African Fish in Trouble
Ann Arbor, MI — A Nile perch species, known as Lates stappersii, is the top predator in the open waters of Lake Tanganyika, and in the past few years its numbers have been declining. Lates stappersii is a very important food fish for the region. It is the second most harvested fish in Lake Tanganyika.
"With two other open water species, the fish makes up about 60% of the local animal protein demands," states Aboubakar Sako, researcher from the University of Cape Town, South Africa.
Although little is known about the causes of this reduction, scientists are studying the life history of Lates stappersii in order to better understand the fish and learn why it is declining and possibly find ways to increase its population.
They know that this Nile perch species is a visual and opportunistic feeder. Juvenile individuals feed on small aquatic plants, and live within the first 10 m (33 feet) depth, whereas at maturity they prey on a sardine-like fish species or on shrimp and live in cool and deep waters.
We studied the otolith (fish earstone) of juvenile and adult fish. We know that the ratio of temperature and oxygen isotopes in the otolith of young fish is similar to the surface temperature of the lake and in adult fish the ratio is similar to the temperatures found in water over 260 feet deep. "The otolith reflects the environment where the fish lives," said Sako.
The South African and American scientists also found that the otolith can tell them what the fish eat at different stages in life. Fish otoliths are also affected by the fish diet, metabolism rate and the quality of the available food. By studying the otolith we can in general determine where the fish lives and what it eats, explained Sako.
"Our study suggested that juvenile Lates stappersii feed on impoverished-carbon (low quality plant material) food items and shift their diet to more enriched-carbon ones (high quality fish) as they mature," said Sako. "Our work revealed three distinct growth stages of Lates stappersii: juvenile, adulthood and the sexual maturity," he added.
Lake Tanganyika is one of the great lakes of Africa and is the second largest lake by surface area on the continent. It is the deepest lake in Africa and holds the greatest volume of fresh water in Africa. The lake is divided between four east African countries - Brundi, Democratic Republic of the Congo, Tanzania and Zambia.
Original Publication Information
Results of this study, "Stable Oxygen and Carbon Isotopic Compositions of Lates stappersii Otoliths from Lake Tanganyika, East Africa," are reported by Aboubakar Sako, Kenneth G. MacLeod and Catherine M. O’Reilly in the latest issue (Volume 33, No. 4, pp. 806-815) of the Journal of Great Lakes Research, published by the International Association for Great Lakes Research, 2007.
Contacts
For more information about the study, contact Aboubakar Sako, Department of the Geological Sciences, University of Cape Town, South Africa, Rondeboch, 7701, skxabo001@uct.ac.za, (021 650 2931).
For information about the Journal of Great Lakes Research, contact Marlene Evans, Editor, National Water Research Institute, 11 Innovation Boulevard, Saskatoon, SK, S7N 3H5, Canada; editor@iaglr.org; (608) 692-1076.
Links
Since 1967, IAGLR has served as the focal point for compiling and disseminating multidisciplinary knowledge on North America's Laurentian Great Lakes and other large lakes of the world and their watersheds. In part, IAGLR communicates this knowledge through publication of the Journal of Great Lakes Research, available to members in print and electronic form. A searchable archive of the journal is available online and includes the abstracts of articles from the journal's inception in 1975 through the most recent issue. In addition, complete articles are available to members who have signed up for an electronic subscription.
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