Effects of weaning age on survival and growth factors of Heterotis niloticus (Cuvier, 1829) larvae

Heterotis niloticus is an important fishery resource because of its individual large size and high commercial value. There is population decline due to habitat degradation and high fishing effort. The rearing of H. niloticus larvae, particularly the weaning phase, is one of the major problems for the successful farming of this species. This research aimed at investigating the age at weaning of H. niloticus larvae and its effects on survival and growth rates. Two independent series of weaning experiments were conducted under natural conditions. The first experiment (early weaning stage) included four treatments, namely: T1 (control) = larvae fed only Artemia nauplii, and T2, T3, T4 corresponding to three weaning ages of 11, 13, and 15 days after hatch (DAH), respectively. The second experiment (late weaning) included three treatments: T1 (control) =larvae fed only Artemia nauplii, and T2, T3 corresponding to two weaning ages, 24 and 26 DAH, respectively. For the early weaning experiment, eight days (9 DAH -16 DAH) of larval rearing resulted in low survival (14.7%, 10.0%, and 14.1%) and low SGR (6.56%.d-1, 6.55%.d-1, and 7.68 %.d-1) for the 11 DAH, 13 DAH, and 15 DAH larvae, respectively. The survival (65.29%) and SGR (12.55%.d-1) from Artemia fed larvae were thehighest and significantly different (

Partial replacement of fish meal with Azolla meal in diets for Nile tilapia (Oreochromis niloticus L.) affects growth and whole body fatty acid composition

The replacement of fishmeal (FM) with Azolla meal (AM) in diets on growth and fatty acid in Nile tilapia, Oreochromis niloticus (initial mean weight: 16.4 g) was studied. Fish were fed with six isonitrogenous (29.2% CP) and isoenergetic (16.9 kJ.g-1) diets containing 0%, 10%, 20%, 30%, 40% and 50% AM respectively, as partial FM substitute. Diet A0 acted as a control. The final weight (51.0-88.3 g), the weight gain (210.8-438.3%) and the SGR (1.3-1.9%day-1) decreased when AM level higher than 10% in diets (P < 0.05). The FCR (1.44 to 2.35) significantly increased when AM level exceeded 20% (P < 0.05). The protein efficiency ratio and the apparent net protein utilization decreased consequently. The fatty acids in the whole fish were significantly influenced by the diets. The replacement of FM by AM produced fish with lower (n-3) highly unsaturated fatty acids mainly docosahexaenoic acid, and higher arachidonic acid (P < 0.05). The n-3/n-6 fatty acid ratio ranged from 0.08 to 0.17, and was lower in fish fed A40 and A50 (P < 0.05). The findings suggest that high AM in diets affects Nile tilapia growth and the fern could be incorporated in diets up to 30% without any adverse effect on fish fatty acid composition. However, alternative additional source of lipid must be used to improve the quality of fish fed AM-diets.Keywords: Azolla, Fish meal replacement, Growth, Fatty acids, Oreochromis niloticus, bioconversio

Importation, releases, and establishment of Neochetina spp. (col., curculionidae) for the biological control of water hyacinth, Eichhomia crassipes CLil., Pontederiaceae), Benin, West Africa

Water hyacinth,Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae) was first reported in Bénin in 1977 and about 10 years later became the major floating water weed in the south east, obstructing boat traffic and fisheries. Water hyacinth multiplies in permanently fresh water in the swampy upper reaches of the Sô River and in tributaries of the Ouémé River. From there it is moved by wind and water flow to the coastal lagoons. The coastal lagoons are brackish during the dry season and water hyacinth eventually dies. In 1991, Neochetina eichhorniae (Warner) (Col.: Curculionidae) of South American origin was imported from Australia via quarantine in Britain to Bénin. A small infestation of the fungus Beauveria bassiana (Bals.) Vuill. (Hyphomycetes) was eliminated from the colony before release by sterilizing eggs and rearing a fungus-free generation. Between late 1991 and mid 1993, about 23, 900N. eichhorniae were released at 11 localities along the Ouémé River and in the head waters of the Sô River. Regular monitoring revealed feeding scars by adults on leaves and tunnelling by larvae in petioles at all release sites. By October 1993,N. eichhorniae had spread up to 20 km from some release sites.Neochetina bruchi Hustache was imported in 1992. A total of about 5,700 weevils have been released in six localities since mid 1992. Recoveries of offspring were made in all but one locality. Despite the negative impact of water flow, wind, penetration of salt water, and removal of infested water hyacinth by fishermen,N. eichhorniae andN. bruchi are established in Bénin in a situation typical for coastal West Africa