Farm Fugitives Feasting On Fiji's Fish?

Tilapia has quickly risen the ranks as an important aquaculture fish. It's third in production behind carps and salmon, with over 1,500,000 metric tons produced every year. They're ideal fish farm species because they're omnivorous, fairly big, quick-growing, tolerate high densities quite well and are mighty tasty.

More than anything else, tilapia are hailed as one of aquaculture's greatest successes. Cheap and easy, they breed well and are considered far more environmentally friendly than other species because they can be fed a vegetarian diet. Conservation organizations have even set up a way that farmers can certify that their tilapia farm is environmentally friendly.

Tilapia species have been in Fiji since at least 1949. At first they were introcued to feed pigs, but once tilapia began to be seen as good for people, too, Fijians began farming them for human consumption. Fish ponds were constructed in the interior areas of Viti Levu and Vanua Levu to provide supplemental animal protein to the protein-deficient inland rural communities. In 1999, fish farms in Fiji produced 300 tonnes of tilapia, and outputs have continued to rise. Since most of Fiji's cultured fish are distributed locally, tilapia are feeding a lot of Fiji's people every year.

But there's a downside to being the perfect fish to farm: tilapia are also a highly damaging invasive species, for many of the same reasons they're so perfectly adaptable to aquaculture. Because they grow fast and eat whatever is available, they're very adaptable to living in just about any freshwater environment that's warm enough. they've invaded the waterways of many of the countries that farm them commercially from accidental or intentional releases from farms. In Fiji, a new study has revealed that these escapees are damaging the natural biodiversity of Fiji's waterways.

Researchers from Wetlands International, the Wildlife Conservation Society, and Conservation International set out to better understand the nature of Fiji's fresh water systems and what forces are damaging them. They sampled native fish from all over the Fiji, including 20 river basins on the major islands of Vitu Levu, Vanua Levu, and Taveuni, and at the same time recorded possible factors that might be influencing native fish density like the presence of invasive tilapia (Oreochromis spp.), forest cover, distance upstream or downstream, and water quality variables.

They found that two things correlated strongly to the diversity of native fish in an area. Firstly, they found that the more an area had been cleared for housing or other reasons, the fewer species of native fish. It was not shocking to find that human activity damaged the native ecosystems, as similar results have been found all over the world. It was the second variable that decimated native fish diversity that made headlines: the presence of tilapia species (genus Oreochromis).

Of the 89 different sample locations surveyed, 85.4% had been invaded by tilapia. The study found that where this occurred, native fish diversity suffered greatly. In areas free of Oreochromis, there were, on average, 16 species of native fish. Where the invaders lived, however, there were only four, a 75% drop in diversity. The the throat-spine gudgeon, the olive flathead-gudgeon, and other gobies were hit the hardest, including species that have been traditionally fished and eaten by Fijians for generations.

Why the tilapia have such a dramatic impact is not certain, but the researchers think that it might be a very direct reason: tilapia are known to eat larvae and juvenile fish, and the researchers think that they might be chowing down on the native young'uns.

In Fiji, the loss of native fish is as much a cultural issue as it is an environmental one. Many native species form an important part of the diet of inland communities, and, in particular, are important in small villages where fish are caught not farmed. The decline in the market trade of traditional Fijian fish damages the poorest who have the most trouble switching to other sources of income.

According to the scientists, "An ecosystem approach to management is required that: (1) incorporates conservation of forests... and (2) actively excludes introduction of Oreochromis spp." In other words, protect the forests and get rid of the tilapia.

It's a good plan for the native fish, but as the human populations continue to expand, countries like Fiji will, out of necessity, rely more and more heavily on compact, efficient means of producing food like fish farms. As they do so, it will be harder and harder to keep the fish in the farms from escaping and damaging the surrounding habitat. While this paper serves to warn of the downsides of aquaculture, it unfortunately doesn't provide a solution to the underlying problem of too many mouths to feed on limited resources. I fear that even the most eco-friendly methods of feeding the world come at a price that may be too steep for many native species.

Citation: Jenkins, A., Jupiter, S., Qauqau, I., & Atherton, J. (2009). The importance of ecosystem-based management for conserving aquatic migratory pathways on tropical high islands: a case study from Fiji Aquatic Conservation: Marine and Freshwater Ecosystems DOI: 10.1002/aqc.1086