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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