Introduction
Currently many small and medium sized tilapia farms in developing countries underperform in terms of feed conversion ratio (FCR), despite the use of genetically improved strains of tilapia such as GIFT. The variable and often lower production in commercial environments compared to nucleus selection environments is a major bottleneck to increase productivity in tilapia farming. Feed cost is the major variable cost in fish farming (El-Sayed, 1999, Craig, 2009) and therefore this underperformance affects the profitability of fish farms negatively. This so-called yield gap has severe consequences for the effectiveness of breeding programs. The objective of this review was to analyse the most important environmental and management factors with significant contributions to growth, increased mortality and reduced feed efficiency of Nile tilapia.
Materials and methods
We conducted a systematic literature search on two data bases ASFA and CAB-Abstract on the 7th of July 2016. The search resulted in 1973 potentially relevant articles from which 59 articles met the inclusion criteria. After categorizing the studies based on the factors they investigated, we extracted data on the following variables: stocking density, photoperiod, culture unit, strain, whether aerated or not, feeding rate, feeding frequency, level of crude protein (CP), stocking weight (IBW) and harvest weight, water temperature, pH, salinity, feed conversion ratio (FCR) here defined as the ratio of total feed given / total biomass of fish harvested, dissolved oxygen (DO), survival and calculated thermal growth coefficient (TGC).
Results
Using linear regression we found significant effects of strains (Abassa, FaST, GIFT, Imaela, Maryut and unspecified strain), culture unit, study length, IBW and feeding rate on FCR, survival and growth, calculated as TGC. The effect of strain was also significant on FCR and survival. FCR, survival and TGC increased with higher levels of dissolved oxygen (DO) and with higher levels of CP in the diet. Increased water temperature (range 21.5 to 28.5oC) favourably reduced FCR and improved survival (range 10 - 37oC). Higher salinity (range 0 to 30 ppt) reduced the survival of fish and growth (range 0 - 24ppt). Increased feeding rate improved survival and TGC while FCR was negatively affected by increased feeding rate. Increased dark photoperiod had a significant negative effect on TGC. The effect of stocking density on FCR, survival and TGC was not significant for the range of densities reported in the literature.
Discussion
We found that the strains Abassa, FaST, GIFT, Imaela and Maryut were better in FCR than the unspecified strain. The probable reason could be that FaST, GIFT, Abassa, Maryut and Imaela strains have been selected for growth, which may have improved FCR as well. Ridha (2006) reported FaST and GIFT strains performed significantly better than unselected strains in terms of FCR. Furthermore, the strong negative genetic correlation of close to minus one between growth and FCR (Thoa et al., 2016) reported for Nile tilapia indicates that selecting for growth improves FCR. Selection for improved growth in Atlantic salmon also resulted in better FCR (Thodesen et al., 1999). In agreement with our study, stocking density was not significant in rainbow trout (Boujard T. et al. (2002) and in Oreochromis spilurus Cruz and Ridha (1989), while in African catfish (Clarias gariepinus) FCR was found to improve significantly with increasing stocking density from 4 fish m-3 to 8 fish m-3 (Toko et al., 2007).
In line with our result Abdel-Tawwab (2012) found fish fed with a higher CP level dealt better with stress than fish fed with a lower CP level. The effect of salinity on survival and the effect of salinity and dark photoperiod on TGC was negative and significant. The effect of salinity could be explained by high energy cost of osmoregulation (Prunet and Bornancin, 1989) which reduces available energy for growth. The negative effect of dark photoperiod could be explained by the fact that tilapias are diurnal feeders and an increased dark photoperiod could affect the feed intake negatively. We found also the effect of feeding rates was significant. Survival and TGC were improved with increased feeding rate, while the best FCR was at lowest feeding rate.
Aquaculture farms should give emphasis to the major environmental and management factors to minimize or avoid the yield gap. This could be achieved by optimizing management and optimizing breeding programs for the environments in which commercial fish will perform, especially if some variables are beyond management control and subject to genotype by environment interaction.
References
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Boujard, T., Labbé, L. & Aupérin, B. (2002) Feeding behaviour, energy expenditure and growth of rainbow trout in relation to stocking density and food accessibility. Aquaculture Research, 33, 1233-1242.
Craig, S. (2009) Understanding Fish Nutrition, Feeds, and Feeding, pp. 4. Virginia Polytechnic Institute and State University.
Cruz, E.M. & Ridha, M. (1989) Preliminary study on the production of the tilapia, Oreochromis spilurus (Günther), cultured in seawater cages. Aquaculture Research, 20, 381-388.
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Thodesen, J., Grisdale-Helland, B., Helland, S.J. & Gjerde, B. (1999) Feed intake, growth and feed utilization of offspring from wild and selected Atlantic salmon (Salmo salar). Aquaculture, 180, 237-246.
Toko, I., Fiogbe, E.D., Koukpode, B. & Kestemont, P. (2007) Rearing of African catfish (Clarias gariepinus) and vundu catfish (Heterobranchus longifilis) in traditional fish ponds (whedos): Effect of stocking density on growth, production and body composition. Aquaculture, 262, 65-72.