic level, whereas mercury values in the predatory C. gariepinus were unexpectedly low. This probably relates to the high growth rate of this species resulting in biodilution of mercury. Accumulation of lead, selenium, chromium, arsenic and cadmium did not differ between species, indicating that these elements are not biomagnified in the food chain. Values of cadmium, selenium and arsenic were highest in fish livers, while lead and chromium levels were highest in the gills, which could be related to the uptake pathway. A significant impact of the industrial discharges on the occurrence of metals in the lakes could not be detected, and the respective concentrations in fish do not pose a public health hazard.

Assessment of sustainable yield and optimum fishing effort for the tilapia ( Oreochromis niloticus L. 1758) stock of Lake Hawassa, Ethiopia

The tilapia (Oreochromis niloticus, L. 1758) stock of Lake Hawassa, Ethiopia, was assessed to estimate sustainable yield (MSY) and optimum fishing effort (fopt) using length-based analytical models (Jones cohort analysis and Thompson and Bell). Pertinent data (length, weight, catch, effort, etc.) were collected on a daily basis for 514 days during 27-12-2003 to 24-05-2005 at the major landing site known as Fish market or AmoraGedel. The sampling days fell into two periods demarcated by the date (08-04-2004) when a management action was implemented, which reduced fishing effort by half (from1954 gillnets/day to below 800 nets/day). Thus, data obtained during 27-12-2003 to 08-04-2004 pertained to the period prior to effort reduction whereas the rest pertained to the period after the reduction. The two data sets were analyzed separately to get a basis to evaluate the effect of the management action of effort reduction. The assessment gave an estimate of current yield of 526.8 t/year for the period before effort reduction whereas 441.6 t/year afterward. The predicted MSY was 514.5 t/year and 441.6 t/year for the period before and after effort reduction, respectively. The respective F-factor is estimated to be 0.5 and 1.0.This suggested that the fishing effort before the reduction of effort (1954nets/day) was very high and, as already implemented, should have been reduced by half(i.e., an F-factor of 0.5).Therefore, the implemented management measure to reduce effort below 800 nets/day is appropriate. Likewise, since the estimated MSY of441.6 t/year for the period after the reduction would be obtained at an F-factor of 1.0, it was concluded that the current level of fishing effort of 696 gillnets/day can be maintained as fopt for sustainable exploitation of the stock.

Feeding habits and trace metal concentrations in the muscle of lapping minnow Garra quadrimaculata (Rüppell, 1835) (Pisces: Cyprinidae) in Lake Hawassa, Ethiopia

Diet composition and trace metal concentration in the muscle of the lapping minnow Garra quadrimaculata (Ruppell, 1835) was investigated to study the trophic status of the species as well as to assess the level of bioaccumulation of heavy metals in the body of the fish. The study was conducted based on 328 gut samples collected from February to March (dry months) and from August to September (wet months) of the year 2011. Frequency of occurrence and volumetric methods were employed in this study. Detritus, fish eggs, macrophytes, phytoplankton and insects occurred in 54.9%, 16.2%, 43.9%, 56.4% and 26.6% of the guts, respectively and comprised 27.1%, 22.2%, 18.2%, 18.2% and 14.1% of the total volume of food, respectively. The proportions of different food items consumed varied during the dry and wet months. Fish eggs and detritus were the dominant food items during the dry months. Macrophytes and insects were also common in the diet. During the wet months, phytoplankton was the most dominant food item (33.5% by volume). Macrophytes, detritus and insects were also important in the diet. The mean volume of fish eggs and phytoplankton decreased as the size of fish increased while the contributions of detritus, insects and macrophytes increased with fish size. The present study clearly indicated that G. quadrimaculata in Lake Hawassa is omnivorous in its feeding habits. Out of the seven analyzed trace metals (Cr, Mn, Zn, Cu, Cd, Pb and Ni), Cd, Pb, and Ni were below detection limits. There was a significant difference (p<0.05) between the mean concentrations of Zn and Cr in the muscle of the fish in the dry and wet months. The concentration of Zn in the dry month was significantly lower than its concentration in the wet months (p<0.05). By contrast, the concentration of Cr in the dry months was significantly higher than its concentration in the wet months. There were no significant variations in the concentrations of Mn and Cu between dry and wet months. There was no sign of bioaccumulation of the investigated trace metals as the fish grew older. The detected elements were compared with different standards and found to be below the maximum permissible limits for human consumption. Keywords : Feeding habits, G. quadrimaculata, Ontogenetic diet shift, heavy metals, Lake Hawassa.