Fish Biology and Fishery Management of Commercial Stocks in a Tropical Rift Valley Lake, Lake Langeno, Ethiopia

The study of status and trends of fish and fisheries in Lake Langeno (also known in the literature as Langano), Ethiopia was conducted from March 2014 to February 2016. The basis for this study was that fish stocks were subjected to uncontrolled fishing over many years and available data on the status and trends of fishes in the lake has become unreliable for management and sustainability of the sector. In order to examine the diversity, habitat preference and seasonal distribution, spatial and seasonal data were collected from six different sampling sites by using gillnets of various mesh sizes and long lines. Data were analyzed by Canonical Correspondence Analysis (CCA), SPSS software and by various descriptive statistics. The results indicate physico-chemical parameters showed no significant spatial variation, but high seasonal variation. A total of seven fish species dominated by family Cyprinidae were identified (H’=1.264). Carassius carassius, was not reported in previous studies, now comprise 0.64% of the total collected specimens. All the fish species were collected from all of the sampling sites, except at one site (Middle), where Carrasius carassius and Garra dembecha were absent. Index of relative importance (IRI) was essentially similar for all the sampling sites, but differed in the relative importance of each species with the high importance of Oreochromis niloticus. Principal Component Analysis (PCA) did not produce distinct habitat-associated species patterns across the sampling sites. However, temperature and depth seemed to be the key environmental factors determining fish community structure in the lake. The results showed that the composition of the fishes has undergone some changes during data collection from what was reported in the literature. The Length-weight relationships of O. niloticus, E. paludinosus, C. gariepinus, L. intermedius and C. carpio were curvilinear (b=2.872, 2.554, 2.823, 2.771 and 2.919, respectively), and statistically significant (p>0.05) except for Enteromius paludinosus. For both sexes, the mean Fulton Condition Factor values of Oreochromis niloticus, B. paludinosus, Clarias gariepinus, Labeobarbus intermedius and C. carpio were 1.77±0.37, 1.06±0.44, 0.60±0.5, 1.33±0.44 and 1.47±0.83, respectively. A better body condition was recorded in the dry seasons for all species except for O. niloticus. All fish species had relatively smaller maximum size and poor body condition than what was reported for fishes in most of the Ethiopian water bodies. Females were more abundant than males for all fish species, and the sex ratio of O. niloticus (1.13:1)and E. paludinosus (1:.24:1) was significantly different from the hypothetical distribution of 1:1 (χ, p<0.05. The identified peak breeding time was AprilJune, May-July, June-July and March-May for O. niloticus, C. gariepinus, L. intermedius and C. 5 carpio, respectively. Their respective length at L50 for females and males during these breeding times were 16.4 cm & 15.8 cm TL, 28.5 cm & 29.5 cm TL 30.5 cm & 29.5 cm FL and 28.2 cm & 27.6 cm FL. The mean fecundity of these fish species were 463.83±114, 141,466±40,982, 3,055±2,234 and 105,631±46,680, respectively. The results also showed significant seasonal variations of GSI for all fish species (ANOVA, P< 0.05). In addition, seven food items, namely phytoplankton, zooplankton, insects, detritus, macrophytes, fish parts and nematodes were identified from the stomach contents of O. niloticus. Phytoplankton was the main food grazed followed by detritus, zooplankton and macrophytes. The other food items were occasionally and randomly consumed. The volumetric contribution of these food items showed high variation in the study months (p<0.05). Phytoplankton and detritus were the dominant food items in the dry season and zooplankton and macrophytes were the highest in the wet months. The contribution of phytoplankton, zooplankton and insects were slightly high in small sized groups (<10 cm), whereas detritus, macrophytes and fish parts were the highest in larger size groups (>20 cm). Generally, food items of plant origin dominated the stomach contents, typically associated with less protein content than food item of animal origin. The result also showed that the lake supported small-scale beach seine fishery for small income generation and alternative employment opportunity of fishermen. The catch composition was dominated by O. niloticus (87.58%) followed by C. gariepinus (5.89%). Fisheries value chain was also developed and major related issues were addressed, fishers’ perceptions about the resources condition and management status were identified and discussed. Generally, the total annual fish production from the lake was estimated to be 1,137.67 tonnes/ year, and the mean gross annual revenue was about 8,622,158 ETB. Extrapolation of annual production and annual revenue from the sector showed a significance variation among the landing facility owners and those that do not have landing facilities (ANOVA, p<0.05), and between the fulltime and part time fishermen (ANOVA, p<0.05). The estimated annual per capita consumption of fish/ person was about 23.65 kg. The majority of the fishermen had positive attitude towards the fish resources, but due to lack of knowledge and follow up, the current harvest system and fishing methods put these resources at risk. Therefore, appropriate management action should be in place in order to sustain the fish resources in Lake Langeno.