Factors affecting the efficiency of genetic selection of the Kuruma prawn, Penaeus japonicus

Coman, Gregory John. (2003). Factors affecting the efficiency of genetic selection of the Kuruma prawn, Penaeus japonicus PhD Thesis, School of Biological Sciences, The University of Queensland.

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Author Coman, Gregory John.
Thesis Title Factors affecting the efficiency of genetic selection of the Kuruma prawn, Penaeus japonicus
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 2003
Thesis type PhD Thesis
Supervisor Associate Professor Don Fielder
Mr Peter Crocos
Dr Nigel Preston
Total pages 200
Collection year 2003
Language eng
Subjects L
270200 Genetics
630399 Fish not elsewhere classified
Formatted abstract

Global aquaculture production is currently growing at more than 10% per year, compared with 3% for terrestrial livestock and 1.5 % for capture fisheries (FAQ 2001). The growth of aquaculture is expected to continue, whereas captive fisheries are considered to have little potential for further growth. Thus, the global demand for seafood will increasingly be supplied from aquaculture. This has several implications for global efforts to balance the exploitation and conservation of the world's seafood supplies. One of the critical implications is that any increase in aquaculture production should be made through improved efficiency, rather than expansion in production area. The history of terrestrial livestock and plant industries has clearly shown that one of the most effective ways of improving production efficiency is through the domestication and selective breeding of the target species. However, in comparison with these terrestrial industries, most aquaculture industries have been relatively slow to exploit the benefits of domestication and selective breeding.

Among the various aquaculture sectors most advances in domestication and selective breeding have been achieved with finfish. The domestication and selective breeding of crustaceans has been comparatively slow. Although the total global production share of crustaceans, most of which is from farmed prawns, comprises only 6% by tonnage, their relative contribution to total value is 24% (FAO 2001). The slow progress in the domestication and selective breeding of farmed prawns can be attributed to a number of factors including the abundance and availability of wild broodstock and postlarvae, a lack of understanding of their reproductive biology and perceptions of low potential for genetic improvement. However, there is a growing awareness that the current reliance on wild broodstock is a high-risk practice and that domestication and selective breeding will both enhance production efficiency and assist in controlling diseases in prawn aquaculture.

As the penaeid aquaculture industry explores the potential to increase production through selective breeding, quantitative information about a range of traits and parameters of interest is required. Accordingly, the objectives of this study were to improve our understanding of some of the key factors that determine the efficiency of genetic selection in farmed prawns. The factors that I investigated were; genotype x environment interactions for rearing temperature and stocking density; the correlation between harvest weight and weights at earlier and later ages; the effect that behavioural interactions between different sized individuals have on population size variation; the relationship between growth and food conversion ratio; and the correlation between growth and age of sexual maturity. The species I used in this study was the Kuruma prawn, Penaeus japonicus. This species has a number of useful attributes as a model species for other penaeids and also has the highest value of all the species of farmed prawns.

Variations in the response of different genotypes when reared across different environmental conditions can result in genotype x environment interactions, which can affect the efficiency of genetic selection in breeding programs. In the present study, I examined the performance of different families (genotypes) of P. japonicus reared across a range of rearing temperatures and densities in controlled laboratory experiments.

In juvenile P. japonicus, the growth, survival and increase in biomass of different families were not consistent across a wide range of rearing temperatures (24°C to 30°C). There were significant genotype (family) x temperature interactions, with some families tolerating a broader range of temperatures than others. As a result, the ranking of families in relation to growth, survival and increase in biomass changed between temperatures. This effect was more pronounced for survival than for growth. Over a narrower temperature range (27.5°C to 31.2°C), no significant family x temperature interaction was found.

In postlarval P. japonicus, the growth, survival, increase in biomass and size variation of different families were investigated over a range of temperatures (24°C to 30°C), Survival of the postlarval families was not consistent when reared across the range of temperatures, resulting in a significant family x temperature interaction. However, no significant family x temperature interaction was found for growth, biomass increase or size variation in the postlarval prawns.

Growth of juvenile P. japonicus from different families was not consistent when reared at two stocking densities (48 and 144 individuals m"^), resulting in significant family x density interaction. The inconsistent growth of the families across these different densities resulted in a change in the relative ranking of the families at each density. Survival of the juveniles was high (>90%) at both the densities used in this study.

The significant interactions found between family (genotype) and the two environmental parameters examined in this study (temperature and density) indicate that genetic variation in the response of farmed prawns to different environmental conditions needs to be accommodated in prawn breeding programs. However, the absence of significant genotype x environment interactions when differences between rearing environments were small suggests that these interactions will have a negligible effect on the selection efficiency where differences between the selection and grow-out environments are only minor.

The ability to efficiently select for weight at the time of harvest (harvest weight) at different ages throughout the life history of the prawns could increase the range of options for selective breeding programs. However, selection at earlier or later ages will only be efficient if weights at these other ages are highly correlated with harvest weight. In this study, I examined the correlation between harvest weight and weights at both earlier and later ages. Harvest weight was strongly correlated (r2>0,8) with weights at ages within 1.5 mo from the harvest age (before and after the time of harvest). Moderate correlations (r2<0.8>0.6) were found between harvest weight and weights measured within 4.5 mo after the harvest age, and only weak correlations (r2<0.6>0.4) were found beyond 4.5 mo. The relationship between harvest weight and weights measured at 2.5 mo prior to the harvest age varied considerably between years (r2 ranged from 0.28 to 0.74), and no correlation was found when weights were measured 4 mo or more before the harvest age. These results suggest that selection based on the relative weights of individuals at broodstock age (typically 12 mo of age) or mid-way through the production cycle will not be an effective strategy of optimising growth to harvest.

The use of stocks genetically selected for lower size variation (increased growth uniformity) could increase the value of penaeid crops. However, the value of such selection will depend on the effect that behavioural interactions between different sized individuals have on growth of the prawns. I assessed this effect by comparing the growth of P. japonicus graded into different size (weight) fractions, at two different ages, with prawns that were not graded. Whilst differences were found between the different separate size fractions (size-grade treatments), the mean weights and coefficients of variation did not differ between the ungraded and combined graded treatments (net performance of all size fractions). The similar performance of the prawns in the graded and ungraded treatments indicated that behavioural interactions between different sized individuals had no effect on the size variation present in the population. The negligible effect that these interactions had on prawn growth indicate that size-grading early in the production cycle will have little effect on size variation, and suggest that any improvements in the size variation of stocks achieved through genetic selection should be maintained through to harvest.

Variation in growth rates between individual prawns may be due to variations in their ability to more effectively obtain and consume a greater amount of the available food, their ability to convert food to tissue (feed conversion), or a combination of both. Maximum production efficiency will be gained by optimising the feed conversion ratio (FCR). If variation is size among prawns is predominantly determined by variation in FCR, selection for improved growth rate could also reduce feeding costs. Conversely, if variation in size is predominantly determined by consumption, feeding costs will not be improved by selecting for improved growth rates. I examined the relationship between growth and FCR in three families of P. japonicus by comparing the growth of prawns from the lower 20 % ("runts") and upper 20 % ("runners") of the weight distribution when fed sub-satiatally and satiatally. Food conversion ratios (FCR's) were determined from the sub-satiatal treatment, using the weight of the food provided as a measure of the food intake. The mean growth of the runners was 0.050g week* faster than the runts in the sub-satiatal treatment and 0.095g week"' faster than the runts in the satiatal treatment. The growth advantage that the runners had over the runts was 18.4% in the sub-satiatal treatment and 23.1% in the satiatal treatment. By comparing the growth advantage that the runners had over the runts in the sub-satiatal and satiatal treatments, I calculated that food conversion accounted for 79.7% of the growth difference between the runts and runners. Consequently, differences in food conversion had a large contribution to the different growth of the runts and runners. These results suggest that genetic selection for growth will improve the FCR of farmed prawns.

Selection of stocks for reduced age at sexual maturity can facilitate increases in the rate of genetic improvement through a reduction in the generation interval. If age at sexual maturity (first maturation) is correlated with growth, a reduction in the generation interval could be obtained indirectly through selection for growth, I examined the relationship between age and weight at first maturation (first sign of ovarian development to stages 3 & 4) and growth in P. japonicus over a period of 7 months. Growth to first maturation was negatively correlated with age at first maturation, indicating that faster growing females matured at an earlier age than slower growing females. Additionally, faster growing females reached sexual maturity at a lower weight than slower growing females. These results suggest that selection of prawns for fast growth will produce stocks that mature at a younger age and lower weight.

The results of this study demonstrate that the design and application of selective breeding programs for P. japonicus needs to accommodate genotype x environment interactions and selection for traits at the appropriate age. My results have shown that there is little to be gained by size-grading prawns early in the production cycle. However, the results have demonstrated that selection for improved growth is also likely to result in improved feed conversion ratios and an earlier age of sexual maturity. The application of this knowledge in the development and implementation of selective breeding programs should significantly enhance their efficiency.

Keyword Penaeidae

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
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Created: Fri, 24 Aug 2007, 18:05:40 EST