Cross breeding of different domesticated lines as a simple way for genetic improvement in small aquaculture industries: Heterosis and inbreeding effects on growth and survival rates of the Pacific blue shrimp Penaeus (Litopenaeus) stylirostris

Two populations of the Latin American shrimp Penaeus (Litopenaeus) stylirostris domesticated in Hawaii and in New Caledonia were previously shown to be genetically differentiated and proven highly inbred. In New Caledonia, where different Vibriosis affect shrimp production and antibiotic use is banned in growing ponds, the Hawaiian population was introduced to increase the allelic variability available for local shrimp farmers and start a genetic improvement program. Growth and survival rates of the two pure populations and the two-way F1-hybrids obtained by breeding Hawaiian animals with New Caledonian animals were assessed in several simple experiments (earthen ponds, floating cages and experimental infection challenges) during two years on two successive generations. Results were very consistent: F1-hybrids growth rates in earthen ponds were 37% (± 7% SD) higher than for pure populations. Cage experiments demonstrated no competition between the different populations when reared together or separately in a common environment. The F1-hybrids also showed better survival rates in all experiments. Combining the results on growth and survival rates leads to the conclusion that biomass production is much higher with F1-hybrid populations than with pure populations using the same quantity of juveniles stocked: biomass production in ponds was increased 1.4 and 2.3 times on year 1 and year 2 respectively, and 1.9 times in floating cages. The advantage of growing F1-hybrids appeared proportionally higher when environmental and sanitary conditions led to poorer survival (34% in year 2 vs. 56% in year 1).These results are a good example of performance improvement by heterosis effect and/or of performance loss due to inbreeding in the pure populations. This study demonstrates that aquaculture industries which cannot afford large selection programs may benefit from using two different inbred parental stocks to produce F1-hybrids for each commercial growout. This is notably true when only inbred populations are available, or when introduction of genetic variability from the wild or from other genetic resources represents a zoo-sanitary risk. In our case, the expected increase in L. stylirostris production could be around 85% (according to our average results) if producers keep stocking their ponds at their current densities using F1-hybrids. However, for sustainability reasons, it is advisable to stock F1-hybrid animals at lower densities, the gain in performance allowing producing the same amount of biomass with less input.