Age, growth reproductive biology and fishery of rastrelliger Kanagurta (Clivier, 1817) in Zanzibar, East Africa

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University of Dar es Salaam
A total of over 4500 specimens of Rastrelliger kanagurta (Scombridae) were obtained from two of Zanzibar's most important landing sites for small pelagics, ZAFICO and Malindi Jetty and were used to investigate aspects of their reproductive biology and growth in Zanzibar waters. The largest female and male R. kanagurta recorded from the purse seine fishery were 26.4 cm (183.9 g) and 24.9 cm (157 g) respectively, whereas the smallest fish were 12 cm (15.4 g) for females and 12.5 (17.5 g) for males. The largest R. kanagurta, however, from a different fishery was 36 cm (501 g) female fish. The smallest R. kanagurta during the whole study period had a length of 9.3 cm with a weight of 7.5 g (O-group). Spawning was found to occur throughout the year with two major peaks occurring in March/ April and September/October. Two recruitment peaks were also observed probably resulting from these two spawning peaks six months later. The gonad somatic index was found to be a good indicator in determining the spawning season with higher values coinciding with the period when the fish were about to spawn. The monthly size progression of the ova is illustrated by frequency polygons, which indicate that the eggs are shed in batches. Sex ratio was 1:1 in all sizes in all the months except in the bigger sizes above 22 cm, where females were slightly more than males. Fecundity was found to relate to size and bigger females produced more eggs with the average number of eggs in one ovary ranging from 41,883 to 184,328. The mean size at which 50% of the fish mature was found to be 19 cm for males and 20 cm for females. The length-weight relationship did not show any significant difference between the sexes throughout the year. The "b" values for males and females were 3.2 and 3.0 respectively. Age was determined using the indirect length frequency method ELEFAN 1 sub routine from the FiSAT software. The fish was found to be short-lived about 2-3 years and very rarely reached 5 to 6 years. Direct ageing with otolith and scales was also tried and these were observed to possess circuli, which in otoliths were found to increase in number with the size of fish. The process of observing scales and otolith was tedious and difficult and further studies are required on this aspect, but the work indicated that daily growth ring analysis might be possible. The ELEFAN 1 analysis was also used to estimate growth rate. The parameters of the Von Bertalanffy growth curve were found to be 36 cm for L¥ and 0.63 for K. The ELEFAN 1 analysis in conjunction with Pauly's method for calculation of instantaneous natural mortality rate indicated an M of 1.29. Catch curve analysis provided an estimate of instantaneous total mortality rate Z of 3.79. Instantaneous fishing mortality rate was 1.63 (Z-M). The fishery operates throughout the year with the greatest catch during the North East Monsoon (November to March). The fishery starts on the 17th or 18th day from the sighting of the new moon to the 8th or 9th of the next new moon. This period is locally known as the "Bamvua". The surplus production analysis indicated that fishing effort has exceeded the effort which would provide maximum sustainable yield. Current effort should be reduced by 50% to maximize productivity and avoid further declines in landings.
Available in print form, East Africana Collection, Dr. Wilbert Chagula Library, Class mark (THS EAF QL639.25.A353J52 )
Fishes, Age determination, Fertility, Reproductive, Zanzibar, East Africa
Jiddawi, N. S. (2000) Age, growth reproductive biology and fishery of rastrelliger Kanagurta (Clivier, 1817) in Zanzibar, East Africa, Doctoral dissertation, University of Dar es Salaam. Dar es Salaam