Tuesday, 29 May 2012

Four continents and a fish

As a child in the seventies, I remember clutching my grandfather’s arm and marvelling at Tilapia being reared in a shallow pond in his land in a South Indian village. Many years later, one summer in the late nineties, in an oriental store in Ohio, USA, I was introduced to a packet of frozen fish which turned out to be Tilapia. Needless to say, I enjoyed it much and felt that it was one of the most delicious fishes I had ever eaten.

Fast forward to 2010… in a lakeside restaurant by the Sea of Galilee, in Israel, we were served St. Peter’s fish which was delectable. St. Peter’s fish is so named after the account in the gospel of St. Matthew where Jesus asks His disciple Peter to catch a fish, open the mouth of the fish, and find the coin that is to be paid as taxes. It turns out that St. Peter’s fish was indeed Tilapia. The species of the fish in Israel carries its small young in its mouth until large enough to survive on their own. The fish also seems to have a penchant for shining objects and is known to pick up small pebbles and bottle caps from the lake floor. In the idyllic restaurant by the Sea of Galilee, our tour guide repeatedly threw pieces of bread into the water and we could see Tilapia shoaling in and catching the food, vying with hoards of kittens and cats perched on the rocks that had learnt to traverse the shallow waters. Later I learnt that St.Peter’s fish has been used in small-scale low-technology commercial or subsistence fishing practices in that area for thousands of years.

I had forgotten all about Tilapia until recently I read the correspondence in Nature by Vitule et al arguing against the proposed law that, if implemented, would lead to the culture of invasive non-native species of Tilapia and other fishes in cages in dammed areas in Brazil.

The name ‘Tilapia’ is used loosely to describe over hundred species of cichlid fish from the tilapine cichlid tribe. Cichlids are most diverse in Africa and South America. The native cichlids in Asia are restricted to very few species in Israel, Iran, India, Syria and Sri Lanka. Other cichlids include fishes commonly grown in aquariums like the angel fish and discus. Tilapia seems to be highly resilient in the nature of its habitats-living in ponds, streams, lakes and rivers. Due to their ease of farming, size, protein content and taste they are among the top ten fish used in farming. Tilapia meat which is a good protein source, low in saturated fat, and calories does not accumulate mercury. Tilapia can effectively controls algae which they feed upon and replaces the need for the use of chemical algaecides. In Africa, they control pests such as mosquitos. Whilst native species are rich in nutrients, farmed species have low levels of omega 3 fatty acids due to their diet. Additionally, farmed fish are fed with hormones such as testosterone which may have endocrine disrupting effects in the food chains.

Whilst tilapia require warm waters to survive and cannot live in natural temperate habitats (indicating that farming of tilapia in controlled conditions may not disrupt the native species if accident release does take place), such a situation could be catastrophic in the long run in the tropical areas by leading to the disruption of native fish species. In Lake Victoria, introduction of the Nile Tilapia led to the disappearance of two native species. The alien species has a higher growth and fecundity rate compared to the native species. Rightly, Tilapia is listed in IUCN’s compendium of invasive alien species.

Farming of native species of Tilapia is beneficial as long as it is subsistence and sustainable farming. However, current aquaculture practices of non-native fishes often involves aggressive farming methods. Further culture of non-native species in cages in natural water bodies might pose significant risk to containment which could be compounded in sub-tropical and tropical environments where escape would lead to colonisation and disruption of the already fragile ecosystems. J.R.S. Vitule, the lead author of the correspondence in Nature from Laboratório de Ecologia e Conservação, Paraná, Brazil, whose research focuses on the conservation of the endemic freshwater fishes of the Atlantic rain forest and the management of non-native fish species that threaten their survival, says ‘Fish escapes are inevitable, and cage aquaculture may create a constant flow of propagules into the wild, establishment, spread and invasions’ (Personal correspondence with Dr. Vitule). Other issues relate to the release of nutrients such as fish feed that can lead to water pollution and also potential endocrine disruption (Ortiz -Rodas et al, 2008), a topic requiring further investigation. The debate continues……


Image source: Sarah Stephen (image shows a typical St. Peter’s fish lunch in the Sea of Galilee)


David said...

A survey of 19 cage fisheries in a hydroelectric reservoir in north Brazil, all growing Nile tilapia (Oreochromis niloticus), reported that every single one had experienced fish escapes into the reservoir. This was regarded as inevitable during the management process, and was described as "highly frequent".
Growing, losing or introducing? Cage aquaculture as a vector for the introduction of non-native fish in Furnas Reservoir, Minas Gerais, Brazil
de Azevedo-Santos et al (2011). Neotropical Ichthyology, 9, 915-919.

Ruth Stephen said...

Thank you David. It appears that containment of alien fishes is almost impossible.


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