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July 22, 2015
New Caledonia and Tahiti
Inbred Penaeus stylirostris
Alain Michel (email@example.com): On July 15, 2015, a crop of Penaeus stylirostris was harvested in Tahiti that originated from 35 years of inbreeding, starting from a population that may have consisted of only two couples originating from postlarvae introduced into New Caledonia in 1980. These populations of P. stylirostris in New Caledonia and in Tahiti are still reproducing and growing well. When you let the normal pressure of selection do its job in a local environment, you reduce the detrimental effects of inbreeding?
For high fecundity species like shrimp, it’s time to reconsider the mammalian genetic paradigm that inbreeding is bad. Inbreeding could be the best way to select the most productive animals.
John Birkett (firstname.lastname@example.org): Alain, inbreeding isn’t a deadly monster. In Ecuador we had to achieve almost a 0,50 Fis inbreeding factor to be able to develop whitespot tolerant P. vannamei. When you have stable conditions (which nature and climate seldom provide) inbreeding could work in your favor for a specific trait, but what you have to keep in mind is that in shrimp ponds, we control very few parameters, and when mother natures unbalances things like the weather, pathogens, or your water quality parameters, inbreeding becomes a major problem. I bet that when New Caledonia and Tahiti face mother natures’ unbalances, those “stylis” will have problems. So where do we draw the line with inbreeding? That, my friends, is something that remains to be determined and something that we all need to work on.
Matt (email@example.com): Alain, are the inbred blue shrimp in New Caledonia really doing well? A glance at the latest Fishstat figures from FAO show that New Caledonia’s production peaked in 2005 at close to 2,500 metric tons a year, and then declined precipitously to about 1,500 tons by 2012. I understand that this year production was a disaster due to unknown problems, mostly with broodstock dying even before they could reproduce. Is this really a good example of inbred animals doing well?
Alain Michel (firstname.lastname@example.org): Hi Matt, my answer is definitely yes. Looking at all the data available and taking into account the detrimental effects of inbreeding, there is no sign that inbreeding in New Caledonia or Tahiti caused production to drop. The data concerning the performance of female shrimp in terms of egg numbers and hatching rate have been stable for many years. For example, this year one hatchery was able to produce one batch of 35 million nauplii using eyestalk ablation and artificial insemination. In ponds, the growth rate has not changed.
But you are right to ask if the production problems were caused by inbreeding. Production dropped last year because hatcheries did not produce enough seedstock, not because of inbreeding. There was a high turnover of hatchery technicians and water quality problems and dirty filters caused many larvae to die. External pollution was also a problem. In addition, the broodstock that were harvested from the ponds were not all that healthy. Farmers think that if you raise broodstock at a very low density, they will harvest good females and males without taking a lot of care. That’s a big mistake. Broodstock are at the start of the production chain, and if they are not high quality (a high survival rate after eyestalk ablation in the maturation tanks), PL production will be low.
In the growout ponds, Vibrio outbreaks, caused by seasonal temperature changes, caused the low production.
It is interesting to note that the broodstock are not selecting for Vibrio resistance, and in the low-density broodstock ponds the Vibrio mortalities are not noticed.
Source: The Shrimp List (a mailing list for shrimp farmers). Subject: P. Stylirostris. July 16–22, 2015.
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