Reproductive biology and growth of Oreochromis (Nyasalapia) karongae in ponds and open water in Malawi
The general objective of the thesis is to investigate the reproductive and growth potential of Lake Malawi tilapia, Oreochromis karongae, subgenus Nyasalapia in aquaculture. Using f’ (Log10K+2Log10L¥), an overall growth performance index, it was shown that wild stocks of Oreochromis lidole (f’=2.79) were faster than Oreochromis karongae (f’=2.76) and Oreochromis squamipinnis (f’=2.58). The growth of Oreochromis karongae in captivity of f’= 3.03 (0.42-1.70% specific growth rate day -1) was comparable to Oreochromis niloticus and Oreochromis aureus but superior to Oreochromis shiranus.
Breeding in ponds was generally low achieving yields of 4.97±4.09, 0-21 fingerlings per female per month) 0.08-0.61 fingerlings per square metre per month. Use of net enclosures (hapas) improved yields to 24.7±27.9, 0-354 fingerlings per female per month. Brood-fish of 99-195g initial weights and 200-411g final weights, stocked at 1.0 fish per square metre, produced the highest number of juveniles of the three densities (0.7, 1.0, 1.7 fish [per] m2) tested. Production at higher densities was constrained by poor water quality as shown by low dissolved oxygen and pH values.
Rainfall, radiation and maximum water temperature were identified, using multiple regression analysis, as the predictor variables which significantly (P < 0.001) influenced production of young. Mathematical models isolated the following predictor variables as influencing growth the greatest in the various treatments: dissolved oxygen and initial body weight, adjusted R2 = 0.391, P < 0.032; dissolved oxygen, phosphorus and water temperature, adjusted R2 = 0.591, P< 0.0259; dissolved oxygen and water temperature. Adjusted R2 = 0.598, P < 0.011; conductivity, adjusted R2 =0.05, P< 0.010. A high ionic composition (163m mho per cm) and relatively high temperatures (20-29°C) recorded at Kasinthula (southern Malawi) enhanced fish growth to 0.97g per day compared with 0.28 g per [day] attained at Domasi (95m per cm; 15-28°C water temperature). Fish survival in ponds was adversely affected when morning dissolved oxygen values dropped below 5.0 mg per litre. Thus, compared to other tilapias, Oreochromis karongae was found to be less tolerant of low dissolved and poor water quality.
More nutritious diets (chicken layers mash vs. maize bran), exogenous hormones (pimozide, D -Ala-LH-RHa) and depth of water ( 0.8 vs. 1.25 metres) significantly (P<0.05) increased spawning success. However, irrespective of the hatchery system that was used, spawning was seasonally restricted to November-February except when exogenous hormones were applied which improved spawning to a maximum of 998 fingerlings per female per month. The sizes at first maturity was recorded at 114g (16.1 cm) showing that stunting was not extensive in this species. Histology showed that the fish could potentially spawn several times in a season[;] the failure of some to develop normally was ascribed to atretic oocytes which lacked yolk vesicles and vacuoles. Environmental factors were indirectly implicated in the poor development of oocytes.
Preliminary feeding trials conducted in concrete ponds showed that diets formulated solely
from plant proteins could support growth of juvenile Oreochromis karongae while more
appropriate feed formulations are being devised. Fish fed diets containing pigeonpeas
(Cajanus cajan). Cowpeas (Vigna unguinculata) and soybeans (Glycine max)
grew better than those which were fed on diets comprising velvet beans (Mucuna pruriens).
The crude protein levels in the diets were low: 9.0-20.0% 12.2-26.0 % (analytical values)
and 14.7-20.2% and 15.3-25.5% (literature values). Growth rates of 0.15-0.48% and 0.56-1.05%
specific growth rate per day were recorded using these plant protein diets in concrete tanks
and net enclosures, respectively.