Recently the PD/A CRSP received an inquiry from an aquaculturist in Panama troubled by the lethal white spot syndrome virus, which has caused shrimp production in Panama to drop; he was seeking advice on converting from shrimp culture to tilapia culture or shrimp-tilapia polyculture. The CRSP contacted program participants Kevin Fitzsimmons, who has assisted farmers with transitions from shrimp to tilapia production or shrimp-tilapia polyculture, and Carole Engle, who is investigating the development of domestic tilapia markets in Central America, to respond to this inquiry. Recent news stories dealing with Ecuadorian shrimp producers converting to tilapia culture (see article, p. 4) and a discussion on the tilapia and shrimp e-groups discussion lists (online at <www.egroups.com/group/tilapia> and <www.egroups.com/group/shrimp>) prompted us to invite Engle and Fitzsimmons to contribute articles discussing the considerations that shrimp farmers should weigh in deciding whether to switch to tilapia production or shrimp-tilapia polyculture.
by Kevin Fitzsimmons
White spot and other diseases have affected marine shrimp production in several countries, including Ecuador, Panama, Mexico, and Thailand, in recent years. In response, some shrimp farmers have turned to polyculture or crop rotation with tilapia as an alternative production system. Polyculture options include growing the fish and shrimp loose in the pond, keeping tilapia in floating net pens in the pond, and sequential polyculturegrowing shrimp and tilapia in separate ponds or raceways and exchanging water between the ponds. Crop rotation involves stocking tilapia into ponds between shrimp crops. There are several advantages and disadvantages to consider before attempting shrimp-tilapia production.
In addressing disease issues, tilapia-shrimp polyculture provides advantages in several ways. Growers in Ecuador have reported that tilapia will consume dead or moribund shrimp in polycultured ponds. Cannibalism is one of the primary vectors for transmission of viral shrimp diseases. Tilapia, which do not appear to be susceptible to or carriers of these viruses, limit cannibalism as a mode of transmission. Tilapia also consume small crustaceans, which are of concern as potential disease vectors, in shrimp ponds. Stocking tilapia directly in the ponds, or alternating tilapia with shrimp in a crop rotation, can be effective in reducing crustacean populations. The incidence of bacterial infections also may be lessened by polyculture. Vibrio and most other bacterial pathogens common in shrimp culture are gram negative, while waters which have been used for finfish culture tend to be predominated by gram-positive bacteria. Using water from a fish culture pond seems to reduce the prevalence of bacterial infections in shrimp ponds. Growers in Asia and South America have reported increases in shrimp production due to higher survival in some of these polyculture systems; however, carefully controlled and replicated trials are needed to better study these systems and confirm these reports.
There may also be physical factors that improve shrimp survival and growth in polyculture and crop rotations. The effect of bottom sediment disturbance by tilapia is not well understood and will require close attention and careful experimentation. Tilapia disturb bottom sediments to a greater degree than shrimp, both in foraging and nest-building activities, which may be beneficial in several ways. Disturbing the bottom may improve oxidation of the substrate and interrupt life cycles of shrimp pathogens and parasites. It may also release nutrients into the water column that could improve algae blooms. However, it is also possible that these activities are detrimental. Disturbing bottom sediments may negatively impact water quality by lowering dissolved oxygen levels, increasing turbidity from sediments, and reducing algae blooms. It also makes it difficult to remove fish and shrimp, and most certainly increases the need to repair pond bottoms between crops.
Nutritional impacts are another aspect of shrimp-tilapia polyculture that should be considered. Several farmers already practicing polyculture report that tilapia aggressively feed on shrimp pellets, which may impact the shrimp growth. Freshwater shrimp (Macrobrachium rosenbergii)tilapia (Oreochromis niloticus) polyculture reduces the yield of prawns compared to monoculture, but increases total yield of fish and prawns (Garcia-Perez et al., 2000). Penaeid (saltwater shrimp)tilapia polyculture seems to be subject to the same effect. Cage culture of the tilapia, sequential polyculture, and crop rotation would avoid this problem. Considering that both shrimp and tilapia are opportunistic detritovores, wastes and feed from either may provide food for the other.
The harvest of shrimp-tilapia polyculture ponds presents several difficulties. Separating fish and shrimp if hey are grown together is time consuming and requires graders or hand sorting. Netting fish from a pond before the shrimp harvest is not practical because tilapia
It should be noted that the first serious outbreak of Taura Syndrome in the shrimp industry in Ecuador, in 1992, contributed to the first boom in tilapia production. Following that boom, prices for tilapia products softened in the US and production stagnated. The outbreak of white spot in 1998 led to the current rapid increase in tilapia production. If shrimp farmers learn to manage white spot, we may see a similar plateau in tilapia production. Farmers in Thailand, Mexico, and Brazil are also experimenting with shrimp-tilapia polyculture (Fitzsimmons, 2000).
Given the relative lack of understanding of the costs and benefits of shrimp-tilapia polyculture, growers should carefully weigh the pros and cons (Table 1) before committing to a change in production.
Fitzsimmons, K., 2000. Tilapia: The most important aquaculture species of the 21st century. In: K. Fitzsimmons and J. Carvalho Filho (Editors), Tilapia Aquaculture in the 21st Century: Proceedings from the Fifth International Symposium on Tilapia Aquaculture. American Tilapia Association and DPA/MA, Rio de Janeiro, Brazil, pp. 38.
Garcia-Perez, A., D. Alston, and R. Cortes-Maldonado, 2000. Growth, survival, yield, and size distribution of freshwater prawn Macrobrachium rosenbergii and tilapia Oreochromis niloticus in polyculture and monoculture systems in Puerto Rico. J. World Aquacult. Soc., 31(3):446451.
Kevin Fitzsimmons, of the University of Arizona, is a member of the PD/A CRSP Technical Committee, and Vice President of the American Tilapia Association.