Monday, December 15, 2008

A long post on tilapia

You've probably all seen tilapia as a popular item in grocery stores, usually in the frozen fish section. What many people don't realize is how important an aspect of the world's food economy it is, why it's becoming increasingly important, and what some of the advantages of tilapia are from the point of view of sustainable food production. A famous (or, alternatively, infamous, depending on whether you believe it or not) Nature paper from two years ago claimed that currently the world supplies of large pelagic fish were at about 10% of their pre-modern abundance. This is frightening enough, even if one can quibble with the methodology (what counts as 'large pelagic'? How do you really assess how many fish are out there?) It was widely sold however as saying that eating fish is bad for the environment. Well, as usual it's more complex than that. I gave a presentation on overfishing and sustainable fish production at a teacher training a while ago (as part of my government science-education fellowship) so I'd like to just just run over a few of the points we covered; I also learned a bit about tilapia raising when I lived in Madagascar, so I want to mention what's interesting and useful about tilapia.

Tilapia are any of a number of species (about 100) in the cichlid family, native to Africa and known for its capacity for rapid diversification. By legend, tilapia fisheries are what St. Peter the Apostle specialized in by trade (what with moringa and now tilapia, Scripture turns out to be a veritable handbook for sustainable food production). They are tropical fish, that vary in size up to 1 or 2 pounds (but often quite smaller), and don't survive well in water that gets below 68 degrees F- their lethal temperature is 50 degrees F. The most commonly raised species are in the genera Oreochromis, Tilapia and Sarotherodon.

One of the biggest advantages of tilapia as a farm-raised fish is that unlike most fish, at least unlike most fish that people eat, tilapia are largely herbivorous. Another Nature paper from, I believe, last year, classifies different kinds of edible fish and aquatic animals by their 'trophic level'- in layman's terms, how high on the food chain they eat. Tuna have a trophic level of about 4.2- top predators, like eagles or wolves. Sea turtles, being mostly plant eaters, have a trophic level of about 2.0, as do mollusks (plants would be at 1.0). Tilapia are at 2.7, meaning that they're largely plant eaters although they do supplement their diet with small crustaceans, etc. In fact, they will eat a lot of green matter that isn't useful for much else (algae, floating phytoplankton) and unlike, say, pigs or chickens don't need to be fed on relatively high-quality food. (Of course, in the U.S. with subsidized grain production, we do feed corn to both tilapia and cows, which are both adapted to feed on green matter- this is neither good for us, the animals, or the environment.)

Trophic level is actually extremely important in determining how efficient any type of livestock is from a natural-resource perspective. Most terrestrial birds of mammals only use about 10% of the energy they consume as food (actually, between 5% and 20%) to construct their body tissues; the rest goes into metabolism, to keep the animals' bodies functioning and to keep them warm. Ultimately this energy is 'lost', mostly as heat but some in the animal wastes. This means that when we eat a cow fed on grain, we ultimately 'lose' about 90% of the total food energy that was in the grain. That's why meat is often relatively expensive (especially from those animals like pigs or chickens that eat grains, roots, and other calorie-dense foods). Of course this isn't a direct tradeoff- cows can be raised fairly sustainably by eating grass, which obviously isn't a food that humans can eat, and in this way they serve the valuable function of converting food energy that isn't usable at all by humans, into a form where it is usable- this is a win from our standpoint, even if 90% of the energy is not usable it's better than a grass field where none of the energy is available to us in an edible form. Nevertheless, trophic level is pretty critical in determining the efficiency of different types of food production. Raising carnivores for food, since you have to feed them meat or fish, would involve even greater energy losses- on the order of about 99% of the total energy in the grass is lost if we first feed grass to, say, rodents, then feed the rodents to hawks and eat the hawks. That's why no culture has ever raised hawks for food. (The few cultures that raised dogs for food, like the Polynesians, fed them largely on plant matter).

We do, however, raise (and catch from the wild, in vast numbers) carnivorous fish- tuna, salmon, etc. The energy tradeoffs here are somewhat less acute- being cold-blooded, fish,mollusks and crustaceans convert about 10-40% of the energy they consume into body tissues, instead of only 5-20%. This is related to the fact that they don't spend much energy regulating their body temperature ("poikilothermic" is the term these days for what we used to call "cold blooded") and can use more for building tissue- edible reptiles like turtles and iguana are also considered highly efficient converters of fodder energy to human food. It's generally a big problem that so many of the fish and crustaceans we like to eat are high on the food chain (salmon, tuna and shrimp are all about 4.0), because their abundance in the wild is relatively low, and because when we raise them, it costs a lot of energy and protein to feed them, and places heavy pressure on wild fisheries (the Peruvian anchovy fishery, for example, largely gets turned into feed for the world's shrimp farms.)

Tilapia, on the other hand, are largely herbivorous fish that don't need to be fed on fishmeal, and don't contribute to the depletion of natural fisheries. They are incredibly prolific, and have a short life cycle of only about 3 months (roughly synchronous with rice crops which makes them useful as an adjunct to rice farming). Tilapia are a highly nutritious, low-fat fish; though there have been some concerns that they're much less nutritious when fed on corn, they are extremely nutritious when fed on green matter as they should be. Farmers often raise them by 'fertilizing' their ponds with submerged compost heaps or additions of manure: the nutrients will support algal growth, and the fish then eat the algae. You can also supplement their algal diet with additions of grain (corn, etc.), rice hulls, green leaves like sweet potato or taro leaves, etc. The nice thing about feeding tilapia is that much of their food, like algae or rice hulls, is inedible by people, so it's not like they are competing for food.

If left to themselves in a pond, tilapia won't ever grow very big, since they put most of their energy into reproducing. You can solve this problem by treating the pond with a sterilizing hormone. Methallibure (a dithiobiurea derivative) will reduce reproduction in tilapia while not eliminating it completely. But if you'd prefer not to, you can just settle for smaller fish. Tilapia can also be good for the environment. A recent study found that tilapia (and other herbivorous fish like catfish and carp) can actually significantly improve water clarity and reduce algal blooms in eutrophic waters. 'Eutrophic' by the way, means waters that have been subjected to abnomally high nutrient enrichment (usually from agricultural or industrial runoff): eutrophic waters are dangerous because they can result in a sudden explosion of the algal population,l which in turn attracts zooplankton and fish: when these animals die they decay and this decomposition reduces oxygen in the area, resulting in a 'dead zone' where little can grow. Temporary explosion of life, followed by prolonged barrenness. We already knew that some mollusks, like oysters in Chesapeake Bay, could help clean up algal blooms in eutrophic waters, but this is nice scientific proof of the same thing (Ask me for the reference, I have it on hand but can't look up articles on this computer right now.)

In tropical countries, tilapia are raised in all kinds of aquatic systems- small lakes, ponds, canal systems, rice paddies- and in much of Africa exist in the wild too. In the U.S., they can only really be raised indoors (and perhaps, outdoors in parts of the South). Nevertheless, many people do raise tilapia in indoor systems in Northern states. I've seen a guy in Massachusetts raising catfish before in 55 gallon barrels.....the little fellows literally swam around and around, and appeared to be doing OK. It was a low tech system, he changed the water by hand. No doubt tilapia could probably be raised in a similar low-tech way. All in all, they are definitely a fish that I would encourage trying to raise, on a small scale, to educate yourself a little in what a sustainable food production system looks like, and perhaps to get a tasty meal at the end of the day as well. (Tilapia, I should add, is a lean white fish that tastes rather 'earthy' and goes well with all kinds of recipes, but especially green vegetables and is quite tasty smoked or dried as well as fresh.) Tilapia are widely eaten throughout much of the developing world- in Central America, Southeast Asia, South America, West and East Africa, and Madagascar at the very least- and are particularly important in the diets of peasant families and poorer people.

I didn't actually get to build a fish pond in my village in Madagascar, as there wasn't enough water available (this was a dry forest region, with an average of about 800 mm annually, and sometimes less than half as much, and with hardly any elevation change.) I did see some beautiful terraced fish pond systems in the plateau though, including on the road from the west coast up to the capital (somewhere south of the capital and north of Antsirabe, I believe, some enterprising farmer has a series of ponds proceeding down a hillside that empty into each other.) I did help my host family during training (up in the plateau) dig an area for a fish pond, and I observed and offered technical advice to people on building them. This is something of a side interest of mine and if I work in the tropics in future, which I hope to do, I'd like to do what I can to encourage incorporation of tilapia raising into integrated farming systems.

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