Peter Edwards is a consultant, part time Editor and Asian Regional Coordinator for CABI’s Aquaculture Compendium, and Emeritus Professor at the Asian Institute of Technology where he founded the aquaculture program. He has nearly 30 years experience in aquaculture from being based in the Asian region. E-mail addresses:firstname.lastname@example.org.
Towards the end of last year I was invited to give a short course on Integrated Aquaculture for Rural Development at CINVESTAV, the Center for Research and Advanced Studies of the National Polytechnic Institute in Merida, Yucatan. The objectives of the course were to review Asian practice in integrated aquaculture; and to assess its relevance for the local Mayan community. I was interested also to see if the ancient Maya had been involved in aquaculture. I took the opportunity to visit the famous chinampas or “floating gardens??? in Mexico City as I was interested in exploring any possible role of aquaculture in this ancient and unique form of aquatic agronomy.The Maya
The height of Mayan culture was over a thousand years ago from 250-900 AD, the Classic Period, when many cities with large pyramids were built in Central America. Soon after this period the Mayan civilization collapsed, with hundreds of cities becoming overgrown with forest. The decline of the Mayans had ecological roots as by the 9th century they had degraded their environment through deforestation and erosion so that it could no longer sustain an extremely high population. An unusually severe drought from 800-1050 AD is now believed to have been the final factor in their downfall. However, there was continued urban development in the northern lowlands of the Yucatan peninsula which I visited although it was conquered by Toltecs from central Mexico during the Post Classic period which continued until the arrival of the Spanish in the 16th century following which most indigenous civilizations in Mexico went into decline. Photo: El Castillo, the largest pyramid at Chichen Itza supporting a flat-topped temple.
The descendants of the founders of the Mayan civilization, the present-day Maya, are relatively poor. In Yucatan most live in rural areas in traditional thatched houses with walls of vertical poles or twigs and dried soil. They farm as did their ancestors by slash-and-burn agriculture to produce maize, beans, chili peppers, squash and tomatoes. Household gardens also provide a great variety of food plants: annual root crops and vegetables and perennial fruit trees and vines.
Mayan farmers in the past also grew crops more intensively in raised fields in some areas, similar to the chinampas of central Mexico. Long and narrow rectangular ridges were raised above low-lying seasonally inundated land along rivers or in swampy depressions by heaping up rich, fertile soil. Channels between the raised fields provided irrigation water, drainage and fertile soil periodically scooped up to renew the cropped bed. According to a standard textbook on the Maya (Sharer, R.J. 1994. The Ancient Maya. Stanford University Press, Stanford, USA. 892 pp. Fifth edition), the channels between the raised fields may have been sources for harvest of fish, molluscs or other aquatic life. Sharer further wrote: “artificial channels raise the possibility that fish were raised there or in artificial ponds … or may even have been stocked??? but his case for aquaculture by the ancient Maya, further supported by his statement “cultivation and stocking of this sort are employed in many areas of the world today???, is essentially unsubstantiated. Sharer also wrote that there might have been artificial ponds to raise fish along the western coast of the Yucatan peninsula. Coastal fishing in coastal lagoons by ancient Maya is well documented by net, hook and line, and bow and arrow; and in inland highland streams by use of stupefying drugs.Northern Yucatan
The Yucatan peninsula juts into the Gulf of Mexico, which borders it on the west and north, with the Caribbean Sea lying to the east. Northern Yucatan is one of the most inhospitable places for inland aquaculture that I’ve seen. It is covered with porous limestone rock with barely enough soil for the growth of scrub forest, although there were large trees in previous times. As it is essentially a riverless plain without surface water, the major sources of water are cenotes, circular sinkholes formed by the collapse of underground caves, which are filled with water percolating through the limestone. The ancient Maya also excavated underground bottle shaped cisterns to store rainwater.Promotion of rural aquaculture
It is not possible to dig fish ponds in most of northern Yucatan as it has little to no soil above the limestone bedrock. Although I never expected to see concrete tanks replenished with pumped groundwater as a culture system for rural aquaculture, it may be the only option under these very difficult circumstances. CINESTAV and Marista University, Merida have developed an integrated system based on what they referred to as the “Thailand model??? of integrated poultry and fish in association with derelict rainwater storage tanks. These had been built by the government but abandoned, as they were dangerous for children. Photo: Poor Mayan children in their dormitory at the fosterhouse.
There were a total of 32 concrete tanks with a volume of 40-60 m3 in Yucatan province. Besides a visit to the prototype tank system at Marista University, two communities were visited where the concrete storage tanks were being used to raise fish to provide food for Mayan children. A foster house has been set up in each community on a farm with a converted rainwater storage tank. Poor children from neighbouring villages eat and sleep at the foster house and attend school in the same village, returning home at weekends.
Nile tilapia were stocked at 1,000-1,200 per tank, equivalent to 17-30/m2. Thirty poultry (muscovy ducks or chickens) were raised in a poultry shed constructed over the tank so that spilled feed and droppings would provide inputs for the fish, although light penetration into the water column was restricted by the poultry shed. Water was sprayed on to the surface of the water in the tank to provide supplementary aeration and to top up the water level. The fish were netted monthly so that the tank could be cleaned. Fish were fed about one 25 kg sack of pelleted feed monthly or about 1 kg feed per day, as well as a green fodder crop, chaya. Fish grew to 300-400 g in six months with almost no mortality. Yields of 250-400 kg were reported each crop, or a final fish density of 6-7 kg/m3. These densities of tilapia are much lower than those from continuously aerated tank or cage systems of at least 20-30 kg/m3 but are much higher than those of 0.5-1 kg/m3 from static water earthen ponds.
Left: Concrete tank surmounted by a poultry shed and surrounded by land irrigated with effluent from fish culture at a fosterhouse. Right: View inside the poultry shed with muscovy ducks.
The water level in the tank was lowered to 30 cm twice a week as the fertile water was also used to irrigate vegetables. Vegetables were sold to provide money to purchase pelleted feed.
I made a visit to two of the more than 100 commercial farms that have established circular tanks made specifically to rear tilapia in Yucatan and which used mainly pelleted feed. The first used a similar but larger system than the two foster houses, but without poultry. Use of chaya was reported to reduce the need for pelleted feed by 30-40%. The second was a rural cooperative society run by Mayan farmers. While the farmers reported no technical or marketing problems, they were concerned about their large debt with a 6-8% annual interest payment. Should they experience a major problem e.g., disease, they could become seriously indebted.Chaya
The tree spinach, Cnidoscolus chayamansa
, family Euphorbiaceae, is a fast growing shrub from the Yucatan peninsula of Mexico to Honduras and Cuba. The leaves were being harvested periodically from plants grown on the farm and fed to tilapia to reduce the cost of purchased feed. I have never seen tilapia consume green fodder so quickly. It appears to be a highly effective fish feed although there do not appear to be any experimental data. Although it is fed fresh to tilapia, it needs to be cooked before being consumed by humans to inactivate toxic hydrocyanic residues. Young shoots and tender leaves are cooked and eaten like spinach. It has a higher crude protein content than spinach (6% on a fresh and 30% on a dry matter basis) as well as being a good source of dietary minerals (Ca, K, Fe) and vitamins (ascorbic acid, beta-carotene).
As the main leafy green vegetable for the Maya today as well as in the past, it is an important dietary staple. Recently it has been introduced into the southern USA for potential use as a leafy vegetable by the Hispanic population. I was able to obtain the plant from a horticultural source in Thailand where it had been introduced from Florida. Currently it is under trials in my garden in Bangkok (but as a human vegetable rather than fish fodder). Left: Chaya, the tree spinach. Right: Chaya leaves being consumed by tilapia.Chinampas
The chinampas or “floating gardens???, as the guides colourfully describe them, are a unique form of land reclamation for agriculture in shallow lakes. They may have a 2,000-year history in the Valley of Mexico where Mexico City is located today. The capital city of the Aztecs, Tenochtitlan, was surrounded by chinampas until the Spanish conquistadors destroyed it in the 16th century. It was described by the Spanish as another Venice. At the time of the Spanish conquest, it was the largest city in the world with 200,000-300,000 people. The chinampas used to produce at least 50% of the food for the capital city. Photo: Transporting ornamental plants along a chinampa canal in Xochimilco.
Only a fraction of the former extent of the chinampas exists today with the most famous being in the town of Xochimilco, south of Mexico City. Xochimilco is a tourist area with brightly painted flat-bottomed boats plying the canals propelled by boatmen using long poles, some with mariachi bands to entertain the tourists. Most of the produce is now flowers and ornamental plants for the markets of Mexico City.
The chinampas were reclaimed from the marshy shallows along the shores of lakes and around the island city of Tenochtitan. Long and narrow rectangular enclosures were staked out in the swampy lakebed. The stakes were joined with fences of woven branches and filled with mud and decaying vegetation with narrow canals left in between. Tall slender willow trees were planted around the perimeter, which developed a dense root system that anchored the retaining walls. As well as being fertilized periodically with mud scooped up from the bottom of the canals, which was spread on the plot before planting a new crop, the Aztecs used nightsoil transported from the city in canoes.
Fish abounded in the canals as recently as 40 years ago (Coe, M.D. 1964. The chinampas of Mexico. Scientific American 211:90-98) and were netted or speared by the chinamperos, the chinampa farmers. The axolotl, a large aquatic salamander was also prized for its tender meat. It is known that the Aztecs caught fish with bag shaped nets woven from cactus fibre and also with hooks, lines and harpoons. There does not appear to be documentary evidence for the Aztecs farming fish but the ruling class had pleasure gardens with ponds containing fish.