The impact of gillnet selectivity on immature Nile tilapia (Oreochromis niloticus L.) (Pisces: Cichlidae) in LakeHawassa, Ethiopia
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Abstract
The impact of gillnet selectivity on immature Nile tilapia (Oreochromis niloticus L.) in Lake Hawassa was studied based on length distribution of 10,274 fish samples obtained from the landings of the fishermen from March- May 2012. The aim of the study was to know the effect of the mesh size used by the fishermen on immature fish before they reach their spawning size. The mean length of the fish caught was 22.6 ±0.2 cm. The sizes at first maturity (Lm50) of 1,141 male and1,447 female O. niloticus were determined using logistic curve and found to be 16.8 cm and 19.8 cm TL, respectively. In order to find the girth circumference of the fish at a given length, linear regression line was fitted by using the total length (TL) as abscissa and the girth circumference (GC) as ordinate of 366 fish samples. The relationship was described by the formula: GC = 0.749TL – 0.054, r2 = 0.987. Since the GC is twice the length of the mesh size of the gillnet used in capturing the fish, it is possible to calculate the minimum
mesh size that should be used to capture fish at its size of Lm50. Accordingly 7.5 cm stretched mesh size was found to be the minimum mesh size of the gillnet that should be used in tilapia fishery in Lake Hawassa. This mesh size is slightly lower than the mesh size used to capture the mean length of 22.6 cm TL which is 8.4 cm stretched mesh size. The current mesh size used in the lake is slightly higher than the minimum mesh size used to capture tilapia at its Lm50 but care should be taken not to lower the mesh size any further otherwise immature fish will be affected by the fishery. The current fishing activity captures about 20% of fish that are below the Lm50. This information is vital for management purpose that intends to minimize the impact of the gillnet selectivity on immature tilapia in Lake Hawassa.
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Alemayehu Negassa and Prabu, P.C. (2008). Abundance, food habits, and breeding season of exotic Tilapia zillii G. and native Oreochromis niloticus L. fish species in Lake Ziway, Ethiopia. Maejo International Journal of Science and Technology, 2(2), 345-360.
Askey, P.J., Post, J.R., Parkinson, E. A., Rivor, E., Paul, A. J., & Biro, E. A. (2007). Estimation of gillnet efficiency and selectivity across multiple sampling units: A hierarchical Bayesian analysis using mark-recapture data. Fisheries Research, 83, 162-174.
Ayaz, A., Kale, S., Cengiz, O., Altinagac, U., Ozekinci, U., Ostekin, A., & Alfin, A. (2009). Gillnet selectivity for Bogue Boops boops caught by drive-in fishing method from Northern Aegean Sea, Turkey. Journal of Animal and Veterinary Advances, 8, 3537-2541.
Bjørkli, S.G. (2004). The fisheries in Lake Awassa, Ethiopia; estimation of annual yield. M.Sc. thesis, Agricultural University of Norway.
Demeke Admassu (1994). Maturity, fecundity, brood-size, and sex ratio of tilapia (Oreochromis niloticus L.) in Lake Awassa, Ethiopia. SINET: Ethiopian Journal of Science, 17, 53-69.
Demeke Admassu (1998). Age and growth determination of tilapia, Oreochromis niloticus L. (Pisces: Cichlidae) in some lakes of Ethiopia. PhD dissertation, Addis Ababa University, Addis Ababa.
Elias Dadebo (2000). Reproductive biology and feeding habits of the catfish Clarias gariepinus Burchel (Pisces: Claridae) in Lake Awassa, Ethiopia SINET: Ethiopian Journal of Science, 17, 53-69.
Fabi, G., Sbrana, M., Biagi, F., Grati, F., Leonari, I., & Sartor, P. (2002). Trammel net and gill net selectivity for Lithognathus mormyrus (L. 1758), Diplodus annularis (L. 1758), and Mollus barbatus (L. 1758) in the Adriatic and Ligurian Seas. Fisheries Research, 54, 375-388.
Fasil, D., Gashaw, T., & Fekadu, F. (2012). Study on the adaptability status and reproductive success of Oreochromis niloticus L. (Pisces: Cichlidae) and carp (Cyprinus carpio L. 1758) in a tropic reservoir (Fincha, Ethiopia). International Journal of Aquaculture, 2(10), 65-71.
Hamley, J.M. (1975). Review of gillnet selectivity. J. Res. Board Can., 32, 1943-1969.
Holden, M.J., & Raitt, D.F.S. (1974). FAO Manual of fisheries science. Part 2. Methods of Resource Investigation and their Application. Fisheries Technical Paper No. 115. Rome, Italy.
Karakulak, F.S., & Erk, H. (2008). Gill net and trammel net selectivity in the northern Aegean Sea, Turkey. Scientia Marina, 72, 527-540.
Lamb, H. F., Teleford, R. J., Leng, M. J., Mohammed, M. U., & Lamb, A. L. (2011). Climatic and non-climatic effects on the delta O-18 and DeltaC-13 compositions of Lake Awassa, Ethiopia, during the last 6.5 Ka. Quaternary Science Review, 21, 20-22.
LFDP (1996). Lake Fisheries and Development Project, Fisheries Statistical Bulletin, No 2 FRDD, MOA. 1-35 p.
Njiru, M., Okeyo-Owuor, J.B., Muchiri, M., & Cowx, I.G. (2004). Shifts in the food of Nile tilapia, Oreochromis niloticus (L.) in Lake Victoria, Kenya. African Journal of Ecology, 42, 163-170.
Park, C.D., Jeong, E.C., Shin, J.K., An, H.C., & Fujimori, Y. (2004). Mesh selectivity of encircling gill net for gizzard shad Konosirus punctatus in the coastal sea of Korea. Fisheries Science, 70, 553-560.
Shipton, T., Tweddle, D., Watts, M. (2004). Introduction of the Nile tilapia (Oreochromis niloticus) into the Eastern Cape. Enviro-Fish Africa (Pty) Ltd, Ocean Terrace Park, East London, 22 pp.
Stergiou, K. I., & Brizini, K. (2002). Comparative fixed gear studies in the Cyclades (Aegean Sea): Size selectivity of small-hook longlines and monofilament gillnets. Fisheries Research, 58, 25-40.
von Brandt, A. (1975). Enmeshing nets: gillnets and entangling nets- the theory of their efficiency. EIFAC Tech. Paper. 1:96-116.
Wootton, R. J. (1998). Ecology of Teleost Fishes, 2nd ed. Kluwer Academic Publishers, London.
Yirgaw Teferi, Demeke Admassu, & Seyoum Mengestou (2000). The food and feeding habit of Oreochromis niloticus L. (Pisces: Cichlidae) in Lake Chamo, Ethiopia. SINET: Ethiopian Journal of Science, 23(1), 1-12.
Yosef, T.G., & Casselman, J.M. (1995). A procedure for increasing the precision of otolith age determination of tropical fish by differentiating biannual recruitment. In: Recent Developments in Fish Otolith Research, pp.247-269, (Secor, D.H., Dean, J.M., and Campana, S.E., eds). University of South Carolina Press, Colombia.
Zerihun Desta, Børgtrom R., & Zinabu, Gebre-Mariam (2008). Habitat use and trophic position determine mercury concentration in the straight fin barb Barbus paludinosus, a small fish species in Lake Awassa, Ethiopia. Journal of Fish Biology, 73, 477-497.
Zinabu, Gebre-Mariam (1988). Dynamics of heterotrophic bacterioplankton in an Ethiopian Rift Valley lake (Awassa). Ph.D Thesis, Canada: University of Waterloo.